Cushings disease occurs in horses, dogs and even people. However, the symptoms are different, depending on the species. In horses it results in increased hair growth (longer, thicker, curly) whereas in dogs it results in hair loss and even bald spots. The horse version of this illness is called ECD (Equine Cushings Disease).
As a horse ages, there is an increasing tendency for the pituitary gland to develop tumors or to grow too large, resulting in it producing more hormones. This in turn stimulates the adrenal glands to produce additional steroids and when the level of steroids becomes excessive it results in cushings. Since these changes are age related, ECD is most often found in horses over 15 years but has been found in horses as young as seven.
Symptoms/diagnosis of Cushing's syndrome
Cushing's syndrome is the term used to define the clinical effects of enhanced glucocorticoid action on the body. While equine Cushing's syndrome has been associated with both the exogenous administration of potent glucocorticoids (e.g. corticosteroids) and with endogenous glucocorticoid substances produced by the pituitary tumour, there are other potential mechanisms that could explain the increased glucocorticoid effect.
The concentration of cortisol is also controlled locally within the tissues of the horse's body. Cortisol exerts its effect on the functioning of the cell by entering the cell and interacting with a receptor. The precise concentration of cortisol within the body’s cells is critical to their normal functioning. An enzyme called 11-beta hydroxysteroid dehydrogenase (HSD) is responsible for maintaining the local concentration of cortisol within the optimal limit.
Under normal circumstances, the concentration of cortisol within the cell is adjusted by HSD within the cell itself. This ensures that the requirements of the cell at any given time are met. It has been suggested that Cushing’s syndrome may sometimes be attributed to abnormal HSD activity within the cells. (Human beings whose cellular cortisol concentration is increased due to abnormal HSD activity, develop symptoms that are similar to Cushing's syndrome.)
Increased HSD activity in the cells leads to increased cortisol within the tissues. This increase is believed to give rise to clinical signs of Cushing's syndrome. In an attempt to determine if laminitis associated with obesity can be a manifestation of abnormal HSD activity in the peripheral tissues of affected horses.
The etiology of Cushing's syndrome
The most common symptoms of Cushing’s syndrome are:
· Sudden-onset polydipsia. An affected horse may drink as much as 80 litres of water a day (as opposed to an average 20 – 30 litres). This condition is usually accompanied by polyuria.
· Abnormal hair growth and shedding. Affected horses may develop a growth of heavy, coarse, often curly hair, which does not shed in the summer. This may be accompanied by sweating and seborrhea.
· Development of a swayback stance and a pot belly.
· Filling above the eyes caused by the deposition of fat.
· A general appearance of malaise, with dull eyes and drab coat.
· Increased appetite (usually with no accompanying weight gain).
· Chronic laminitis.
· Loss of muscle over the topline.
· Compromised immune system. This gives rise to a host of conditions/diseases which are often passed off as old age. These include respiratory disease, skin infections, abscesses of the foot, buccal ulcers, and periodontal disease.
In order to confirm a diagnosis of Cushing's disease, specific hormone tests are normally undertaken. While the symptoms of Cushing's syndrome are attributed to excessive production of cortisol by the adrenal glands, the level of cortisol in the blood varies throughout the day. This makes it difficult to differentiate between levels of cortisone concentration in the normal horse and a horse with Cushing’s syndrome.
The most widely used test for classic equine Cushing's syndrome associated with a pituitary tumor is the dexamethasone suppression test (DST). This test determines the effect of dexamethasone on the blood cortisol concentration. Dexamethasone blocks ACTH secretion by the healthy pituitary gland and significantly reduces circulating cortisol. Since dexamethasone is unable to block ACTH production by the pituitary tumor, it does not suppress blood cortisol levels in the horses affected with classic Cushing's syndrome.
High cortisol levels interfere with the action of insulin, causing elevated insulin levels in horses with Cushing's syndrome. Some veterinarians therefore also measure insulin to support their diagnosis.
Treatments for Cushing's syndrome
Blood and urine tests are normally employed to diagnose Cushing’s disease. Repeat testing may be necessary, as preliminary tests do not always produce conclusive results. Some cases of Cushing’s syndrome are so obvious (polydipsia, dense hair growth, etc.), that treatment is prescribed in advance of testing.
Once a positive diagnosis has been reached, appropriate treatment can begin. When symptoms are caught early, treatment can be extremely successful, returning the horse to normal health for many more years.
Helping the horse with Cushing's syndrome
No treatment for Cushing’s will cure the disease. Therefore, the owner’s role will be to focus on careful health management and preventive treatment as follows:
· Avoiding stressing the horse. The hormonal profile of many horses with Cushing’s already indicates high stress levels, so reducing stress is critical.
· Providing a safe, comfortable “sanctuary” for the horse.
· Sticking to a strict routine, which will help minimize stress.
· Keeping water and feed conveniently located and in the same place.
· Clipping the horse in warm weather; using blankets when it is cold.
· Keeping up grooming to minimize skin diseases.
· Keeping hooves in good shape.
· Checking teeth regularly and having them checked by a professional twice a year.
· Avoiding turning the horse out with aggressive horses.
· Avoiding contact with horses from a new location.
· Keeping immunizations to a minimum/ making sure all necessary shots are given.
· Deworming regularly (usually every 30 – 45 days).
· Providing an appropriate diet for the horse. This will usually involve elimination of simple carbohydrates.