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α-MSH - α-melanocyte-stimulating hormone

α-MSH is a POMC-derived peptide hormone produced in the pars intermedia. α-MSH has effects on skin and coat pigmentation, metabolism, food intake and obesity, and has anti-inflammatory effects, although may produce hyperalgesia.

α-MSH may be a useful diagnostic test for PPID.

J Am Vet Med Assoc. 2012 Jul 15;241(2):241-8. (PubMed)
Seasonal variation in results of diagnostic tests for pituitary pars intermedia dysfunction in older, clinically normal geldings.
Schreiber CM, Stewart AJ, Kwessi E, Behrend EN, Wright JC, Kemppainen RJ, Busch KA.
ACTH, α-MSH, insulin and cortisol were measured every month in 15 clinically normal geldings (median age 14 years).
"A significant time effect was found between months for α-MSH and insulin concentrations. Endogenous cortisol and ACTH concentrations remained within existing reference ranges..."Seasonally specific reference ranges are required for α-MSH and insulin concentrations, with significantly higher concentrations detected in the fall. Practitioners should be advised to submit samples only to local laboratories that can provide such reference ranges for their local geographic region."

Paradis MR, McFarlane D, Costa L, Hermida P, Sanchez A, Mahon J
Comparison of melanocyte stimulating hormone (MSH) and adrenocorticotropin hormone (ACTH) levels in 179 horses in New England
2011 Equine Endocrinology Summit

Beech J, McFarlane D, Lindborg S, Sojka JE, Boston RC.
α-Melanocyte--stimulating hormone and adrenocorticotropin concentrations in response to thyrotropin-releasing hormone and comparison with adrenocorticotropin concentration after domperidone administration in healthy horses and horses with pituitary pars intermedia dysfunction.
J Am Vet Med Assoc. 2011 May 15;238(10):1305-15. (PubMed)
"ACTH and α-MSH concentrations increased in all horses after TRH administration, with a greater and more prolonged increase in horses with PPID. Percentage increase was significantly greater for α-MSH concentration than for ACTH concentration."
"Use of an α-MSH concentration ≥ 30 or 50 pmol/L did not appear superior to use of an ACTH concentration ≥ 36 pg/mL for the diagnosis of PPID, either before or 30 minutes after TRH administration."

McFarlane, D., Paradis, M., Zimmel, D., Sykes, B., Brorsen, B., Sanchez, A. and Vainio, K.
The Effect of Geographic Location, Breed, and Pituitary Dysfunction on Seasonal Adrenocorticotropin and α-Melanocyte-Stimulating Hormone Plasma Concentrations in Horses
Journal of Veterinary Internal Medicine, 25: 872–881 (2011)
"The fall peak plasma α-MSH concentration occurred earlier in horses residing at more northern locations." "Concentrations of both hormones (α-MSH and ACTH) were high in the fall compared with the spring in horses with PPID."

McFarlane D, Donaldson MT, McConnell SM, Cribb AE
Effects of season and sample handling on measurement of plasma α-melanocyte-stimulating hormone concentrations in horses and ponies
AJVR, Vol 65, No. 11, November 2004
Equine α-MSH was measured by radioimmunoassay designed for use with human plasma. Blood was collected into EDTA and separated by centrifugation. Hemolysis did not significantly affect α-MSH concentrations. There was no significant difference in plasma α-MSH concentrations between males and females. It was recommended that plasma should be separated within 8 hours of blood sample collection and stored frozen (at -80'C) until assay, and as α-MSH does not appear to have a circadian rhythm, samples can be collected at any time of day.
α-MSH showed a distinct seasonal rise in the autumn in healthy horses and ponies, with concentrations peaking in September in stabled horses in Canada (9.5 - 15.9 pmol/l April compared to 2.7 - 58 pmol/l September) and ponies at grass in Pennsylvania (3 - 19 pmol/l May compared to 40 - 185 pmol/l September). 6 out of 13 healthy ponies had plasma α-MSH concentrations in excess of that considered diagnostic of PPID (> 91 pmol/l) in September, therefore if plasma α-MSH concentrations are to be used to diagnose PPID, seasonally adjusted reference ranges should be used.

J Am Vet Med Assoc. 2009 Sep 15;235(6):715-22. (PubMed)
Evaluation of plasma ACTH, alpha-melanocyte-stimulating hormone, and insulin concentrations during various photoperiods in clinically normal horses and ponies and those with pituitary pars intermedia dysfunction.
Beech J, Boston RC, McFarlane D, Lindborg S.
"Plasma alpha-MSH and ACTH concentrations increased as daylight decreased from summer solstice (maximum daylight hours) to 12 hours of daylight."

Domest Anim Endocrinol. 2006 May;30(4):276-88. Epub 2005 Aug 9. (PubMed)
Alpha-melanocyte stimulating hormone release in response to thyrotropin releasing hormone in healthy horses, horses with pituitary pars intermedia dysfunction and equine pars intermedia explants
McFarlane D, Beech J, Cribb A.

Vet Clin North Am Equine Pract. 2011 Apr;27(1):1-17
Hypothalamic-pituitary gland axis function and dysfunction in horses
Hurcombe SD

There is a complex relationship between β-endorphin and alpha-MSH:
Endocrinology. 2012 Sep;153(9):4246-55 (PubMed) (Full paper)
β-Endorphin antagonizes the effects of α-MSH on food intake and body weight
Dutia R, Meece K, Dighe S, Kim AJ, Wardlaw SL

Am J Vet Res. 2004 Nov;65(11):1469-73
Correlation between plasma alpha-melanocyte-stimulating hormone concentration and body mass index in healthy horses
Donaldson MT, McFarlane D, Jorgensen AJ, Beech J
"Horses in the upper quartile of BMI had significantly greater plasma alpha-MSH concentration (median, 9.1 pmol/L; range, 2.0 to 95.3 pmol/L) than horses in the lowest quartile of BMI (median, 70 pmol/L; range, 3.6 to 15.7 pmol/L)" - NB the median figures are presumably either the wrong way round or mistyped.
"CONCLUSIONS AND CLINICAL RELEVANCE:A correlation exists between plasma alpha-MSH concentration and BMI in horses. Further study is required to determine whether melanocortin receptor defects underlie this correlation or, alternately, whether plasma alpha-MSH concentration is simply a correlate of adiposity".

AAKG

AAKG (arginine alpha ketoglutarate) is a salt of the amino acid arginine, and alpha-ketoglutaric acid. Arginine is a conditionally essential amino acid - in mature healthy horses arginine appears to be synthesized in adequate quantities to meet demands, but ill or injured horses may need a dietary source of arginine. Important functions of arginine include increasing the secretion of hormones such as insulin, glucagon, prolactin and growth hormone, improving immune function, increasing nitric oxide production, improving endothelial function, and nitrogen metabolism and excretion in the urea cycle.

Good sources of arginine include protein concentrates (pea, whey, soya), whole soya beans, copra meal, linseed, legume pasture and grass pasture.

Arginine has a relationship with insulin, but findings in research appear to differ, with some papers finding that supplementing with arginine stimulates insulin secretion (which might not be ideal for horses with insulin dysregulation and already abnormally high insulin levels?), others finding that chronic supplementation with arginine leads to insulin resistance (in rats - Salgueiro et al. 2017) or improves insulin sensitivity in humans (Bogdanski et al. 2012).

Risk assessment of "other substances" - L-arginine and arginine alpha-ketoglutarate - Norwegian Scientific Committee for Food Safety 2016

AAKG is a support nutrient for the synthesis of nitric oxide and there have been suggestions that it may help horses that suffer foot pain in cold weather. See the ECIR Group file on AAKG for more information before using as AAKG is contra-indicated in certain situations (including horses with acute laminitis, uncontrolled PPID, melanomas/ tumours or horses receiving certain drugs).

Winter care of the Insulin resistant horse
Dr. Kellon: "When more help was needed, we added the amino acid arginine, in the form of AAKG - arginine alpha-ketoglutarate, to support nitric oxide sythesis. Nitric oxide keeps blood vessels dilated. All horses made it through the winter without their usual hoof pain."

Abbreviations

α-MSH - alpha-melanocyte-stimulating hormone
AAKG - arginine alphaketoglutarate
ACTH - adrenocorticotropic hormone (stimulates cortisol release from adrenal glands)
AIRg - Acute insulin response to glucose
ALC/ALCAR - Acetyl-L-carnitine
AMPK - adenosine monophosphate kinase / AMP-activated protein kinase
AUC - area under the curve
BCS - body condition score
BM - body mass
BW or bwt - bodyweight
CBC - complete blood count
CGIT - combined glucose-insulin test
CK - creatine kinase (a muscle enzyme, frequently slightly raised with acute laminitis)
CLIP - corticotropin-like intermediate lobe peptide (a POMC derived peptide)
CRH - cortrosyn releasing hormone (stimulates release of ACTH)
DDFT - deep digital flexor tendon
DM - dry matter
DMI - dry matter intake
DST - dexamethasone suppression test
ECD - equine Cushing's disease (PPID)
EDTA - ethylenediaminetetraacetic acid (an anticoagulant for blood samples, particularly for ACTH and CBC)
EMS - equine metabolic syndrome
ESC - ethanol-soluble carbohydrate (simple sugars; a sub-set of WSC and NSC)
ET - endothelin (e.g. ET-1, endothelin-1, a cytokine that is a potent vasoconstrictor)
FSIGT - frequently sampled intra-venous glucose tolerance (test)
GI - gastrointestinal
GLUT - glucose transporter
I:G ratio - insulin:glucose ratio
IL - interleukin (e.g. IL-1, interleukin-1, an inflammatory cytokine)
IR - insulin resistance
IV - intravenous
IVGT - intravenous glucose tolerance (test)
J-herb - jiaogulan (gynostemma pentaphyllum)
MIRG - modified insulin-to-glucose ratio
MMP - matrix metalloproteinase
NFC - nonfibre carbohydrates
NO - nitric oxide
NSAID - non-steroidal anti-inflammatory drug (e.g. Bute)
NSC - nonstructural carbohydrate (WSC + starch)
OGT - oral glucose tolerance (test)
P3 - third phalanx/coffin bone/pedal bone
PLMS - pre-laminitic metabolic syndrome (now replaced by EMS)
POMC - pro-opiomelanocortin
PPID - Pituitary pars intermedia dysfunction (Cushing's disease)
PU/PD - polyuria/polydipsia (increased urination and increased thirst)
QUICKI - quantitative insulin sensitivity check index
RIA - radioimmunoassay
RISQI - reciprocal of the square root of insulin
Sg - glucose effectiveness - the ability of glucose to mediate its own disposal, independently of insulin
SI - insulin sensitivity - the sensitivity of target tissues to insulin-mediated glucose disposal
S/S - sugar/starch (as in low sugar/starch diet)
T2DM - type 2 diabetes mellitus
T3 - triiodothyronine (a thyroid hormone)
T4 - thyroxine (a thyroid hormone)
TNF-alpha - tumor necrosis factor-alpha
TRH - thyrotropin releasing hormone
TSH - thyroid stimulating hormone
WBC - white blood cell
WSC - water-soluble carbohydrate (ESC plus fructans; a sub-set of NSC)

Abscesses (foot)

Horses that have had rotation and/or distal descent frequently develop an abscess (pocket of infection) anything from a few days to several weeks after the initial laminitis. Pockets of blood/serum and dead tissue between the hoof wall/sole and P3 may become infected. The abscess grows, compressing surrounding live tissue and often causing intense pain, and will usually eventually break out through the coronary band, white line or above the heel bulbs. Pressure necrosis from thin soles, stone bruises and puncture wounds may also cause sole abscesses - hoof boots with thick pads should be used to protect thin soles (and the trim and diet examined to work out why the sole is thin).

The first sign of an abscess brewing is increased pain/lameness which may progress to non-weight bearing, usually in one leg, often with a bounding digital pulse and heat. There is sometimes swelling at the back of the leg above the fetlock, and there may be tenderness at the coronet or heel bulbs if the abscess is near the skin surface.

Abscesses can infect joint capsules or the pedal/coffin bone, causing osteomyelitis, and can lead to euthanasia. It is important that abscesses discharge and drain.

How do you tell the difference between an abscess and laminitis?

If the lameness, bounding digital pulse and heat are only in one leg, it's more likely to be an abscess. Abscesses can form in more than one foot at the same time (after all, the damage from the laminitis likely occurred in more than one foot at the same time), and laminitis can be worse in one foot than others (particularly if one foot was less correctly balanced), but as a general rule, lameness in just one foot following laminitis has a good chance of being an abscess.
If the initial laminitis has been controlled and nothing has changed, it's likely to be an abscess. Check that diagnosis, diet, management and trim are all as perfect as possible - if they are, look for an abscess.
If the lameness appears worse than the original laminitis, it is likely to be an abscess - owners often think their horse has broken a leg when first presented with an abscess, it can be that painful.
Consider the timescale - if the horse appeared to start to recover from the initial laminitis, then suddenly became lame usually a couple of weeks to a couple of months later, it's likely to be an abscess. Abscesses are often triggered by a trim - has the horse been trimmed recently, if so, it could be an abscess.
Radiographs (x-rays) sometimes show the position of an abscess- see x-rays of sub-solar abscess (inclusion of link does not imply endorsement of treatment methods used). Radiolucency (black areas on an x-ray) can be caused by gas once an abscess has drained, whereas an unopened abscess usually had a density similar to tissue so will appear grey (ref: MSD Veterinary Manual).
Hoof testers or a hammer may help locate the position of an abscess (be sure to differentiate whether the pain is in the wall or the sole).
Thermal imaging may also help to locate heat (and therefore an abscess) within the foot.

More information about abscesses following laminitis is available from Dr Eleanor Kellon's Cushing's and Insulin Resistance course.

How should an abscess be treated?
Plenty of controversy here! To drain or allow to break out naturally, to soak or to keep dry...?

Tetanus - the horse should always be vaccinated against tetanus. If tetanus cover is not up to date, tetanus antitoxin should be given.

Anti-inflammatory pain killers (NSAIDs) - if the pain from an abscess is severe, NSAIDs may be necessary on welfare grounds. However some vets suggest NSAIDs should not be used if possible (particularly if allowing the abscess to erupt naturally), as inflammation is needed to mobilize the abscess and cause it to erupt, and using anti-inflammatories can slow or prevent the abscess from maturing, thereby preventing healing. As soon as an abscess has burst/been released the pain will usually subside and use of NSAIDs can be stopped.

Antibiotics - the use of antibiotics to treat a foot abscess before it has burst is controversial, as systemic antibiotics are unlikely to be able to reach the bacteria in the centre of an abscess, and treating an abscess with antibiotics may slow resolution of the abscess. The bacteria involved in a foot abscess are likely to be anaerobic, commonly used antibiotics tend to be effective against aerobic bacteria.
Local antibiotics may be useful immediately following a sole penetration or if an abscess has been opened and damaged tissue exposed.

John Stewart has an excellent section on foot abscesses in Understanding the Horse's Feet p 176 onwards.

According to Pete Ramey (Care and Rehabilitation of the Equine Foot p 298), whilst draining the abscess usually provides immediate relief for the horse, it is usually impossible to avoid the risk of infection and to provide acceptable sterile care, and recurrence of the abscess and secondary infection becomes likely. Allowing the abscess to break out on its own means the abscess cycle is less likely to repeat. If an abscess is to be drained, it should be through the white line or by a hole drilled through the wall, NEVER through the sole. Whether the abscess erupts naturally or is drained, antiseptic treatments like Clean Trax or White Lightning, or 50:50 apple cider vinegar:water soaks are recommended.

How to soak and wrap a hoof - Genesee Valley Equine Clinic

Linda Cowles Hoof Care Abscess Page
Abscesses - Linda Cowles www.healthyhoof.com

Hoof Abscesses are Painful! - www.ironfreehoof.com
Hoof Abscesses - Stephen E. O'Grady, DVM, MRCVS

Subsolar Abscess and Coffin Bone Osteomyelitis - ACVS

Managing Hoof Abscesses - Stephen E. O'Grady, DVM, MRCVS

Pus and Us

Equine Foot Wounds - Alberta Agriculture and Rural Development

Other suggestions include:
To encourage mobilization of the abscess:
Warmth and movement encourage mobilization. Thoroughly clean the foot and poultice with hot Animalintex or another drawing poultice, or wrap the foot in cotton wool periodically soaked with warm water, or soak the clean foot in warm water and Epsom salts (magnesium sulphate) several times a day. Or keep the foot dry but use leg wraps or bandage over a dry poultice or nappy, or some suggest bandaging on warm towels or gently heated microwave hot pads.
Some owners swear by ichthammol, which (the website claims) "works to soften the tissues surrounding an injury and to help relieve swelling and draw out the abscess contents", and can be used to pack the hoof once an abscess has started to drain.

Once the abscess has erupted or been drained:
Draw the infection and keep scrupulously clean. Poultice the hole with hot Animalintex, or soak the clean foot in warm water and Epsom salts (magnesium sulphate) several times a day, or Clean Trax, White Lightning or 50:50 apple cider vinegar:water as above, to draw out all the infection. Apply antiseptic to the wound e.g. betadine - do not use anything that is not suitable for use on live tissue. Keep clean and dry - dry Animalintex or nappies make suitable dressings bandaged on to the foot, and poultice boots can be invaluable to keep dressings in place.

Photos of hooves with abscesses - www.barefoothooves.net

This abscess burst out at the coronary band 03 September.

The same abscess 6 weeks later (17 October) - the exit hole slit grows down with the hoof wall, with no further lameness seen.

Below - this sequence of x-rays and photos shows an abscess in the front foot of a pony with long-term chronic laminitis. The vet or farrier made a hole in the sole (blue arrows) to try to relieve the abscess in month 1, but this did not resolve the abscess despite CleanTrax soaks and poulticing. In the top left x-ray you can see that the abscess is trying to burst out at the heels (red arrow), and this happened shortly afterwards. Flushing with dilute iodine was then advised, but the abscess did not fully resolve until the hole in the sole grew over. The exit hole at the heel bulbs caused no problem at all, and just over 4 months after the first x-ray the foot looked fairly normal (although still needing a good realigning trim). Might this abscess have resolved much sooner if the sole had not been opened up?

Osteomyelitis/Septic Pedal Osteitis
Long-term abscessing can lead to osteomyelitis/septic pedal osteitis - the inflammation of bone due to bacterial infection - purulent exudate (pus) will be present. Although rare, osteomyelitis of the pedal/coffin bone can follow laminitis with severe rotation/sinking, usually after penetration of the sole or long-term (usually sub-solar) abscessing which extends into the bone. See Osteomyelitis/Septic Pedal Osteitis

Research & Articles
Cole SD, Stefanovski D, Towl S, Boyle AG
Factors associated with prolonged treatment days, increased veterinary visits and complications in horses with subsolar abscesses
Veterinary Record 2019 184(8):259. Published Online 04 December 2018. doi: 10.1136/vr.104138

Lame Horses: Examine Entire Hoof reports from Cole et al. 2019 that

The presence of a drainage tract and higher lameness score was associated with shorter treatment times; and
Complications included cellulitis (infection of the skin and underlying tissues), osteitis (inflammation of the lining of the bones in the foot), hoof cracks, septic joints, and recurrence. Each of these prolonged treatment times.

and “In terms of treatment, researchers found that antibiotic and nonsteroidal anti-inflammatory drugs, two medications frequently used in horses with infections, could actually be contraindicated. These medications prevent the abscess from maturing, thereby allowing the microorganisms responsible for the abscess to invade surrounding tissues”.

Draper Janna
Characterization and Identification of Bacterial Flora from Infected Equine Hooves
2013 Senior Honors Theses

Onishi JC, Park JW, Häggblom MM, Fennell MJ, Fugaro MN
Chronic laminitis is associated with potential bacterial pathogens in the laminae
Vet Microbiol. 2012 Feb 21 [Epub ahead of print] (PubMed)
"A common sequella of chronic laminitis in horses is repeated abscesses with variable lameness and drainage. It is unclear whether the exudate represents the debridement phase of a non-septic inflammatory process involving clearance of laminar tissue damaged during the acute episode of laminitis, or a response to a microbial infection developed by ascent of microbes from the environment to the tissue via the white line. The objective of this study was to evaluate the possibility that an undiagnosed microbial infection in laminar tissue is present in laminar tissue collected from chronically laminitic horses without an active hoof abscess. Methods to collect laminar tissue, aseptically, from control (non-laminitic) horses and those with chronic/recurrent laminitis are described. Laminae homogenates were evaluated for the presence of bacteria. Bacteria were identified using biochemical tests and sequencing of 16S rRNA and virulence genes. Laminae from chronically lamintic horses revealed 100-fold higher levels (P=0.002) of bacteria compared to control, non-laminitic horses. Although environmental organisms were identified, potential pathogens were identified. Included were Gram positive bacteria, Brevibacterium luteolum, coagulase-negative Staphylococcus spp. as well as Gram negative bacteria, enterohemorrhagic Escherichia coli and Alcaligenes faecalis. Further research is warranted to evaluate the role of bacteria in equine chronic laminitis."

ACP (Ace, Acepromazine, Sedalin)

Acepromazine (ACP) is a phenothiazine derivative and dopamine antagonist commonly used to sedate horses - www.drugbank.ca - Acepromazine.

NOAH datasheet for Sedalin

In the Foreward to Equine Laminitis (2017) edited by JK Belknap & RJ Geor, published by John Wiley & Sons, Chris Pollitt said
"Recent research indicates that drugs used to promote digital vasodilation for decades, including acepromazine... are ineffective", suggesting that mechanically loading or unloading the foot is more likely to affect lamellar circulation than medical treatment.

Pequito M, Amory H, Serteyn D, Busoni V, de Moffarts B, Sandersen C
Comparison of the Sedative and Hemodynamic Effects of Acepromazine and Promethazine in the Standing Horse
Journal of Equine Veterinary Science - 30 April 2012 (10.1016/j.jevs.2012.03.011)

Vet Surg. 2007 Dec;36(8):717-23. (PubMed)
Effects of intramuscular administration of acepromazine on palmar digital blood flow, palmar digital arterial pressure, transverse facial arterial pressure, and packed cell volume in clinically healthy, conscious horses.
Leise BS, Fugler LA, Stokes AM, Eades SC, Moore RM.
"CONCLUSION: IM acepromazine causes hypotension and increases palmar digital blood flow over time but the magnitude of the effect on digital blood flow was not sufficient to yield differences compared with saline-treated horses."

Vet Surg. 1999 May-Jun;28(3):154-60. (PubMed)
The effect of oral isoxsuprine and pentoxifylline on digital and laminar blood flow in healthy horses.
Ingle-Fehr JE, Baxter GM.
"Horses also received acetylpromazine as a positive control (0.066 mg/kg, intravenously)....Acepromazine resulted in a significant increase (P = .0007) in DBF" (Digital Blood Flow) "for approximately 75 minutes beginning 15 minutes after treatment. A mild but insignificant increase in LP" (Laminar Perfusion) "was identified after acetylpromazine treatment....Acepromazine caused an increased blood flow to the digit. Based on the results of this study acetylpromazine potentially would have a greater effect on improving digital blood flow than oral isoxsuprine or pentoxifylline when treating ischemic conditions of the foot in horses."

Can ACP be given to a horse that is taking Pergolide?

Generally no, unless specifically instructed by a vet. ACP is a phenothiazine, a dopamine antagonist. Pergolide is a dopamine agonist. The two drugs have opposing effects. Therefore ACP could reduce the effectiveness of pergolide/Prascend.

Ranvet - Equine Cushing's Syndrome:
"Phenothiazine tranquilizers such as acepromazine may interfere with the action of pergolide and dopamine antagonists should not be administered together with pergolide because they reduce its efficacy."

From the Irish Medicines Board datasheet for Sedalin oral gel:
"Acepromazine is a phenothiazine derivative. This group of molecules belongs to the neuroleptics: they depress the
central nervous system and exert associated effects on the autonomic system. These effects are due to their interference
with different neurotransmitter receptors (dopaminergic, adrenergic) and to their interference with hypothalamic
performance."

From the NOAH datasheet for Prascend (1mg pergolide tablets):
"Dopamine antagonists, such as neuroleptics (phenothiazines), domperidone, or metoclopramide ordinarily should not be administered concurrently with pergolide mesylate (a dopamine agonist); these agents may diminish the effectiveness of pergolide mesylate."

ACTH - Adrenocorticotropic Hormone

ACTH (also known as corticotropin) is a hormone, secreted from the pituitary gland, that stimulates the adrenal glands to release cortisol (and to a lesser extent, aldosterone). ACTH is derived from a hormone precursor protein, POMC (proopiomelanocortin). In healthy horses nearly all plasma ACTH is produced from POMC in the pars distalis in response to CRH (corticotropin releasing hormone) or AVP (arginine vasopressin) secreted by the hypothalamus. When cortisol reaches a certain level in the blood, a negative feedback signal causes a decrease in the secretion of CRH, AVP and ACTH.
Although in the healthy horse ACTH is also produced from POMC in the pars intermedia, most of the PI ACTH is converted into α-MSH and CLIP.

Horses with PPID have a reduced amount of dopamine acting as a break on the production of POMC in the pars intermedia, causing increased amounts of ACTH and other hormones to be secreted.

Vet Clin North Am Equine Pract. 2011 Apr;27(1):1-17
Hypothalamic-pituitary gland axis function and dysfunction in horses
Hurcombe SD

Corticotrophin (ACTH) edited by Choh Hao Li (1987)

Is the ACTH in PPID horses bioactive?
Horses with PPID often have high plasma ACTH concentrations but normal resting serum cortisol concentrations, and enlargement of the adrenal glands (from overproduction of cortisol) is not commonly seen in horses with PPID. Recent research by Cordero, Shrauner and McFarlane suggests that plasma ACTH from PPID horses is less biologically active than plasma ACTH from normal horses:

Cordero M, Shrauner B, McFarlane D
Bioactivity of Plasma ACTH from PPID-affected Horses Compared to Normal Horses
2011 Equine Endocrinology Summit
See Forum for more information and to comment.

Orth DN, Nicholson WE
Bioactive and immunoreactive adrenocorticotropin in normal equine pituitary and in pituitary tumors of horses with Cushing's disease
Endocrinology. 1982 Aug;111(2):559-63 (PubMed)
"Equine Cushing's disease is caused by hypersecretion of ACTH by hyperplasia or adenomas of pars intermedia (PI) cells, in contrast to human Cushing's disease, which is caused by hyperplasia or adenomas of pars distalis (PD) ACTH-secreting cells. We assayed both bioactive and immunoreactive (IR) ACTH in two normal equine pituitary glands and in the PD, PI, and pars nervosa of four such glands, as well as in the PI adenomas of five horses with Cushing's disease. In normal horse pituitaries, as in those of other species, most of the bioactive and IR-ACTH was found in PD, much less in PI, and only traces in pars nervosa. In PI adenomas of horses with Cushing's disease, bioactive ACTH concentrations were similar to those in normal PI, but the total tumor content of bioactive ACTH exceeded that of normal whole pituitary. IR-ACTH concentrations were even higher in PI tumors, suggesting that some of the tumor ACTH was biologically inactive. Plasma IR-ACTH, which, like the PI adenoma tissue, presumably included a major fraction of bioactive ACTH, was greatly elevated in five horses with Cushing's disease and would account for the adrenal hyperplasia and hyperfunction observed in these animals."

Reasons for ACTH to be above normal other than PPID
Illness (Stewart et al. 2019)

Stewart AJ, Hackett E, Bertin FR, Towns TJ.
Cortisol and adrenocorticotropic hormone concentrations in horses with systemic inflammatory response syndrome.
J Vet Intern Med. 2019 Sep;33(5):2257-2266. doi: 10.1111/jvim.15620. Epub 2019 Sep 12. PMID: 31512777; PMCID: PMC6766528.

ACTH testing

ACTH - to convert pg/ml to pmol/L, multiply pg/ml x 0.2222, so 45 pg/ml = 10 pmol/L
- to convert pmol/L to pg/ml, divide pmol/L by 0.2222, so 10 pmol/L = 45 pg/ml
- pg/ml = ng/L

An ACTH blood test can be used to diagnose PPID and is the diagnostic test recommended by The Laminitis Site. Advantages of the ACTH test over the DST are:
- no risk of laminitis, nothing is injected into or given to the horse
- less expensive as single vet visit/one blood collection
- can be used at any time of day and year for any horse
- the only test that is diagnostic during the autumn seasonal rise (if seasonally adjusted reference ranges are used)
- the results give an indication of the severity of the PPID and can be used to monitor pergolide effectiveness

See the "How is PPID Diagnosed" video at www.prascend.co.uk
Andy Durham of Liphook Equine Hospital on PPID diagnostic tests: "the main one undoubtedly is looking at the levels of a hormone called ACTH in the horse's blood stream, which is a fairly simple test, ...it just requires a one-off blood test to look at the hormone level and it can be done at any time of day in any time of year on any individual."

Labs testing equine ACTH
Liphook Equine Hospital and Laboratory (UK) - instructions for ACTH testing
Lyon (France) (NB TLS does not consider this protocol satisfactory - please refer to Liphook's instructions for keeping the blood cool during transport)
Idexx-Alfort (France)
Laboratoire Frank Duncombe, Caen (France)
Cornell (USA) - instructions for ACTH testing
UC Davis (USA)

Protocol for measuring ACTH:
- collect blood into EDTA tube
- separate plasma either by gravity or centrifuge
- chill within 3 hours of collection
- keep chilled en route to laboratory

However check with the individual lab for their handling protocol.

ACTH is affected by season, age, diet and stress - see below.

Are ACTH levels constant or are pulsatility/ultradian rhythms an issue?
ACTH is released in a pulsatile manner and ACTH concentrations can vary significantly ("There is significant hourly variation in ACTH in horses/ponies with very high initial values" - Thomas Divers), therefore some vets recommend taking more than one blood collection when testing ACTH.

Rendle DI, Duz M, Beech J, Parkin T, Durham AE
Investigation of Single and Paired Measurements of Adrenocorticotropic Hormone for the Diagnosis of Pituitary Pars Intermedia Dysfunction in Horses
Journal of Veterinary Internal Medicine Volume 29, Issue 1, pages 355–361, January/February 2015

Can ACTH be tested during the seasonal rise?
Yes, definitely - in fact according to Liphook this may be the best time to test - "it is apparent that testing in the autumn may actually allow the greatest differentiation between PPID cases from normal horses and there is no reason at all to avoid testing for PPID in the autumn."
- as long as a seasonally adjusted reference range is used. Liphook tested ACTH from 156 normal horses every month for a year and plotted the upper limit reference interval. August, September and October had a mean upper limit of 47 pg/ml with September having the highest at over 50 pg/ml. The mean upper limit for the rest of the year was 29 pg/ml with January, March, July, November and December all having mean upper limits over 30 pg/ml.

Cathy McGowan echoed this in her 11 August 2011 Prascend webinar - "if you test ACTH in the autumn with seasonally adjusted ACTH reference ranges, you have an increased chance of picking up PPID cases", and Cathy suggested that if an ACTH test has not previously been diagnostic, it could be worth retesting ACTH in the autumn.

See Seasonal rise in ACTH

Doesn't the blood for ACTH testing need special handling?
Liphook have found that delayed separation of plasma has no significant effect on ACTH levels, and centrifugation before arrival at the laboratory may not be necessary - "Gravity separated plasma may be sent chilled to the laboratory as long as it is centrifuged by the testing laboratory before analysis."

However they also found that temperatures of 20'C were associated with a significant decrease in ACTH over time (around 20% decrease in 24 hours) and specify that samples must be chilled within 3 hours of collection. See Investigation of the in vitro stability of ACTH in horses, Durham A, Copas V

Rendle DI, Litchfield E, Gough S, Cowling A, Hughes KJ
The effects of sample handling and N-phenylmaleimide on concentration of adrenocorticotrophic hormone in equine plasma
Equine Veterinary Journal Volume 47, Issue 5, pages 587–591, September 2015, first published online: 7 OCT 2014
"Clinically useful results are still obtained if ACTH concentration is measured up to 48 h after sample collection. Allowing samples to separate by gravity rather than centrifugation did not have a significant effect on ACTH concentration".

Equine Vet J. 2002 Nov;34(7):679-85.
Plasma adrenocorticotropin (ACTH) concentrations and clinical response in horses treated for equine Cushing's disease with cyproheptadine or pergolide.
Perkins GA, Lamb S, Erb HN, Schanbacher B, Nydam DV, Divers TJ.
"The ACTH was stable when whole blood was collected and held in plastic tubes for 8h before separating the plasma."

Clinical Chemistry February 2007 vol. 53 no. 2 358-359

Preanalytical Stability of Adrenocorticotropic Hormone Depends on Time to Centrifugation Rather than Temperature
Reisch N, Reincke M, Bidlingmaiera M
Mean (human) ACTH concentrations in whole EDTA blood stored at room temperature decreased by no more than 10% in 24 hours (using chemiluminescent assay).
ACTH in uncentrifuged samples decayed more rapidly than immediately centrifuged samples.
ACTH concentrations in some samples decreased far more than others.
Centrifugation and separation of plasma is recommended within 4 hours of sample collection.

Hu K, Stewart AJ, Yuen KY, Hinrichsen S, Dryburgh EL, Bertin FR
The effect of freeze-thaw cycles on determination of immunoreactive plasma adrenocorticotrophic hormone concentrations in horses
J Vet Intern Med. 2020 May;34(3):1350-1356. doi: 10.1111/jvim.15771. Epub 2020 Apr 7. PMID: 32255541; PMCID: PMC7255672
Freezing and thawing caused a significant decrease in ACTH compared to unfrozen samples, with an average 17.4% decrease in measured ACTH after one freeze-thaw cycle. One freeze-thaw cycle did not affect diagnosis of PPID, but two or more freeze-thaw cycles did. ACTH from horses with PPID was more affected by freeze-thaw cycles than ACTH from healthy horses. The paper concludes that "if PPID is suspected, an unfrozen sample should be used for analysis."

What can cause ACTH to increase or decrease?
Any form of stress - illness, pain, e.g. from laminitis, excitement, exercise, travelling, use of a twitch, veterinary procedures, e.g. dental work, fear of vet/needles - could increase ACTH from the pars distalis.  Some medicines increase ACTH, e.g clenbuterol (Ventipulmin), and some sedation drugs may affect ACTH (both increase and decrease), insulin or glucose. See Does a high ACTH result mean my horse has PPID?

Using a twitch on donkeys resulted in a significant increase in mean plasma ACTH concentration in Hendrike Vreeman's PhD thesis The Twitch in Donkeys

"Isolation distress and mild to moderate illness and pain do not appear to affect plasma ACTH. General anaesthesia, strenuous exercise, sedation, severe illness and severe pain may all increase plasma ACTH." From Dealing with Equine PPID in Equine Practice

Vet J. 2010 Apr;184(1):100-4 (PubMed)
Plasma levels of heat shock protein 72 (HSP72) and beta-endorphin as indicators of stress, pain and prognosis in horses with colic
Niinistö KE, Korolainen RV, Raekallio MR, Mykkänen AK, Koho NM, Ruohoniemi MO, Leppäluoto J, Pösö AR.
"Plasma beta-endorphin was related with severity of colic and survival, as well as with plasma cortisol, ACTH and lactate concentrations, heart rate, PCV and pain score."

Vet Clin Equine 27 (2011)
Hypothalamic-Pituitary Gland Axis Function and Dysfunction in Horses
Hurcombe SDA
Endotoxaemia, sepsis, critical illness, acute abdominal pain, colic and proinflammatory cytokines may increase ACTH.

Vet Clin Equine 27 (2011) 35-47
Endocrine Dysregulation in Critically Ill Foals and Horses
Toribio RE
Endotoxaemia in mares induces AVP, oxytocin and ACTH release (Alexander et al. 1996). Cortisol concentrations were elevated in acutely laminitic horses (Clarke et al. 1982).

Freezing the blood sample before it has been separated by centrifuge can cause falsely high ACTH concentrations.

Feeding and diet may affect ACTH:
Horses eating a starch diet had significantly higher ACTH than horses on a low sugar/starch diet:
Jacob SI, Geor RJ, Weber PSD, Harris PA, McCue ME
Effect of dietary carbohydrates and time of year on ACTH and cortisol concentrations in adult and aged horses
Domest Anim Endocrinol. Published online 2017 Nov 1;63:15-22. doi: 10.1016/j.domaniend.2017.10.005

Diez de Castro E, Lopez I, Cortes B, Pineda C, Garfia B, Aguilera-Tejero E
Influence of feeding status, time of the day and season on baseline ACTH and the response to TRH-stimulation test in healthy horses
Domestic Animal Endocrinology published online 07 March 2014
50 healthy horses had baseline ACTH of 17.1 pg/ml before feeding and 46.1 pg/ml 2 hours after feeding.
6 healthy horses had TRH stimulated ACTH (10 minutes after being given TRH) of 124.1 pg/ml before feeding and 192.6 pg/ml 2 hours after feeding, and TRH stimulated ACTH (30 minutes after being given TRH) of 40.1 pg/ml before feeding and 73.2 pg/ml 2 hours after feeding .
"In addition to season, feeding is a potential confounding factor when measuring baseline or stimulated ACTH in horses. In conclusion, feeding status should be standardized for the diagnosis of equine pituitary pars intermedia dysfunction."

Breed may affect ACTH results:
Bamford NJ, Harris PA, Bailey SR
Circannual variation in plasma adrenocorticotropic hormone concentrations and dexamethasone suppression test results in Standardbred horses, Andalusian horses and mixed‐breed ponies
Australian Veterinary Journal published online 01 October 2020. https://doi.org/10.1111/avj.13022
Healthy Standardbreds, Andalusians and ponies, aged 5-15 years, had ACTH and DST tests carried out 10 times over 18 months
"Higher plasma ACTH concentrations and more false‐positive DST results were obtained during autumn in ponies and Andalusian horses when compared with Standardbred horses. Potential differences between breeds should be considered when interpreting test results for horses and ponies that are evaluated for PPID. Further work is recommended to establish population‐based reference intervals and clinical cut‐off values for ACTH in different equine breeds."

What are typical ACTH results and normal reference ranges?
ACTH concentrations vary widely, even in healthy (non-PPID) horses. However it is almost impossible to be confident a horse does not have PPID without a post mortem to examine the pituitary gland. Here are some examples:

Healthy horses
From Donaldson et al. 2005 using chemiluminescent immunoassay normal ref range 9 - 35 pg/ml - all horses and ponies were supposedly healthy (non-PPID) with no symptoms of PPID and negative DST in January (some of the September results would be considered diagnostic of PPID using current seasonally adjusted reference ranges!).
January horses range from 13.4 to 32.4 pg/ml
January ponies range from 8.1 to 36.9 pg/ml
May horses range from 13.1 to 22.2 pg/ml
May ponies range from 12.0 to 33.1 pg/ml
September horses range from 25.6 to 140.0 and the following September from 20.5 to 236.0 pg/ml
September ponies range from 34.9 to 479.0 and the following September from 30.6 to 192.0 pg/ml

See Donkeys and mules for donkey ACTH reference ranges.

Are the blood tests for PPID 100% accurate?
No - none of the laboratory tests for PPID are 100% accurate. Clinical signs should always be considered as well as laboratory results.
Some horses seem to show clinical signs of PPID but repeatedly test negative. In these cases:
- a trial of Pergolide could be considered;
- ACTH could be retested during the seasonal rise (usually August - October in the northern hemisphere);
- a TRH stimulation test, testing ACTH, could be carried out (but probably not during the seasonal rise).

An assessment of measurement of ACTH in human plasma in 35 laboratories using 7 different assays (2 CLS, 5 IRMA):
Eur J Endocrinol. 2011 Apr;164(4):505-12. Epub 2011 Jan 20 (PubMed) Full Paper
Assessment of ACTH assay variability: a multicenter study.
Pecori Giraldi F, Saccani A, Cavagnini F; Study Group on the Hypothalamo-Pituitary-Adrenal Axis of the Italian Society of Endocrinology.
"Field ACTH assays have to be interpreted with caution as they are burdened by high variability and often fail to correctly identify patients with suppressed ACTH secretion."

ACTH can be measured by RIA or CIA - click here for definitions of RIA and CIA.

McGilvray TA, Knowles EJ, Harris PA, Menzies-Gow NJ
Comparison of immunofluorescence and chemiluminescence assays for measuring ACTH in equine plasma
Equine Vet J. published online 18 Jan 2020. doi: 10.1111/evj.13227

ACTH assay methods used in published research:
Immulite - chemiluminescent immunoassay - New York State Animal Diagnostic Lab - ref range 9 - 35 pg/ml - Donaldson et al. (2005)
Immulite - chemiluminescent immunoassay - Purdue University - ref range 10 - 59 pg/ml - Miller et al. 2008
Diagnostic Products Corp - chemiluminescence - AHDC at Cornell (sequential immunometric assay using chemiluminescence for signal generation. ACTH assay generally specific for ACTH fragment 1-39 but has approx. 12-14% cross-reactivity with fragment 18-39) - Beech et al. (2011)

Research - ACTH testing

Durham AE, Potier JF, Huber L
The effect of month and breed on plasma adrenocorticotropic hormone concentrations in equids
The Veterinary Journal August 2022 Vol 286 105857.  https://doi.org/10.1016/j.tvjl.2022.105857.  View preprint full paper on SSRN.

Tatum RC, McGowan CM, Ireland JL.
Evaluation of the sensitivity and specificity of basal plasma adrenocorticotrophic hormone concentration for diagnosing pituitary pars intermedia dysfunction in horses: A systematic review.
Vet J. 2021 Sep;275:105695. doi: 10.1016/j.tvjl.2021.105695. Epub 2021 May 21. PMID: 34099343.

Horn R, Stewart AJ, Jackson KV, Dryburgh EL, Medina-Torres CE, Bertin FR
Clinical implications of using adrenocorticotropic hormone diagnostic cutoffs or reference intervals to diagnose pituitary pars intermedia dysfunction in mature horses
J Vet Intern Med. Published online 24 Dec 2020. doi: 10.1111/jvim.16017

Durham AE, Clarke BR, Potier JFN, Hammarstrand R, Malone GL
Clinically and temporally specific diagnostic thresholds for plasma ACTH in the horse
Equine Veterinary Journal published online 29 May 2020. https://doi.org/10.1111/evj.13292

Acupuncture (for laminitis)

"Alternative therapies such as massage, acupuncture and chiropractics may also help to minimize and ameliorate the negative impact of forelimb pain on the muscles and joints of the back and pelvic limbs." - General Supportive Care for the Lamnitis Case, Ch 30 p 273 Equine Laminitis (2017), Floyd AE and Taylor DR

TLS NB: endocrinopathic laminitis is likely to involve vasodilation/increased blood flow to the foot, therefore therapies that aim to increase vasodilation may be contra-indicated, at least at some points in the treatment following laminitis.

Acupuncture Can Help Horses With Chronic Laminitis - Alexandra Beckstett, May 2019, www.thehorse.com

Faramarzi B, Lee D, May K, Dong F
Response to acupuncture treatment in horses with chronic laminitis
Can Vet J. 2017 Aug;58(8):823-827

Bowker R, Isbell D, Lancaster L
Preliminary findings from a pilot study on equine foot blood flow in response to acupuncture.
Presented at the American Academy of Veterinary Acupuncture Annual Conference Boston March 2015 (ResearchGate)

Lancaster LS, Bowker RM
Acupuncture Points of the Horse’s Distal Thoracic Limb: A Neuroanatomic Approach to the Transposition of
Traditional Points
Animals 2012, 2, 455-471; doi:10.3390/ani2030455

Acupuncture for Laminitis - Dr Lisa Lancaster - Holistic Horse - June/July 2012

Adiponectin

Adiponectin is an adipokine, a cytokine (or cell signaling protein) secreted by adipocytes (fat cells)/adipose tissue. Leptin is another adipokine.
Adiponectin concentrations appear to reflect metabolic problems due to obesity, such as insulin dysregulation, whereas leptin concentrations appear to reflect fat mass.
Adiponectin has anti-inflammatory actions, decreases vascular tone and increases insulin sensitivity.
Obesity and glucocorticoids lower adiponectin concentrations, and hyperinsulinaemia suppresses adiponectin receptor expression, potentially causing adiponectin resistance.

The Potential Usefulness and Applications of Adiponectin Testing in Horses with Suspected EMS - Prof. Andy Durham, Liphook Equine Hospital - 2016

HMW Adiponectin – a new tool for EMS diagnosis - Liphook Equine Hospital Spring 2016 newsletter
"Following research over the last 5 to 10 years, the Liphook Equine Hospital is the first (and only) laboratory internationally to have validated an HMW adiponectin assay for use in equine practice. Adiponectin is best measured in serum (red top) and is stable for at least 48 hours when chilled. There is no circadian pattern or gender/age effects in horses, and adiponectin is not notably affected by feeding or exercise. Weight loss programmes have been shown to restore normal levels."

Meier AD, de Laat MA, Reiche DB, Sillence MN
Glucagon-like peptide-1, insulin-like growth factor-1, and adiponectin in insulin-dysregulated ponies: effects of feeding a high nonstructural carbohydrate diet and association with prospective laminitis
Domest Anim Endocrinol. published online Sept 2019 13;71:106397. doi: 10.1016/j.domaniend.2019.106397

Menzies-Gow NJ, Knowles EJ, Rogers I, Rendle DI
Validity and application of immunoturbidimetric and enzyme-linked immunosorbent assays for the measurement of adiponectin concentration in ponies
Equine Veterinary Journal published online 20 April 2018. doi: 10.1111/evj.12960
Adiponectin concentrations measured by radioimmunoassay (RIA) have been lower in ponies that had had laminitis or ponies that went on to have laminitis, than non laminitic ponies. Immunoturbidimetric (IT) and enzyme linked immunosorbent (ELISA) assays for equine total adiponectin concentrations (TAC) and high molecular weight adiponectin concentrations (HMWAC) were evaluated at RVC and Rainbow Equine Lab. "The IT assay was precise and demonstrated good agreement with the previously validated radioimmunoassay" - IT is suitable for measurement of TAC. Elisa was not suitable for measurement of TAC or HMWAC. Previously laminitic ponies had a mean TAC of 8.9 μg/mL compared to never laminitic ponies mean of 24.2 μg/mL. Of the never laminitic ponies, those that went on to develop first time laminitis within a year had a median TAC of 4.8 μg/mL (interquartile range 2.65-13.4 μg/mL), whilst those that remained laminitis free had a median TAC of 19.9 μg/mL (interquartile range 9.95-31.5 μg/mL. Total adiponectin concentration was significantly associated with laminitis occurrence within 12 months.

Menzies-Gow NJ, Harris PA, Elliott J
Prospective cohort study evaluating risk factors for the development of pasture-associated laminitis in the United Kingdom
Equine Vet J. 2017 May;49(3):300-306. doi: 10.1111/evj.12606. Epub 2016 Aug 25
446 ponies aged 7 or above with no history of laminitis had body condition score, height, weight, crest height and thickness measured and adiponectin, leptin, triglyceride, basal insulin, insulin post-dexamethasone, insulin-like growth factor 1 (IGF-1), IGF binding protein 1 (IGFBP-1), IGFBP-3, C-reactive protein, von Willebrand's factor, soluble E-selectin and P-selectin concentrations tested. 72.2% were overweight (BCS 7/9 or above), 27.3% were ideal weight (BCS 4-6/9), and 0.5% underweight (BCS 1-3/9).
After 1 year 4% of ponies were reported to have had laminitis; after 2 years 6.7% and after 3 years 9.9%.
Adiponectin, resting insulin and insulin post-dexamethasone were significantly associated with laminitis occurrence.
Conclusion: Risk factors for future laminitis prior to disease occurrence include low plasma adiponectin and high serum basal insulin or insulin post-dexamethasone concentrations.
TLS note: dexamethasone increases insulin and should not be given to horses at risk from insulin dysregulation/laminitis.

Bamford NJ, Potter SJ, Harris PA, Bailey SR
Effect of increased adiposity on insulin sensitivity and adipokine concentrations in horses and ponies fed a high fat diet, with or without a once daily high glycaemic meal
Equine Vet J. 2016 May;48(3):368-73. doi: 10.1111/evj.12434. Epub 2015 May 13

Wray H, Elliott J, Bailey SR, Harris PA, Menzies-Gow NJ
Plasma concentrations of inflammatory markers in previously laminitic ponies
Equine Vet J. 2013 Sep;45(5):546-51. Epub 2013 Feb 19 (PubMed)
The inflammatory profile of previously laminitic ponies was no different to never laminitic ponies:
"Conclusions: There were significant effects of season, gender and bodyweight on a number of proinflammatory mediators or markers of inflammation. The only marker influenced by laminitis status was adiponectin, and concentrations of this anti-inflammatory marker were lower in previously laminitic animals.
Potential relevance: Recurrent laminitis may be associated with reduced anti-inflammatory capacity rather than a proinflammatory state."

Ungru J, Blüher M, Coenen M, Raila J, Boston R, Vervuert I
Effects of body weight reduction on blood adipokines and subcutaneous adipose tissue adipokine mRNA expression profiles in obese ponies
Vet Rec. 2012 Nov 24;171(21):528. doi: 10.1136/vr.100911. Epub 2012 Oct 7
15 obese ponies were classified either insulin resistant or insulin sensitive by FSIGT and placed on restricted energy diets. Significant weight loss occurred when ponies were fed 7 MJ DE per 100 kg bodyweight. Serum leptin was similar in obese insulin resistant and obese insulin sensitive ponies, and decreased significantly with weight loss. Obese insulin resistant ponies had significantly lower plasma adiponectin levels than obese insulin sensitive ponies.
"Plasma adiponectin was strongly related to IR, whereas serum leptin and RBP4 were closely linked to adiposity, independent of insulin sensitivity."

Wooldridge AA, Edwards HG, Plaisance EP, Applegate R, Taylor DR, Taintor J, Zhong Q, Judd RL
Evaluation of high-molecular weight adiponectin in horses
Am J Vet Res. 2012 Aug;73(8):1230-40. doi: 10.2460/ajvr.73.8.1230
See also:
High molecular weight adiponectin is negatively correlated with body condition score and serum insulin:
Wooldridge AA, Edwards HG, Plaisance EP, Applegate R, Taylor DR, Taintor J, Zhong Q, Judd RL
High molecular weight adiponectin in horses
2011 Equine Endocrinology Summit

Owens, Kellie Marie, MSc Thesis 2009
The Effect of Changes in Body Condition on Insulin Sensitivity, Leptin, and Adiponectin in Horses fed Forage-Only Diets

Adiponectin concentrations were found to be inversely proportional to body fat mass (obesity) in horses:
Vet J. 2006 Nov;172(3):460-5. Epub 2005 Jun 29.
Adiponectin and leptin are related to fat mass in horses.
Kearns CF, McKeever KH, Roegner V, Brady SM, Malinowski K.

Adipose tissue

Reynolds A, Keen JA, Fordham T, Morgan RA
Adipose tissue dysfunction in obese horses with equine metabolic syndrome
Equine Vet J. 2019 Nov;51(6):760-766. doi: 10.1111/evj.13097. Epub 2019 Apr 10

Age

What has age got to do with it? Quite a lot, both for PPID and EMS.

Donaldson et al 2005 found that the sensitivity of the pituitary-adrenal axis decreases with age (ACTH concentrations were generally significantly higher in older (16+) horses), and suggested exposure to oxidative stress as a possible contributing factor to both aging and PPID. However, just because it may be common for aged horses to experience loss of pars intermedia function, this shouldn't be considered normal. The paper suggests that the diagnosis of PPID may not be a "yes" or "no" diagnosis, and rather that PPID may be a slow loss of pars intermedia function which often occurs as horses get older.

Jacob SI, Geor RJ, Weber PSD, Harris PA, McCue ME
Effect of age and dietary carbohydrate profiles on glucose and insulin dynamics in horses
Equine Veterinary Journal March 2018 Volume 50, Issue 2 Pages 249–254
Acute insulin response to glucose (AIRg) was higher in aged 582.0 vs. adult 358.0 horses.
Adult and aged horses had a higher insulin sensitivity on starch (adult 3.3, aged 2.8) and sugar (adult 3.4, aged 4.0) diets compared with control (adult 2.0; aged 1.4).
Feeding a starch or sugar meal resulted in postprandial hyperinsulinaemia:
area under the curve for insulin (AUCi):
starch: adult 21581.0, aged: 35205.0;
sugar: adult 26050.0, aged 25720.0
Rapson JL, Schott II HC, Nielsen BD, McCut
cheon LJ, Harris PA, Geor RJ
Effects of age and diet on glucose and insulin dynamics in the horse
Equine Veterinary Journal published online Jan 2018

Rapson JL MSc thesis Michigan State University 2015
Effects of age and diet on glucose and insulin dynamics in the horse

What about age and inflammation - does age increase inflammation?

Siard-Altman MH, Harris PA, Moffett-Krotky AD, Ireland JL, Betancourt A, Barker VD, McMurry KE, Reedy SE, Adams AA
Relationships of inflamm-aging with circulating nutrient levels, body composition, age, and pituitary pars intermedia dysfunction in a senior horse population
Vet Immunol Immunopathol. 2020 Mar;221:110013. doi: 10.1016/j.vetimm.2020.110013. Epub 2020 Jan 26

Siard, Melissa H
Lymphocyte-mediated inflamm-aging in the horse
PhD dissertation 2017 University of Kentucky

Vet Immunol Immunopathol. 2009 Feb 15;127(3-4):286-94. Epub 2008 Oct 31 PubMed
Effect of body condition, body weight and adiposity on inflammatory cytokine responses in old horses
Adams AA, Katepalli MP, Kohler K, Reedy SE, Stilz JP, Vick MM, Fitzgerald BP, Lawrence LM, Horohov DW
"Advanced age is associated with a low-grade, systemic inflammatory response characterized by increased inflammatory cytokine production both in vitro and in vivo, termed inflamm-aging. It is also known that increased white adipose tissue, associated with obesity, leads to increased production of inflammatory cytokines. To date, it is unknown whether increased adiposity contributes to the age-related increased inflammatory status. Here we show that peripheral blood mononuclear cells (PBMC) from old horses compared to young horses have increased inflammatory cytokine production; moreover, fat old horses compared to thin old horses have even greater frequencies of lymphocytes and monocytes producing inflammatory cytokines. Therefore, we proposed that decreasing adiposity in old horses would reduce age-associated increases of inflammatory cytokines both in vitro and in vivo, and increasing adiposity in old horses would increase these measurements....when weight and fat increased in these old horses there was a significant increase in inflammatory cytokine production. Regression analysis also revealed significant relationships. These findings demonstrate that age-related obesity potentially plays a role in the dysregulation of inflammatory cytokine production by the immune system with age or inflamm-aging in the horse."

AGEs - Advanced Glycation End-products

Glycation describes the non-enzymatic attachment of a sugar (usually glucose or fructose) to a protein (or lipid or nucleic acid). Examples of glycated proteins include fructosamine, HbA1c (measured to diagnose diabetes), and advanced glycation end-products (AGEs).

AGEs can be ingested from the diet (exogenous) or formed in the body (endogenous). Endogenous AGEs formation in tissues and body fluids is a normal consequence of metabolism, but this can be pathogenic (cause disease) if high levels of AGEs accumulate in tissues and the circulation. Hyperglycaemia, hyperlipidaemia and oxidative stress accelerate AGEs formation.

AGEs can be harmful - they accumulate in body tissues and fluids, affect the normal function of other proteins, alter intracellular signaling and gene expression, increase reactive oxygen species (ROS) production/oxidation stress, cause inflammation, and are associated with several diseases/pathologies and ageing.

AGEs are produced in the Maillard reaction, when food is heated and browned.

AGEs are tissue proteins that have been changed permanently by the attachment of a sugar (primarily glucose) molecule, altering the structure and function of the protein. Hyperglycaemia (abnormally high levels of glucose) increases the formation and aging increases the accumulation of AGEs.
In humans with type 2 diabetes, AGEs have been linked with vascular complications and kidney and eye damage.

Melody de Laat and her co-researchers investigated whether AGEs accumulated in the lamellar tissues of horses with hyperinsulinaemia-induced laminitis. Accumulation of AGEs was identified in the lamellar tissue of horses with hyperinsulinaemia-induced laminitis in the acute, but not the developmental, stage, suggesting that AGEs play no significant role in the development of endocrinopathic laminitis.

de Laat MA, Kyaw-Tanner MT, Sillence MN, McGowan CM, Pollitt CC
Advanced glycation endproducts in horses with insulin-induced laminitis
Vet Immunol Immunopathol. 2012 Jan 15;145(1-2):395-401. doi: 10.1016/j.vetimm.2011.12.016. Epub 2011 Dec 26. PMID: 22240145.
"due to the lack of AGE accumulation during disease development and a failure to detect an increase in ROS or upregulation of RAGE, it appears unlikely that oxidative stress and protein glycosylation play a central role in the pathogenesis of acute, insulin-induced laminitis."

Summarized in (see Mechanisms for insulin to cause laminitis):
Katz LM, Bailey SR
A review of recent advances and current hypotheses on the pathogenesis of acute laminitis
Equine Veterinary Journal Volume 44, Issue 6, pages 752–761, November 2012

AGEs in the diet
NB there is evidence (in other species) that AGEs in the diet (from the Maillard reaction) contribute to AGE accumulation in tissues. During the curing process, hay that has been baled with too much moisture can heat up, causing sugars to bond to proteins and resulting in a sweet smelling brown "heat damaged" hay. Food processed at high temperatures may contain AGEs.

AICAR - 5-aminoimidazole-4-carboxamide-1-D-ribofuranoside

de Laat MA, Robinson MA, Gruntmeir KJ, Liu Y, Soma LR, Lacombe VA
AICAR administration affects glucose metabolism by upregulating the novel glucose transporter, GLUT8, in equine skeletal muscle
Vet J. 2015 Sep;205(3):381-6. doi: 10.1016/j.tvjl.2015.05.018. Epub 2015 May 27

ALCAR - Acetyl-L-Carnitine, L-Carnitine

Acetyl-L-Carnitine (ALC / ALCAR) is an ester produced by the human brain, kidney and liver which appears to increase the uptake of Acetyl-CoA into the mitochondria.

Acetyl-L-Carnitine (ALCAR) for neuropathic pain

Nascimento OJM, Pessoa BL, Orsini M, et al.
Neuropathic Pain Treatment: Still a Challenge.
Neurology International. 2016;8(2):6322. doi:10.4081/ni.2016.6322.
ALC has been shown to reduce neuropathic pain and improve nerve fibre regeneration in humans with diabetic neuropathy:
Diabetes Care. 2005 Jan;28(1):89-94.
Acetyl-L-carnitine improves pain, nerve regeneration, and vibratory perception in patients with chronic diabetic neuropathy: an analysis of two randomized placebo-controlled trials.
Sima AA, Calvani M, Mehra M, Amato A; Acetyl-L-Carnitine Study Group.

and to decrease pain and change nerve conduction velocity, also in humans with diabetic neuropathy:
Drugs R D. 2002;3(4):223-31.
Acetyl-L-carnitine (levacecarnine) in the treatment of diabetic neuropathy. A long-term, randomised, double-blind, placebo-controlled study.
De Grandis D, Minardi C.

PLoS One. 2015 Mar 9;10(3):e0119479. doi: 10.1371/journal.pone.0119479. eCollection 2015.
Acetyl-L-carnitine in the treatment of peripheral neuropathic pain: a systematic review and meta-analysis of randomized controlled trials.
Li S, Li Q, Li Y, Li L, Tian H, Sun X.
"In conclusion, the current evidence suggests that ALC seems effective and safe in the treatment of PNP (peripheral neurpathic pain), especially of diabetic PNP. Oral administration of ALC may be recommended due to its similar efficacy but easier administration."

Does supplementation with Acetyl-L-Carnitine (ALCAR) help with weight loss?

Barcelos K, Jordão L, Ferraz G, Lana A, Rezende A
Effect of chelated chromium and L-carnitine on subcutaneous fat deposition in the Mangalarga Marchador breed after training
International Conference on Equine Exercise Physiology June 2014
This research found that "Supplementation with (10 g per day) L-carnitine reduces subcutaneous fat deposition in the lumbar region of Mangalarga Marchador mares after training, which may be indicative of increased use of subcutaneous fat in this region as a substrate for exercise."

Schmengler U, Ungru J, Boston R, Coenen M, Vervuert I
Effects of l-carnitine supplementation on body weight losses and metabolic profile in obese and insulin-resistant ponies during a 14-week body weight reduction programme
Livestock Science, August 2013 Volume 155, Issue 2, 301 - 307
This research found that restricting energy intake to 7 MJ/100 kg BW resulted in body weight losses, which improved insulin sensitivity and glucose metabolism, and that L-carnitine supplementation did not further improve glucose or fat metabolism.

Alfalfa

It is sometimes suggested that alfalfa or lucerne (Medicago sativa), should not be fed to horses prone to laminitis.

Is there any science behind this?

Alfalfa is a legume (as is clover).
Looking at average figures for common feed profiles published by Equi-Analytical, ESC and starch figures are quite similar between legume (mostly alfalfa) hay and grass hay, but crude protein levels are much higher in legume hay than grass hay, and digestible energy is higher in legume hay than grass hay.

Alfalfa - Feedipedia

According to Equine Applied and Clinical Nutrition (2013) p 131, as long as a horse has adequate water there should be "few lasting health consequences of excess dietary amino acid intake" - so the increased protein levels shouldn't be a problem.

In 2016, Dr Kellon suggested "for reasons that are not yet explained alfalfa can cause flares of laminitis pain in some IR horses".
Alfalfa and the IR horse - Dr Eleanor Kellon Jan 2016

Trimmer Linda Cowles of www.healthyhoof.com says that "Alfalfa often results in a more brittle wall and poor wall attachment. I see this on most horses… the feet almost always get better when alfalfa is removed.

Amino acids are shown as % protein on Feedipedia (calculated here as % of the crude protein figure)

4 Misconceptions About Alfalfa - Heather Smith Thomas www.thehorse.com Mar 2017
Note in the above article that there is no evidence that obese horses and horses with laminitis, equine metabolic syndrome (EMS) and PPID need to have NSC levels in their diet monitored - it is likely to be ESC plus starch that horses with laminitis, EMS and PPID must keep low - there is no research showing that naturally ingested fructans cause a significant increase in insulin. Obese horses should have energy/calories limited in their diet.

It has been noted that "alfalfa hay can be especially high in Se if grown on seleniferous soils"
J.G. Davis, T.J. Steffens, T.E. Engle, K.L. Mallow, S.E. Cotton
Preventing Selenium Toxicity
Colorado State University extension services Fact Sheet No. 6.110 2004

AMPK

Burns TA, Watts MR, Weber PS, McCutcheon LJ, Geor RJ, Belknap JK
Effect of dietary nonstructural carbohydrate content on activation of 5'-adenosine monophosphate-activated protein kinase in liver, skeletal muscle, and digital laminae of lean and obese ponies
J Vet Intern Med. 2014 Jul-Aug;28(4):1280-8. doi: 10.1111/jvim.12356. Epub 2014 Apr 20. (PubMed)
"RESULTS: 5'-Adenosine-monophosphate-activated protein kinase was immunolocalized to laminar keratinocytes, dermal constituents, and hepatocytes. A high-CHO diet resulted in significantly decreased laminar [P-AMPK] in lean ponies (P = .03), but no changes in skeletal muscle (lean, P = .33; obese, P = .43) or liver (lean, P = .84; obese, P = .13) [P-AMPK]. An inverse correlation existed between [blood glucose] and laminar [P-AMPK] in obese ponies on a high-CHO diet."

Diabetes Metab Syndr Obes. 2014; 7: 241–253. Published online Jun 24, 2014
AMPK activation: a therapeutic target for type 2 diabetes?
Kimberly A Coughlan, Rudy J Valentine, Neil B Ruderman, and Asish K Saha

Anatomy (hoof/foot and lower leg)

Ramzi Al-Agele, Emily Paul, Valentina Kubale Dvojmoc, Craig J. Sturrock, Cyril Rauch and Catrin Sian Rutland
The Anatomy, Histology and Physiology of the Healthy and Lame Equine Hoof
In Veterinary Anatomy and Physiology Edited by Catrin Sian Rutland and Valentina Kubale published February 2019
DOI: 10.5772/intechopen.84514 (Open Access)

Okanagan School of Natural Hoof Care - Hoof Anatomy and Function

Iron Free Hoof - General anatomy

Horse Engineer: Hoof School parts I to VI

www.all-natural-horse-care.com - Horse Hoof Anatomy via Hoof Dissection

Anatomy of the horse: an illustrated text

The Illustrated Horse's Foot - Christopher C Pollitt

Update on the anatomy of the inner hoof wall - Dr Chris Pollitt

Colour Atlas of the Horse's Foot - Christopher C Pollitt 1995

Sole and Pedal Bone Shape - Monique Craig & Michael T Savoldi 2008

Adams and Stashak's Lameness in Horses - Edited by Gary M Baxter 2011

Equine Anatomy - www.plastinate.com

The Laminitis Laboratory Gallery, New Bolton Center, PennVet

Angiotensin

Am J Vet Res. 1989 Apr;50(4):508-17
In vitro reactivity of digital arteries and veins to vasoconstrictive mediators in healthy horses and in horses with early laminitis
Baxter GM, Laskey RE, Tackett RL, Moore JN, Allen D
"There were no differences in EC50 concentrations and in maximal contractions between forelimb and hind limb arteries and veins for angiotensin II, norepinephrine, and serotonin. Incubation with endotoxin had no effect on the reactivity of arteries and veins to angiotensin II, norepinephrine, and serotonin."

Anhidrosis

Anhidrosis is the partial or complete inability to sweat, and can lead to increased body temperature, increased heart rate and respiration rate as the horse is unable to dissipate heat. It may be seen in horses with PPID , particularly in hot humid climates. Hyperhidrosis - excessive sweating - is also a symptom of PPID.

Anhidrosis in Horses - Clair Thunes www.thehorse.com July 2021

Spelta CW, Axon JE
Case series of equine pituitary pars intermedia dysfunction in a tropical climate
Australian Veterinary Journal Article first published online: 11 OCT 2012
Anhidrosis and heat stress were primary presenting problems, with associated exercise intolerance, in PPID horses in a tropical hot and humid climate. Symptoms resolved when PPID was treated medically.

Understanding Anhidrosis - Martha Mallicote www.thehorse.com Sept 2015

Anhidrosis: Should I Sweat It? by Jeremy Frederick, DVM www.theHorse.com August 01 2009, Article # 14767

Anti-Diuretic Hormone/Vasopressin

Anti-Diuretic Hormone or ADH, also known as vasopressin or arginine vasopressin (AVP), is one of two hormones secreted from the pars nervosa (or posterior pituitary) in the pituitary gland.

Vasopressin - Wikipedia

Mechanism of Action, Clinical Uses and Adverse Effects of Vassopressin (in humans)

Vet Clin Equine 27 (2011) 1–17
Hypothalamic-Pituitary Gland Axis Function and Dysfunction in Horses
Samuel D.A. Hurcombe

Aspirin - Acetylsalicylic Acid (ASA)

Aspirin is used for its antipyretic, analgesic, anti-inflammatory and anti-thrombotic properties in humans. Aspirin is not licensed for use in horses (in the UK).

Aspirin is rapidly metabolized and eliminated and has the weakest anti-inflammatory and pain relieving activity of all NSAIDs in horses, so is rarely used for inflammatory conditions.

Research published in 2017 showed no anti-coagulatory effect in 3 horses, and only short-term effects in 7 horses:
Roscher KA, Failing K, Schenk I, Moritz A
Suspected aspirin resistance in individual healthy adult warmblood horses
J Vet Pharmacol Ther. 2017 Dec;40(6):e16-e22. doi: 10.1111/jvp.12408. Epub 2017 Apr 7
See also: Study: Aspirin Ineffective as a Blood Thinner in Some Horses - Christa Lesté-Lasserre, www.thehorse.com, Feb 2018

When blood vessel walls are damaged and blood escapes, platelets in the blood clump together and form a plug or clot to protect the wound and prevent further blood loss. The activated platelets around the wound release a chemical called thromboxane, which calls more platelets to the wound site, and causes the vasoconstriction necessary for the blood vessel to heal. Aspirin blocks platelet thromboxane production, and therefore thrombus (clot) formation, for many days after aspirin is given to the horse, leading to increased or prolonged bleeding.

Aspirin's anti-clotting action may be useful in treating conditions that involve formation of blood clots. However, the use of aspirin may cause excessive bleeding.

From Sole - necrosis and penetration:
"Slight rotation/descent of P3 .... Blood vessels in the solar corium can be crushed by the sharp bottom edge of P3 and cause haemorrhage (NB use of medications that affect blood clotting, e.g. aspirin, can make haemorrhage more likely to occur)"

AAVPT monograph - Aspirin
"Horses: Due to the rapid elimination of salicylate by horses, it is questionable whether therapeutic yet nontoxic concentrations can be maintained in the horse using conventional dosage intervals".
Horses rapidly eliminate salicylate.
Aspirin is sometimes used to decrease thromboembolism in horses with laminitis. As at 2004 there were no studies that had investigated the safety or efficacy of this in horses.
Use of other NSAIDS at the same time may decrease the anti-thrombotic activity of aspirin.

Does Aspirin have a place in the treatment of laminitis?

When considering any treatment for laminitis, it is essential to consider the type of laminitis (whether endocrinopathic, sepsis-related or supporting limb). Aspirin may be suggested for its (weak) anti-inflammatory effects, or for its antithrombotic effects.

Dr Simon Bailey considers the role of thrombosis and coagulation in equine laminitis in chapter 12 of Equine Laminitis edited by James Belknap (2017), and suggests that there is evidence that coagulation deficits may occur in carbohydrate models of sepsis-related laminitis, that there is no evidence that endocrinopathic laminitis (due to EMS or PPID) involves coagulation deficits, and that it is currently not known whether coagulation abnormalities are associated with supporting limb laminitis.

Endocrinopathic laminitis - around 90% of laminitis is endocrine laminitis, which does not involve significant inflammation nor coagulation abnormalities. Therefore aspirin is not recommended as a treatment for endocrinopathic laminitis.

Sepsis-related laminitis - sepsis-related laminitis does involve inflammation, and carbohydrate-overload forms may involve platelet activation and microthrombus formation, therefore anti-inflammatories and antiplatelet therapies may be used to treat sepsis-related laminitis. Given the weak anti-inflammatory activity of aspirin in horses, NSAIDs other than aspirin are likely to be recommended. Aspirin is not mentioned as an antiplatelet therapy in chapter 12 of Equine Laminitis.

Aspirin is sometimes recommended as a treatment for horses with laminitis in Europe, often concurrently with Equipalazone (Bute), presumably for its anti-thrombotic action. Aspirin should not be used at the same time as other NSAIDs. As described above, there is no evidence that endocrinopathic laminitis - the major form of laminitis, and all laminitis due to grazing, EMS and PPID - involves platelet activation or thrombus formation, and therefore anti-thrombotic drugs are not indicated, and could cause increased or prolonged bleeding, e.g. if rotation and/or sinking has damaged blood vessels, causing bleeding in the feet.

J Vet Intern Med. 2011 Jan-Feb;25(1):116-22. doi: 10.1111/j.1939-1676.2010.0647.x. Epub 2010 Dec 8.
Effects of clopidogrel and aspirin on platelet aggregation, thromboxane production, and serotonin secretion in horses.
Brainard BM, Epstein KL, LoBato D, Kwon S, Papich MG, Moore JN.
Aspirin did not decrease platelet aggregation, but did decrease serum TXB(2).

Modifying the Coagulation Cascade: Available Medications - Wong DM, Brockus C, Alcott C, Sponseller B, Compendium Equine 2009

Am J Vet Res. 2008 Feb;69(2):199-207. (PubMed)
Roles of thromboxane A2 and 5-hydroxytryptamine in endotoxin-induced digital vasoconstriction in horses.
Menzies-Gow NJ, Sepulveda MF, Bailey SR, Cunningham FM, Elliott J.
"Aspirin given 2 hours before LPS administration abolished the increase in plasma TxA2 concentration and significantly attenuated LPS-induced digital hypoperfusion. Aspirin given 4 days before LPS significantly attenuated the increase in plasma TxA2 concentration and digital hypothermia. Aspirin plus GR55562 had a greater effect on LPS-induced digital hypothermia than aspirin alone."

Can J Vet Res. 2003 October; 67(4): 297–302.
Pharmacokinetics and plasma concentrations of acetylsalicylic acid after intravenous, rectal, and intragastric administration to horses
Ted A. Broome, Murray P. Brown, Ronald R. Gronwall, Matthew F. Casey, and Kelly A. Meritt

Equine Vet J. 1991 Mar;23(2):123-7 (PubMed)
Antithrombotic actions of aspirin in the horse.
Cambridge H, Lees P, Hooke RE, Russell CS
"The antithrombotic effects of aspirin at two dose rates (4 mg/kg and 11 mg/kg bodyweight [bwt] were evaluated in normal, healthy ponies by measuring template bleeding time. Inhibition of platelet aggregation in response to adenosine diphosphate (ADP) and collagen was evaluated and cyclo-oxygenase activity was monitored by radioimmunoassay of thromboxane B2 (TXB2), the stable metabolite of thromboxane A2 (TXA2). TXB2 was measured in serum and platelet rich plasma. Bleeding time was prolonged significantly until 48 h after treatment at 12 mg/kg bwt and until 4 h at the lower dose rate. Synthesis of TXB2 and collagen induced aggregation were diminished for much greater periods with similar results at each of the dose rates. The prolonged effects of aspirin on platelet function occurred in spite of a very short plasma half-life of aspirin, because of its irreversible action on platelet cyclo-oxygenase. The results show that low dose aspirin has a potential role in antithrombotic therapy in horses although the relationship between skin bleeding time in normal horses and improvement of clinical conditions requires further research and evaluation in clinical trials. TXB2 measurement appears to overestimate the duration of antithrombotic effects of aspirin in vivo."

Am J Vet Res. 1987 Jan;48(1):13-6
Effect of aspirin on ex vivo generation of thromboxane in healthy horses
Baxter GM, Moore JN

Side effects of aspirin use

The Merck Vetarinary Manual - Nonsteroidal Anti-inflammatory Drugs
Aspirin is not approved for veterinary use, therefore effective dosages have not been established.
Aspirin has caused mucosal erosion and ulceration in dogs at therapeutic doses. Overdose in any species may result it severe acid-base abnormalities, haemorrhage, seizures, coma and death.
A single dose of 20 mg/kg of aspirin may prolong bleeding for 48 hours in horses.

Aspirin - Equimed.com
GI irritation and bleeding are the most common side effects. Frequent use at higher doses may cause kidney damage, bleeding disorders and protein loss. Ponies and older horses may be more at risk of side effects and should be closely monitored with the lowest effective dose being used.
Aspirin should be avoided (or closely monitored) in horses with bleeding disorders, liver, kidney or gastrointestinal problem; it should not be used for pregnant horses. Aspirin should not be combined with other NSAIDs or corticosteroids, or the diuretic Furosemide.
Overdose of aspirin can cause gastrointestinal ulcers, liver and kidney damage and protein loss. Signs of toxicity include loss of appetite and depression.

Current Therapy in Equine Medicine Vol 6 - Robinson, Sprayberry. Chapter 2 NSAIDs - Anthony T Blikslager (p 7)
Aspirin irreversibly inhibits COX-1 and COX-2.