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. Nora Grenager, VMD, Dipl. ACVIM, of the Steinbeck Country Equine Clinic, in Salinas, Calif., updated attendees on Cushing's at the Sept. 17-18 Laminitis West Conference, in Monterey, Calif.
Horses with Cushing's (technically known as pituitary pars intermedia dysfunction or PPID) have an eightfold decrease in levels of dopamine (a hormone and neurotransmitter) and its metabolites (breakdown products) compared with nonafflicted horses of the same age. Grenager reported that this is most likely due to localized oxidative stress and noted that evidence is lacking that this decrease is 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. 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 widely divergent clinical signs.
"The ones where we need help are the ones where (the diagnosis) is not so clear," Grenager said. "The bad news is that we don't yet have a great test for those horses. That's what researchers are working on."
Grenager discussed common Cushing's tests, particularly the dexamethasone suppression test (DST) and the resting endogenous adrenocorticotropic hormone (ACTH) test. Veterinarians usually employ these tests 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 (meaning the horse's response to the test will vary with season; false positives are more likely to occur when testing in late summer and fall). 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 vet visit and no steroids. Veterinarians must handle samples very carefully, collecting them in plastic tubes because ACTH binds to glass, and separating and freezing them quickly.
Grenager likes 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, Grenager 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.
Veterinarians usually prescribe specific diet and exercise regimens, which can help minimize the risk of laminitis. Grenager also recommended an emphasis on good foot and dental care, routine fecal egg count testing (to monitor for parasites), 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.