Published December, 2012

Is your mare behaving more (night)marish lately, or is your gelding acting studly? Before recruiting a behaviorist or administering medication, experts advise ruling out medical issues for any animal experiencing behavior problems.

"A number of conditions can cause hormonal imbalances in horses of either sex that affects their behavior," said Barry A. Ball, DVM, PhD, Dip. ACT, the Albert G. Clay Endowed Chair in Equine Reproduction at the University of Kentucky's Maxwell H. Gluck Equine Research Center.

The problem with diagnosing hormonal imbalances is that many tests are not accurate or sensitive enough to detect small changes in blood levels of certain hormones. Also, many hormone levels fluctuate (in a cyclic manner), depending on time of day and/or season. To date, practitioners have used testosterone, inhibin, progesterone, and estrogenlike hormones to help diagnose ovarian tumors in mares and/or cryptorchidism (undescended testicles).

"Inhibin and testosterone are not ideal because both may be elevated normally during pregnancy in the mare, and pregnancy must be excluded in order to interpret their values," Ball said.

Likewise, using testosterone to detect a retained testis in a "gelding" might require multiple blood samples and possibly a stimulation test involving another hormone, human chorionic gonadotropin.

Ball and colleagues, attempting to find a better hormone test, ultimately "borrowed" a human hormone assay to measure anti-Müllerian hormone (AMH). The researchers used the AMH assay in mares with ovarian tumors and male horses (stallions, geldings, and cryptorchids).

In their studies they determined that one of the most useful situations for AMH is to test geldings with questionable castration history to determine if they are cryptorchids.

"One study demonstrated that traditional testosterone tests for cryptorchids yielded inconclusive results in 14% cases tested," Ball relayed.

AMH, which is produced by specific cells inside testicles, can be detected in a single blood sample. Ball and colleagues from the University of California, Davis, Veterinary Medical Teaching Hospital's Clinical Endocrinology Laboratory found that AMH concentrations were significantly higher in cryptorchids (n=44) than intact stallions (n=15) and geldings (n=48).

"This data support the use of AMH to detect testicular tissue in horses with questionable castration history," Ball added. "Unlike some tests for cryptorchids, AMH requires only a single blood sample."

Ball's team also evaluated using the AMH test in mares with granulose cell tumors (GCT), the most common type of ovarian tumor. Although mostly benign, GCTs can prevent pregnancy and cause stallionlike behavior and other problems in mares. GCTs can be palpated on rectal examination, but hormone testing for a definitive diagnosis is recommended.

Granulosa cells of the ovaries produce AMH; thus, Ball et al. collected blood samples from mares diagnosed with GCTs as well as normal mares to determine if the human AMH test could be employed in mares. The discovered that mares with confirmed GCTs had significantly higher AMH levels than normal (cyclic) mares, pregnant mares, and ovariectomized mares (those without ovaries). Further, they found AMH to be a more sensitive test (i.e., correctly identifies mares with GCTs) than inhibin, testosterone, and a combination of inhibin/testosterone testing.

Another application of AMH testing Ball explored is assessing "follicular reserve" in broodmares.

"Mares only have a certain number of ovarian follicles when they are born," Ball said. "Once the follicular supply is depleted, a mare will no longer be able to become pregnant. Having a test that can determine if a mare has a low follicular reserve could identify which mares might have a shorted reproductive lifespan."

In their research, Ball and colleagues found significantly higher AMH blood levels in young mares (3-15 years) than older mares (16-31 years). In addition, they noted a positive association between the follicle counts (determined via ultrasonography) and AMH concentration.

Research in this field is ongoing and, according to Bell, the AMH test will be available in the near future through Minitube of America Inc., in Verona, Wisc.

Stacey Oke, DVM, MSc, is a freelance medical writer based out of Canada.