Equine metabolic syndrome (EMS)

Equine metabolic syndrome (EMS) is characterised by a dysfunction of insulin production and abnormal distribution of fat.
Insulin is a hormone which is secreted in the pancreas. When it attaches to receptors in cell membranes, it allows glucose to enter the said cells. In the case of EMS, insulin is produced but has no effect on the cell receptors. Glucose then accumulates in the blood and lymphatic fluid circulation.

Typically, horses afflicted with EMS are obese (body condition score >4-5) or show signs of abnormal distribution of fatty deposits under the skin, usually seen on the crest, the shoulders, the girth, the base of the tail, above the eye, the sheath for males, and the teats for females.
Horses have a natural tendancy to be overweight, even when their food and exercise is managed correctly. Some horses can be affected even if their body condition  is deemed correct.
Horses with EMS are also at higher risk of developing chronic or acute laminitis. Sometimes the symptoms of laminitis are discrete, but prominent growth rings can be observed on the hoof wall, as well as a convex sole, signs of chronic laminitis.

Equine metabolic syndrome, obesity and reproduction.

Overweight mares, especially those who present episodes of lameness or bouts of laminitis associted to an endocrine disorder (Cushing’s disease, or equine metabolic syndrome), are subject to orthopaedic pressures which can be particularly difficult to cope with in the last term of gestation when fœtus growth is highest.
Foals born to overweight mares seem to have more trouble suckling due to the excess fatty tissue around the teats making them less prominent and less accessible.
Obesity and EMS contribute to the following :
•    Lengthening the ovulation season : mares stop coming into season later in the year ;
•    Their seasons last longer;
•    Fertility is reduced.

Even if the genes involved are unknown, some equines present a genetic predisposition to EMS. Some breeds are more affected than others : pony breeds in general, the Pure Spanish horse (PRE), miniature horses, Morgan horses and Arab thoroughbreds.
A horse with Cushing’s syndrome (PPID) is also more at risk since this disease also creates a dysfunctioning of insulin production.
A high energy diet with regard to the horse’s needs, high in carbohydrates combined with a lack of exercise are also a high risk factor.

Recurrent bouts of laminitis in an overweight horse, are the first signs which should lead the vet to suggesting a test to confirm an Equine metabolic syndrome diagnosis. Clinical signs may be less obvious to detect, and regular clinical monitoring is recommended especially in predisposed breeds.

A simple insulin blood count may be carried out. This count is however not very sensitive : the insulin measured can appear normal even in a horse affected by EMS. When the result is negative, a dynamic test should be carried out.

There are two different types of dynamic test :

• A comparison of insulin and glucose levels in the blood before and after administration of a sugary feed ;
• A comparison of glucose levels in the blood before and after an insulin injection.

With a horse aged over 10, or presenting signs of Cushing’s syndrome, testing for PPID should also be carried out as it can affect a horse concomitantly with EMS.

Change of feed and living conditions

As a general rule, horses affected by EMS should be fed on a low carbohydrate diet, since it is a carbohydrate metabolism disorder. When these horses are obese or present signs of regional fatty deposits, the energy level should also be reduced.

• Access to pasture should be restricted or even banned. The proportion of soluble carbohydrates present in grass is usually too high, especially in Spring and Autumn. Limiting access to pasture for a few hours a day is not really a solution as it incites the animals with retricted access (particularly ponies) to ingest huge amounts of grass in a short time.
• Any hard feed based on cereal, molasses, sugarbeet, which is rich in soluble carbohydrates, should be banned for horses with EMS.
• Give high fibre, low energy hay (harvested at a late maturity stage).

If the horse has a normal body condition, lipid input (fats) can be increased to maintain the calorie level of the ration (decrease the input of carbohydrates at the same time).

Restore physical activity

Physical exercise is highly recommended for horses affected by EMS, unless locomotor disorders due to laminitis are apparent. Whatever the intensity, physical exercise increases the energy expenditure and will help weight loss. However, more intense work may be necessary to improve the horse’s sensitivity to insulin.
For horses who have been out of work for long periods, work should be resumed gradually.

Living conditions

Ideally, horses with EMS should be kept in groups, to avoid social isolation. :
•    Inside, in barns (be careful of them ingesting too much straw which can lead to colic).
•    Or outside in a bare paddock : track like structures (« paradise paddocks » for example) are well adapted to this type of horse.

Medical traitment

• Administering levothyroxin is indicated if despite diet and exercise the horse does not lose any weight, or to help with weight loss when the horse has acute laminitis. The only medication available is for humans, and is very costly.
• Administering metformin is recommended in the case of persisitant hyperinsulimic horses, despite adapted diet and exercise. Again only human medication is available.
• When Cushing’s syndrome is also present, daily administration of pergolide is recommended.

Monitoring the horse’s body condition and weight as well as regular insulin counts are paramount so as to assess the response to the treatment and adapt it accordingly.