Weight gain is a risk factor for laminitis.  "We are the personal trainer and dietician for our horses".   
Monitor your horse's body condition score (BCS) by getting hands on to feel for fat deposits.  More information here: Body condition scoring.
Monitor weight - with a weighbridge, weight tape, a length of string or girth holes - if the girth isn't going as tight or weight increases on a horse in good condition, take action to get the horse to lose weight straight away.
Ask your vet, farrier, physio to help you assess your horse's body condition.
Assess how much grass or forage your horse is eating.  Dung production can help monitor intake - more dung usually means more food eaten.  Watch how much time your horse has his head down eating grass - a field may look bare, but if a horse is busy eating, he is finding grass!  Fence off a small area of field so that your horse can't graze it and watch how much grass grows - you may think your horse's field is bare until you see how much grass grows when he isn't eating it.
For weight loss, control intake by using slow feeder techniques, prolonging chewing time, and aiming to feed less calories without reducing the amount fed, and increase exercise.

Horses with PPID may be particularly at risk from autumn laminitis.
PPID (formerly called Equine Cushing's disease) is a progressive degenerative disease that becomes more common with age (one study found PPID in 21% of horses older than 15).  PPID has been diagnosed in horses younger than 15 [but the Equine Endocrine Pioneers Circle suggested in 2019 that PPID in a horse younger than 10 is "very unlikely"].  In horses with PPID, degeneration of neurons [in the hypothalamus in the brain] causes incorrect messages to be sent to the pituitary gland, resulting in increased levels of certain hormones, including ACTH, which is the hormone most commonly tested to diagnose and monitor PPID.
Clinical signs of PPID include changes to the coat and delayed shedding of the winter coat, lethargy, muscle loss along the topline, pot belly, abnormal fat deposits e.g. cresty neck and filled supraorbital hollows above the eyes, increased drinking and urination, lowered immunity so increased infections, and laminitis.  
Not all horses with PPID are at greater risk of laminitis - it is horses with PPID and insulin dysregulation that are at risk of laminitis, so insulin should be tested as well as ACTH if PPID is suspected to assess laminitis risk.
The treatment for PPID is Prascend (pergolide).

[Does treatment with pergolide stop/slow progression of PPID?  There has been suggestion that pergolide may reduce the hyperplasia and hypertrophy (increase in number of and size of cells) caused by the excess production of hormones from the pituitary gland, pergolide has been shown to have neuro-protective properties in other species, and Andy Durham from Liphook Equine Hospital has suggested that if PPID is well controlled and hyperplastic activity suppressed, the dose of pergolide may be able to be reduced – there may be one dose for getting PPID under control and then a lower maintenance dose (Source: "A discussion of Equine Cushing’s disease by Andy Durham and Victoria South November 2017).  The Laminitis Site sees many horses with PPID that are able to remain on a low dose for many years, suggesting that progression of the disease is controlled by treatment.] 

[Does treatment with pergolide reduce laminitis risk in horses with PPID?  Pergolide reduces or controls the excess hormone production.  If a horse's insulin dysregulation (ID) is driven by the excess PPID hormones, and treatment reduces/controls these hormones, then it should follow that laminitis risk is reduced.  However, if ID is driven at least partly by EMS factors (being overweight, a diet too high in sugar/starch, insufficient exercise), then those factors must be controlled by management.]

To rug or not to rug?  Horses, particularly native types and ponies may not need to be rugged if they have free access to shelter and forage.  Keeping warm uses up calories and encourages weight loss.  Overweight ponies can even have a small clip (e.g. a bib clip) to encourage weight loss.  Seasonal weight loss is the owner's friend, and good doers should lose weight over the winter. 

Summary:
Laminitis can occur at any time of year - be vigilant all year round.
Autumn laminitis may occur due to seasonal rise hormones, increased grass growth and changes in management such as increasing feed, reducing exercise and rugging unnecessarily.  Monitor weight/body condition throughout the year and take action to prevent unwanted weight gain.
​

Dr Kellon suggests looking out for signs such as "less spontaneous activity, reluctance to make sharp turns, preference for soft ground, muscle tension in the forearms, back and hindquarters, more rigid head carriage (high or low) and a subdued attitude", as these may be indicative of laminitis.  

​She concludes by saying that controlling simple sugars and starch and calories, and providing good levels of other nutrients can be successful in controlling insulin.  

In a retrospective study of 274 horses, researchers in Australia found that the clinical presentation of PPID changed with latitude and climate, with anhidrosis and polyuria/polydipsia more commonly recognised at lower latitudes.

They also found that being a pony, having an adequate body condition score and being treated with pergolide were associated with survival. Laminitis was diagnosed in 89.9% of cases and insulin dysregulation was diagnosed in 76.5% of cases in which they were investigated (note that horses with chronic laminitis may have had previous but not necessarily current insulin dysregulation, so it is possible for horses with signs of laminitis to not test positive to insulin dysregulation - earlier testing before or at the time of the first laminitic changes might have identified insulin dysregulation).
Horn R, Bamford NJ, Afonso T, Sutherland M, Buckerfield J, Tan RHH, Secombe CJ, Stewart AJ, Bertin FR
Factors associated with survival, laminitis and insulin dysregulation in horses diagnosed with equine pituitary pars intermedia dysfunction
Equine Veterinary Journal published online 12 November 2018 https://doi.org/10.1111/evj.13041

See also: 
Equine Cushing's Disease: Australian and North American Differences - KER Dec 2018

Professor Sillence and his research team from Queensland University of Technology are testing drug velagliflozin as a possible treatment for horses and ponies with equine metabolic syndrome to help prevent endocrinopathic laminitis.  Velagliflozin works by causing the kidneys to excrete more glucose in the urine to take the pressure off the pancreas and lower insulin levels.  Velagliflozin has been trialed in Australia and is now being trialed in Europe.

​Meier A, Reiche D, de Laat M, Pollitt C, Walsh D, Sillence M
The sodium-glucose co-transporter 2 inhibitor velagliflozin reduces hyperinsulinemia and prevents laminitis in insulin-dysregulated ponies
PLoS ONE 13(9): e0203655 published 13 Sept 2018. https://doi.org/10.1371/journal.pone.0203655
See also: ​Drug shows promises in preventing laminitis in at-risk ponies - www.horsetalk.co.nz 24 Sept 2018
Giddy up: help for plump ponies is fast on its way - Dec 2018 Queensland University of Technology

From 01 December 2018, horses competing in the USA under USEF rules can apply for a Therapeutic Use Exemption (TUE) for pergolide after submitting proof of diagnosis.  This is a great step forward for the many horses with PPID that continue to compete.  See USEF Announces New Therapeutic Use Exemption (TUE) Status for Pergolide Administration Effective December 1, 2018.

The FEI continue to class pergolide as a prohibited substance - controlled medication. All members of the equestrian community are welcome to submit a suggested change to the Equine Prohibited Substances List - if you feel that the FEI should review their classification of pergolide/Prascend for horses diagnosed with PPID, complete and return the FEI ​Equine Prohibited Substances List submission form.

National Equine Health Survey (NEHS) 2018

The National Equine Health Survey (NEHS) 2018 received information for over 13,000 horses, giving a snapshot of the health of UK horses for one week in May 2018.
38% of horses had one or more health problems recorded.
Of all the disease syndromes recorded, 29% were lameness including laminitis (23% in 2017), and 7.4% were metabolic diseases (8.1% in 2017).
Laminitis made up 5.4% of all individual disease syndromes recorded (6.5% in 2017), and PPID 5.3% (6.0% in 2017).  43% of laminitis cases were first episodes and 57% were repeat episodes (49% and 51% respectively in 2017). 

However, without knowing how many individual syndromes were reported, these figures are not particularly helpful.  All we know from the report is that 38% of 13,873 horses, so 5,272 horses, had 1 syndrome or more.
So a minimum of 2% of the population had laminitis when the survey ran for a week in May 2018 (calculated 38% of 13,873 horses = 5,272 x 5.4% = 285 / 13.873 = 2%).
Similarly a minimum of 2% had PPID, and 2.8% had metabolic diseases - but does metabolic disease include PPID, or does it mean EMS?  How many had EMS AND PPID, and how many had PPID without EMS?

67% of horses were recorded as having a body (fat) score of 3 with 17.8% recorded as being thin (score of 2 and under) and 15.5% recorded as overweight (greater than 3). [However, previous research has suggested that owners tend to under-estimate their horses' body condition scores].

According to this article from Kentucky Equine Staff
Can Body Shape Predict Metabolic Issues in Horses? KER August 2018
reporting on research by Fitzgerald et al. at Queensland University presented at the 2018 Australasian Equine Science Symposium:
Fitzgerald DM, Sillence MN, De Laat MA
Morphometric measurements for identifying equine metabolic syndrome.
Proc. Australasian Equine Science Symposium 2018 7:29
cresty neck score (CNS), but not body condition score (BCS), neck or girth circumference, was positively correlated to a post oral glucose test insulin result that suggested Equine Metabolic Syndrome.

However, 
Coleman MC, Belknap JK, Eades SC, Galantino-Homer HL, Hunt RJ, Geor RJ, McCue ME, McIlwraith CW, Moore RM, Peroni JF, Townsend HG, White NA, Cummings KJ, Ivanek-Miojevic R, Cohen ND
Case-control study of risk factors for pasture-and endocrinopathy-associated laminitis in North American horses
J Am Vet Med Assoc. 2018 Aug 15;253(4):470-478. doi: 10.2460/javma.253.4.470
looked at 199 horses diagnosed with endocrinopathic laminitis after grazing and found that "horses with an obese body condition (i.e. body condition score ≥ 7), generalized or regional adiposity (alone or in combination), preexisting endocrinopathy, or recent (within 30 days) glucocorticoid administration had increased odds of developing PEAL, compared with horses that did not have these findings."

​We are now well into the seasonal rise (in pituitary hormones) in the northern hemisphere.
In the UK the seasonal rise is thought to start around the end of June/start of July, peak in September, with hormones returning to normal around the end of November/start of December.

Three important facts about the seasonal rise:

1. Horses with PPID may need an increase in their pergolide dose during the seasonal rise. Testing resting ACTH in August may help to identify horses that need an increased dose.

2. For horses suspected of having, but not yet diagnosed with PPID, testing resting ACTH in September may be the best time to differentiate horses with PPID from normal horses. Lab fees for ACTH testing are currently free for horses (in the UK) that haven't been diagnosed with PPID through Talk About Laminitis.

3. Horses with PPID may show increased clinical signs of PPID during the seasonal rise. Horses that have unexplained laminitis or infections, particularly from August to October, may need to have PPID eliminated as a contributing factor.

In all cases, if you are worried that your horse may have PPID or needs a change in his/her treatment, speak to your vet.

​(Graph: used with permission from Liphook Equine Hospital)

The autumn seasonal rise is now thought likely to be the best time of year to test resting ACTH for the diagnosis of PPID, but how are blood test results interpreted during this time?
Liphook Equine Hospital is at the forefront of research in this area, having initially suggested that a two-tier reference interval should be used, with a cut-off of 47 pg/ml from August to October, and a cut-off of 29 pg/ml the rest of the year.

However, this was still a rather crude measure, not allowing for the gradual increase and decrease in ACTH through the seasonal rise, and new weekly cutoff values are now being used at Liphook, based on the ACTH results of over 35,000 horses, with ACTH concentrations increasing shortly after the June solstice (when day length starts to decrease), peaking around the September equinox, and declining back to basal levels by the end of November.

Interestingly, Liphook found that insulin and glucose concentrations did not appear to increase alongside ACTH, and if anything insulin and glucose tended to be higher from December to March.
What isn't clear from these new cutoffs though is how the transitional zone around them should be interpreted, as research by David Rendle et al published in 2013 suggested "Our results support previous assertions that ACTH is a reliable means of differentiating non-PPID and PPID horses but indicate that there is a transitional zone, around currently recommended cut-offs for diagnosing the disease, at which further testing should be considered."
Rendle DI, Litchfield E, Heller J, Hughes K J
Investigation of rhythms of secretion and repeatability of plasma adrenocorticotropic hormone concentrations in healthy horses and horses with pituitary pars intermedia dysfunction
Equine Veterinary Journal Vol 46, Issue 1, pages 113–117, January 2014 (published online Aug 2013)

More information:
Seasonal Changes in ACTH Secretion - Liphook Equine Hospital

Further Observations of Seasonality of Pars Intermedia Secretory Function in 30,000 Horses and Ponies - Andy Durham - p 11-12 Dorothy Russell Havemeyer Foundation Equine Geriatric Workshop II 3rd Equine Endocrine Summit 2014

​​Herbie walking comfortably in her Sole Supports.  She had been wearing them for just over a week, with them being removed and reapplied every 2-3 days so that we could keep an eye on her feet.  ​

The Sole Supports appear to be ideal for emergency support for horses with laminitis, and a pair of soft Supports would seem a sensible addition to any laminitis first aid kit.