Cushing’s syndrome

Cushing’s disease or « Pituatary pars intermedia dysfunction (PPID) », is an illness affecting the endocrinal system, i.e caused by a diruption of the hormonal system.

This disease represents a dominant in equine geriatrics : around 20 % of horses over the age of 15, and 40 % of those over 30 are likely to be affected, against 0,5 % of the entire equine population. The disease develops very gradually and it can be many years before the first noticeable clinical signs become apparent.

Many symptoms are associated to Cushing’s disease. When the disease has reached an advanced stage, some of the clinical signs are characteristic and make diagnosis easy. On the other hand the early signs are not so easy to detect. However, early management of the disease is essential.
The main signs to look for are as follows :

Dermatological signs

Extremely characteristic of the disease and easily noticeable :

• Shaggy rough coat (or hypertrichosis) : abnormally long hair, sometimes curly all over the body or on some parts of the body, whatever the season. At the onset of the disease, more subtle changes to the coat can be taken for normal clinical signs of ageing.
• Delayed coat shedding, or unshed winter coat at the change of seasons ;
• Excessive sweating that persists even after clipping;
• More rarely decoloration of the coat.

Locomotor signs

• Recurrent episodes of laminitis ;
• More rarely , deterioration of the suspenory ligamant in the fetlock.

A change in the horse’s general outline

• Muscle shrinkage, visible along the back, but which can easily be mistaken for amyotrophy linked to ageing, but which also gives an impression of a hanging abdominal girth;
• Abnormal distribution of fat with an accumulation around the eye sockets, the crest, above the tail and in the perineal area ;
• Weight loss.

General symptoms

• Loss of performance ;
• General lethargy, difficult to notice and which can be mistaken for normal signs of ageing ;
• Polyuria/Polydipsia : increased need to drink, and urinate.

Partial loss of immunity

This can be the cause of :

• Oppotunistic infections being more frequent, especially skin complaints (dermatophilosis, ringworm) tooth and sinus infections, hoof abscesses, pneumonia. Thses symptoms are frequently under-diagnosed as clinical signs are not as apparent in animals whose immunity is affected. Thses horses are also more subject to worms
• Delayed healing of wounds.

Other signs

• Lowering of the fertility in mares;
• Neurological signs in the most advanced cases : ataxia, loss of sight, convulsions, narcolepsia (uncontrolled drowsiness).

A certain number of these symptoms can be mistaken for the signs of the normal ageing process. If in doubt, it is better to get a veterinary opinion. Bouts of laminitis and abnormal distribution of fat are also signs of Equine Metabolic syndrome. It is not uncommon for horses to be affected by both of theses endocrinal disorders at the same time.

When in the presence of characteristic signs of PPID, a simple blood count of ACTH can be caried out. If the result is inconclusive, or when in presence of more subtle signs of the disease, other tests can be done : the dexamethdone suppression test or TRH stimulation test (thyrotropin-releasing hormone). These tests consist in taking two blood counts, before and after injecting a dose of dexamethdone or TRH, an measuring the ACTH present.
The doses of ACTH present can however be difficult to interpret due to the daily and seasonal variations of ACTH present in the blood.

It is also advisable to test for an insulin disorder to make sure the horse is not affected by EMS (Equine Metabolic Syndrome) at the same time.

Pergolide is a molecule with an identical effect to dopamine, and its secretion regulates the production of ACTH in the hypophses. It is to date the best treatment for horses affected by Cushing’s disease. It is given orally on a daily basis. This treatment will not cure the horse ; it has to be given for life. It improves the horse’s quality of life as it alleviated the symptoms of the disease. The necessary dosage and the response to the treatment will vary from one animal to another : regular monitoring of the ACTH levels and of the improvements should be carried out by the treating vet.

Other preventive measures can restrict the complications due to the disease. Because their immune system is affecte, horses with Cushing’s disease are subject to the following :

• They are more likely to be infested with digestive parasites : monitoring the eggs shed in droppings by means of egg-counts will enable you to determine when and how often to de-worm these horses ;
• They are more prone to opportunistic infections : it is recommended to keep close track of dental and hoof condition with your dentist and farrier, clipping in summer can also be advisable to prevent bacterial skin infections and improve their comfort with regard to heat ;
• Respond less to vaccination boosters, therefore the frequency should be increased :
  • Vaccinating for tetanos : yearly booster jabs;
  • Vaccinating for equine flu and EHV  : booster injections every 3 to 6 months.

Horses affected by Cushing’s disease are prone to bouts of laminitis. It is therefore advisable to do the following :
•    Restrict access to pasture during critical periods (spring and autumn) ;
•    Restrict energy levels in the feed : they can be fed on low energy fodder. e.g hay harvested late in the season. Avoid hard feed as much as possible. If the energy needs are high, the horse can be supplemented by providing fatty acids.