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Cushing’s difficult to diagnose early

by
01 November 2010, at 12:00am

reports on a paper presented at a recent conference on laminitis in the USA

“BECAUSE horses afflicted with equine Cushing’s disease often develop laminitis, advances in diagnosis and treatment of Cushing’s can help veterinarians better prevent and/or treat the laminitis that stems from it,” Dr Nora Grenager, of the Steinbeck Country Equine Clinic, in Salinas, California, told delegates at the Laminitis West Conference held in California in September

Horses with Cushing’s (pituitary pars intermedia dysfunction or PPID) have an eight-fold decrease in levels of dopamine and its metabolites compared with non-afflicted horses of the same age, she said. This was most likely due to localised oxidative stress but she commented that evidence was lacking that this decrease was caused by systemic inflammation or systemic oxidative stress.

Other, less likely, factors that have not yet been well-evaluated could include toxins or pesticides, genetic predisposition, chronic inflammation or abnormal systemic metabolism, and a diet lacking enough selenium or protein.

Cushing’s horses are frequently insulin-resistant, a condition that also can be a factor in non- Cushing’s-related laminitis cases, but a simple blood test can often identify horses with insulin resistance. Part of the problem with Cushing’s is that it can be difficult to diagnose early because horses can present with widely divergent clinical signs. “The ones where we need help are the ones where the diagnosis is not so clear,” Dr Grenager continued. “The bad news is that we don’t yet have a great test for those horses. That’s what researchers are working on.”

She went on to discuss common Cushing’s tests, particularly the dexamethasone suppression test (DST) and the resting endogenous adrenocorticotropic hormone (ACTH) test. These tests are usually used in an attempt to diagnose horses that don’t show dramatic clinical Cushing’s signs.

While the DST can be effective, it requires two veterinary visits and administration of steroids (which can on occasion trigger laminitis in susceptible horses). It can also give early false negatives and show seasonal variations – false positives are more likely to occur when testing in late summer and autumn. A combined DST/thyrotropin- releasing stimulation test might show fewer false positives, but it is a 24-hour test that requires four blood samples.

The ACTH test, another effective test, requires only one veterinary visit and no steroids.

Dr Grenager prefers the newer domperidone stimulation test because it is a safe test that doesn’t take long and has been shown to be more effective year-round, avoiding the seasonal variations in test results. It is more expensive, however, and its reliability is not yet well-documented.

Regarding Cushing’s treatments, the speaker noted that pergolide is the most effective treatment, but it can be expensive; cyproheptadine is less expensive yet not as effective. She said that more studies are needed on both medications, and on newer medications that work on Cushing’s in humans and could potentially be explored for use in horses.

It is usual to prescribe specific diet and exercise regimens, which can help minimise

the risk of laminitis. Dr Grenager also recommended an emphasis on good foot and dental care, routine faecal egg count testing, and body clipping if medication is not controlling the long hair coat. She advised watching for clinical signs and diagnosing and treating affected horses as soon as possible to avoid Cushing’s-associated laminitis.