Addisons Disease in Dogs and Cats

Addison's disease (or hypoadrenocorticism) is a condition of dogs and very rarely cats caused by destruction of part of the adrenal gland. This lack of adrenal function causes a variety of vague symptoms in affected animals but may also cause dramatic signs such as collapse or even sudden death. This article discusses the cause, signs, diagnosis and treatment of hypoadrenocorticism in dogs. Very little information is available on Addison's disease in cats, but it is thought to be very similar in all respects to the syndrome in dogs.

The majority of cases of Addison's disease are caused through an autoimmune process, similar to the development of Hypothyroidism and Diabetes Mellitus. An autoimmune disease is one in which the immune system is mistakenly 'programmed' to attack its own cells as though they were foreign. In the case of hypoadrenocorticism, the cells destroyed are those of the adrenal cortex.

These cells are responsible for producing several vital hormones which may be grouped as glucocorticoids and mineralocorticoids. Glucocorticoids such as cortisol are responsible for helping the body deal with stressful situations such as illness or starvation. An excess of glucocorticoids is the cause of Cushing's Disease, which is in many ways the opposite condition to Addison's. Mineralocorticoids on the other hand are responsible for regulating levels of minerals and salts in the blood, maintaining normal fluid balance and cell function.

Without adequate regulation of salt levels in the blood, dogs with hypioadrenocorticism are liable to develop hypovolemia, a critical reduction in the circulating blood volume. This, coupled with the effects of mineral imbalance on the vital organs such as the heart and kidneys, is what is responsible for causing signs of illness.

Addison's disease typically affects young to middle-aged dogs, with 70% of patients reported to be female. The disease may present as a dramatic acute episode, or more commonly as a milder chronic illness.

In the acute form of the condition (an Addisonian crisis), the following signs are seen:

• collapse
• lack of blood volume with poor pulses
• slow heart rate (less than 50 beats per minute)
• abdominal pain
• vomiting
• diarrhoea
• dehydration
• hypothermia- the gums, feet and ears may feel cold to the touch

Chronic hypoadrenocorticism may be difficult to recognise, as the symptoms are usually vague, mild, and the animal will recover with non-specific treatment. For these reasons, it often takes several bouts of illness before the condition is diagnosed. The most consistent signs are:

• poor appetite
• vomiting
• diarrhoea
• dehydration
• weight loss
• lethargy
• weakness
• increased thirst
• occasional episodes of collapse

Due to the lack of glucocorticoids, the symptoms listed above are usually seen during or immediately after times of stress; for example staying at a boarding facility or after atrip to the veterinary surgery.

An Addisonian crisis is a true emergency, and treatment must be initiated by your veterinarian before a definitive diagnosis is made. Thankfully, most animals are put on very high rates of intravenous fluids immediately when presented to the veterinary hospital, and this is usually sufficient to stabilise them to allow further investigations.

Routine blood tests will show non-specific and potentially confusing changes such as anemia and elevations in kidney values. Urine testing may show a failure to concentrate urine. At this point in the investigation, it is possible that an incorrect diagnosis of kidney failure could be made with disastrous consequences. Your pet will be dependent on your veterinarian's skill and attention to detail to take the next step in diagnosis. As for Cushing's Disease, the definitive diagnosis is made using the ACTH stimulation test, which will show no response by the adrenals to stimulation.

Once diagnosed, treatment of the disease is usually very successful. However, your dog or cat will need to be kept on medication for the rest of his/her life. As mentioned above, your pet may require hospitalisation and intensive care if he/she presents in an addisonian crisis, but follow up treatment does not usually require hospitalisation. In the United States, most veterinarians will choose to use an injectable mineralocorticoid called deoxycorticosterone pivalate (DOCP), which is usually administered once every 3-4 weeks. When using this treatment it is also necessary to give daily glucocorticoid supplementation, usually with prednisone or hydrocortisone.

In the UK and Ireland, most patients receive fludrocortisone (Florinef) tablets daily, which have both glucocorticoid and mineralocorticoid actions. With both DOCP and Florinef, it is advisable to supplement extra glucocorticoids if periods of stress are anticipated. The outlook for most dogs with Addison's disease is excellent.