If your horse tested positive for Cushing’s and the blood was taken in the fall, you may want to test again in the spring. This is especially true if he was tested due to a history of both spring/grass-related and fall laminitis, but he has no obvious symptoms of Cushing’s disease — and isn’t really considered old enough for Cushing’s normally.
Since treating Cushing’s is expensive, and treatments may lose their effectiveness over time, you may want to talk to your veterinarian about tapering off treatment in late April, early May and retesting the horse after two to three weeks off medication. If normal, your horse may only need the treatments to suppress ACTH during the fall, early winter.
A field study by New Bolton Center (University of Pennsylvania Veterinary College) yielded important information about testing for Cushing’s disease: They took 29 ponies (mares and stallions) and 10 horses (mares), all living outdoors, and had ACTH determinations and dexamethasone suppression tests performed in September (at the start and again at the end of the study), January and May. These are the two most commonly performed tests for pituitary tumor Cushing’s disease. None of the animals had signs or symptoms of Cushing’s when tested.
The researchers found that in samples from May, all the animals tested within normal ranges for ACTH. In January, 97% of the results were within normal. However, on the September testings 95% of the horses and ponies showed elevations of ACTH beyond the normal range, some extremely high. There was also a 36% false positive test result when the dexamethasone suppression test was used. In September and January, older horses were found to have less cortisol suppression on dexamethasone suppression tests than young ones and to have ACTH in the higher ranges of normal. However, in May there was no effect of age on test results.
These findings are extremely important because they mean that trying to diagnose or confirm Cushing’s disease by doing ACTH or dexamethasone suppression tests can lead to large numbers of false positives if testing is done in the fall. The study didn’t look at horses on a month-by-month basis, so we don’t really know yet when this seasonal increase in ACTH starts and ends, except that it appears to normalize for the most part by January and disappears entirely by May.
If you have an older horse that has classical symptoms of Cushing’s (e.g. the long, curly coat), odds are your test results are reliable regardless of when the testing was done. However, many horses are tested for Cushing’s when their only symptom is unexplained, chronic or recurrent laminitis. Insulin is almost always elevated as well, which certainly is often the case with Cushing’s but can be caused by insulin resistance not related to Cushing’s, too. It’s possible that some horses with insulin-resistance symptoms — easy keepers, cresty neck, odd fat deposits, prone to laminitis — may be pushed over the edge into laminitis by the seasonal elevations in ACTH and cortisol.
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