Hyperadrenocorticism - Donkey
Introduction
This condition is not as common in donkeys as is usually thought. It is often suspected due to clinical signs, which include laminitis, seborrheoic hirsuitism, muscle wastage and hepatopathy (McGowan, 2003).
Diagnosis
Suspected cases may give an initial blood screen which may show raised AST, LDH, GGT, GLDH, SAP and bile acids. Individuals may be hyperglycaemic and often glucosuric (dipstick urine if available). There may be an increased white cell count and a stress leucogram.
Diagnostic methods recommended by The Donkey Sanctuary are given below:
1. Serum insulin
Serum insulin can be a useful screen but is not recommended as a diagnostic test in its own right.
There are no reference ranges for the donkey, but one can satisfactorily use the accepted equine ranges: normal cases have parameters within the range of 5.4–36 IU/ml whilst Cushing’s syndrome cases have parameters of >50 IU/ml.
2. TRH test
Take a blood sample before injection of 1 mg of TRH i.v. Further blood samples for cortisol assays are taken at 15 and 30 minutes following injection (positive test shows an increased 100% cortisol level at 30 minutes post injection).
This test has low sensitivity and specificity with false positives demonstrated in horses that are unwell for other reasons.
3. Dexamethasone suppression test
This test aims to detect a failure of suppression of cortisol following administration of dexamethasone in an animal suffering from Cushing’s syndrome. Although the test is not affected by the time of day, for convenience it may be run overnight.
A baseline blood sample (serum or heparinised) should be taken at 1700 hours on day 1, when a 40 μg/kg dose of dexamethazone is injected i.m.
A second blood sample should be taken at 1200 hours on day 2 (i.e. 12 to 24 hours post-injection).
Iatrogenic laminitis is not observed due to the low dose of dexamethasone used.
4. Combined dexamethasone suppression/TRH test
This test is more complicated and only suitable with hospital facilities. Although easier to interpret it is more costly in regard to time and money.
- At time zero an i.v. catheter is placed and a baseline sample (as above) collected, followed by an i.m. injection of 40 mg/kg of
dexamethasone
- At time zero +3 hours a second blood sample is taken, followed immediately by an i.v. injection of 1mg TRH
- Blood samples can then be collected +15 mins, +30 mins, +45 mins, +60 mins and +21 hrs post TRH injection
- In donkeys with Cushing’s syndrome there is cortisol suppression for the three hours after the dexamethasone injection but it then increases after the TRH injection
Treatment options
Many cases may be managed satisfactorily by appropriate clipping, diet, farriery and rugs. Treat the laminitis with ACP and phenylbutazone (see Laminitis). Supplementation with electrolytes may be necessary if sweating is excessive.
Pharmacological preparations include two types of drugs, dopamine agonists (pergolide) and cortisol inhibitors (trilostane):
- Pergolide Mesilate. Start treatment per os daily for two weeks then start to reduce the dose gradually if blood glucose shows signs of returning to normal. Side-effects are rare but include diarrhoea, colic, depression and anorexia. Be aware that it is an expensive drug to use for an extended period
- Trilostane. Use once a day. Give in the afternoon or evening. (Note the licence for this drug is only held for dogs.)
Literature Search
Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).
Hyperadrenocorticism in donkeys publications
References
- Sprayson, T. (2008) The care of the geriatric donkey In Svendsen, E.D., Duncan, J. and Hadrill, D. (2008) The Professional Handbook of the Donkey, 4th edition, Whittet Books, Chapter 13
- McGowan, C. (2003). ‘Diagnostic and Management Protocols for Equine Cushing’s Syndrome’. In Practice (2003) 25. pp 586-592.
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