Cushing's disease or as it is also known Pituitary Pars Intermedia Dysfunction (PPID) is primarily a disease of older horses and ponies (15 years and over) but can be seen in younger animals. It is thought to affect over 20% of horses over the age of 15 and is a commonly seen condition in equine practice. PPID occurs due to a benign growth of the pituitary gland located at the base of the brain. This gland is regulated by a substance called dopamine and produces a variety of hormones one of which is ACTH. These hormones are then released into the bloodstream where they are distributed all over the body and are responsible for controlling and maintaining a variety of bodily functions. Horses with PPID do not produce enough dopamine and so the pituitary gland is unregulated and produces too many hormones. Signs that can be seen in horses affected by PPID include lethargy/poor performance, muscle wastage, periorbital fat (around the eyes), excessive drinking/urination, long abnormal hair coat/delayed hair coat shedding, excessive/patchy sweating, recurrent infections e.g. foot abscess, sinusitis and laminitis (history of, active or recurrent). Horses may show none of the clinical signs, one or two of these clinical or multiple clinical signs. Research has shown that PPID and EMS account for 90% of laminitis cases therefore it is worthwhile testing cases for this disease. Testing for PPID is relatively straight forward. The simplest and most common test for PPID is the basal or resting ACTH test. A blood sample is taken which is then sent chilled to a lab and is measured for a baseline ACTH concentration. Horse's that are affected will have higher than normal ACTH levels. The reference range for ACTH changes with the season to allow for the summer/autumn increase in pituitary activity which takes effect from late June until mid November. If pain and stress may have an effect on ACTH levels and so generally it is preferable to test in the absence of pain/stress so results can be validly compared. For horses that come back as borderline or in the 'grey zone' (i.e. neither a definitive positive or negative) your vet may advise re-testing in 3-6 months or performing a different test known as a TRH stimulation test. The latter involves taking a baseline blood sample then injecting a small amount of the TRH hormone and then taking a second blood sample at 10 and/or 30 minutes later. ACTH levels are then measured pre and post hormone injection. The advantage of this test is that it has a slightly higher diagnostic accuracy however it's use is generally avoided during July-Novemeber when the normal seasonal variation occurs. Occasionally drug companies will have offers of free lab testing for PPID so contact your local vet practice to discuss. Once your horse has been diagnosed with PPID a decision can be made as whether to treat the condition. This will depend on the level of increase of the ACTH and the clinical signs exhibited. It is advisable to treat any horse with laminitis. Treatment involves giving a drug called pergolide mesylate ('Prascend') which is currently the only licenced treatment available in the UK. The drug acts on the brain to reduce the release of ACTH which in turn decreases the amount of hormones released. Clipping out long coats can also help with excessive sweating and regular dental care and up to date worming are advised. Treatment is usually successful. In rare cases side effects can occur including inappetance, transient anorexia, mild central nervous system signs (mild depression and ataxia), colic and diarrhoea. If any of these occur often stopping treatment for a couple of days and then restarting at a half dose and increasing back up to a full dose will help negate these side effects.
If you think your horse or pony may have Cushings then please contact your local vet to discuss or visit www.talkaboutlaminitis.co.uk for more information.