HEALTH CARE Challenges of Cushing’s diagnosis

Q:I live in Canada, and I own four horses ranging from 6 to 25 years old. By late spring last year, they had all shed their winter hair and had beautiful slick coats, except for my 22-year-old Trakehner-cross mare. She was recently tested for Cushing’s disease, but the tests were negative. What else could be causing her not to shed?

Name Withheld By Request

A:This is a very important question. Our understanding of equine Cushing’s disease, also known as PPID (pituitary pars intermedia dysfunction), is advancing rapidly, but testing is still imperfect.

The equine version of this disease is different from Cushing’s in other species in that for horses it is caused by

changes in a specific portion of the pituitary gland. The pituitary then showers the body with a variety of hormones without the usual control and feedback. One effect of these hormones is to stimulate overproduction of the stress hormone cortisol from the adrenal glands, which in turn leads to imbalances in many other hormones that cause many of the outward signs of PPID.

A long, shaggy winter coat that is slow to shed in the spring is a hallmark of PPID, but other early signs include decreased athletic performance, lethargy, changes in body shape and persistent, low-grade laminitis. In more advanced cases the horse may develop increased or decreased sweating, stubborn infections, slow-healing wounds or injuries, increased thirst and urination, and a host of other problems. This disease truly affects all aspects of a horse’s health.

Many tests are currently available for PPID, and each has benefits and drawbacks. The ACTH (adrenocorticotropic hormone) test is very common now because it is safe and simple to perform. But because levels of this hormone can fluctuate at different times of year in normal horses, the results must be interpreted carefully.

The most sensitive test we have is the TRH (thyrotropin-releasing hor-