If you have a horse that is in its 20s or 30s, you may worry about him becoming a Cushing’s horse. Look for these common symptoms:
• Long coat that doesn’t shed
• Increased thirst
• Increased urination
• Loss of muscle mass
• A big pot belly
• Fat above the eyes.
These symptoms, in this age group, are certainly classic. Blood work on a classic case will show elevated insulin, elevated glucose and elevated ACTH hormone levels. What if one or more of these symptoms are missing, though' Does the horse or pony still have Cushing’s' How can you tell for sure'Since a diagnosis of Cushing’s usually goes hand in hand with the use of expensive drugs that have a risk of side effects, it’s important to figure out if it’s true Cushing’s before beginning any treatment.
Early Cushing’s disease, or the even the more nebulous terms pre-Cushing’s or peripheral Cushing’s, are being applied with increasing frequency to horses and ponies of all ages. In reality, true Cushing’s disease is caused by a pituitary tumor.
However, horses as young as five are often labeled as Cushing’s or ”Cushingoid.” Many overweight, laminitic animals are called Cushing’s on this clinical picture alone, or in combination with an elevated insulin level. Are these truly early Cushing’s cases or something else' You need the diagnostic tests to find out.
The combination of elevated ACTH, abnormal hair coat and elevated blood sugar are generally accepted as diagnostic. In fact, an elevated ACTH level is virtually diagnostic, making this test a good place to start. (Note: ACTH levels that are done while the horse is on a Cushing’s medicine may not be accurate. You could get a false negative, which means a negative result may not be reliable.)
Unfortunately, the sample must be handled carefully, under controlled temperatures with the serum stored in a plastic container and kept frozen until analyzed, which is impractical in some areas of the country.
There are also some horses with many classic Cushing’s symptoms that don’t have an elevated ACTH or have only a marginal elevation. Whether these horses are only pituitary tumor look-alikes or have a pituitary abnormality that doesn’t involve ACTH remains to be seen.
A variety of stimulation and suppression tests - TRH stimulation, dexamethasone suppression and ACTH stimulation - are also considered highly accurate, at least in the advanced cases with classic symptoms. However, they may carry a risk of inducing laminitis.
Blood cortisol, produced by the adrenal glands when they are stimulated by ACTH, may be elevated in some Cushing’s horses, but not all. In addition, any type of stress can elevate blood cortisol.
For example, transporting the horse and/or confining him to a stall when he is used to constant turnout have been shown to be sufficiently stressful to elevate blood cortisol. We believe this makes cortisol more of a screening test with false negatives and false positives expected.
Cushing’s horses also often have elevated blood-glucose levels. Unfortunately, this symptom usually occurs late in the disease, so it’s possible to have a horse with the classic coat changes that doesn’t yet have an elevated glucose.
Very elevated insulin is another late change and, like the high blood sugar, reflects the insulin resistance in this disease. However, also like the blood sugar, it’s likely a finding that becomes progressively worse over time, so earlier cases might not have dramatic elevations.
Of the two tests, insulin and glucose, far more suspected Cushing’s cases have a normal or slightly elevated glucose but elevated blood insulin. High insulin does raise the suspicion of Cushing’s disease, but other conditions besides true Cushing’s can cause it.
A current popular test is the Cushing’s blood panel from BET Labs (have your veterinarian contact them at www.betlabs.com or 859/273-3036). This panel looks at T4, insulin and cortisol levels in a morning and an evening sample.
In most normal horses, morning and evening cortisol levels will vary by at least 30%. Loss of this variation can occur in horses with Cushing’s disease, and finding elevated insulin as well would increase the level of suspicion.
Unfortunately, cortisol diurnal variations are influenced by more than just Cushing’s disease, so this test isn’t 100% accurate either.
According to data presented by Dr. R. H. Douglas, director of BET Laboratories, the sensitivity of this test in detecting Cushing’s disease is about 68% (32% false negatives), while the false positives are 40%. As many as 56% of horses housed in stalls 16 hours a day will give false positive results on this test.
We realize there may not be a truly 100% accurate way to diagnose true pituitary-tumor Cushing’s disease in all its stages. We worry that Cushing’s is becoming a ”trash-can” or ”catch-all” diagnosis.
Some horses with chronic laminitis and weight problems may indeed be early Cushing’s cases. Others with the same symptoms could have a host of other metabolic problems that result in insulin resistance, even hypothyroidism. You’ve got to work to decide what’s most likely.
Perhaps someday imaging studies, like MRI scanning or CAT scans, will be common enough for horses that we can diagnose even small pituitary tumors without the ambivalence of blood tests. Until then, carbohydrate-restricted diets, mineral balancing, correction of concurrent hormonal problems like hypothyroidism, and regular exercise should all be tried first when dealing with insulin resistance. With severe or refractory cases, other hormonal or drug therapies may be indicated.