What Does AMH Say About Your Stallion or Mare?

AMH appears to be a reliable marker for cryptorchidism, granulosa-cell tumors in mares, and, possibly, mare fertility.

Photo: Erica Larson, News Editor

Researchers at the University of Kentucky Gluck Equine Research Center, in Lexington, have spent the last decade studying a hormone that’s linked to a number of reproductive conditions. It’s called anti-Mullerian hormone (AMH), and it’s proved a reliable marker for cryptorchidism in stallions, granulosa-cell tumors in mares, and, possibly, mare fertility.

At the 2016 Theriogenology Conference, held July 27-30 in Asheville, North Carolina, the Gluck Center’s Barry Ball, DVM, PhD, described current applications of AMH.

AMH in Stallions

In the normal male horse, AMH is produced is high concentrations in the fetal testes, with concentrations declining as the horse hits puberty, said Ball.

This is because the blood-testis barrier forms at puberty, causing AMH to decline in the blood and begin appearing in the seminal fluids. It continues to show up in lower concentrations in blood, as well as in seminal plasma, in the adult horse. As with other reproductive parameters, AMH concentrations in blood vary with season.

Cryptorchids (stallions with one or more retained testicles) are the outliers here. Their blood-testis barrier is altered, allowing AMH to continue circulating.

With that in mind, “can we use AMH as a marker for crytorchid testes?” Ball asked.

To answer this question, he described at 2013 study in which he and colleagues compared AMH levels in 41 geldings, 15 intact stallions, and 41 cryptorchids with one retained testis.

“AMH concentrations were significantly higher in cryptorchids versus geldings, and AMH concentrations were higher in cryptorchid stallions versus intact stallions,” he said.

This information is useful for verifying the presence of undescended testicles as well as determining whether a cryptorchid still has functional testicular tissue.

AMH in Mares

Each mare’s number of oocytes–called the follicular reserve--is fixed at birth. This reserve varies greatly between individuals and likely determines a mare’s reproductive longevity (the larger the reserve, the longer she can reproduce), Ball explained.

Studies conducted in mice, women, and cattle all show a relationship between AMH (which, in mares, is expressed in the follicular wall as follicles grow), age, and follicular reserve.

From these studies researchers have learned that “concentrations of AMH are closely related to the total number of antral follicles present on the ovaries,” said Ball. “Antral follicle counts (AFC), in turn, are closely related to the number of primordial follicles remaining on the ovaries. Therefore, peripheral AMH concentrations appear to be a good marker for follicular reserve.”

Ball tested this theory in a 2014 study of young, middle-aged, and old mares and found that serum AMH and total AFC were, indeed, closely related. Both declined significantly with age, he said.

In another retrospective study of 419 Thoroughbred mares, Ball and colleagues found that low AMH levels were associated with an increased chance of nonpregnancy.

In summary, he said, “Because follicular reserve undergoes depletion at different ages in the mare, it appears that AMH and AFC may be a better predictor or reproductive longevity in mares than their calendar age.”

AMH in Tumors

Anti-Mullerian hormone levels might also help veterinarians diagnose certain tumors. In stallions, for instance, equine testis tumors called Sertoli cell tumors express AMH.

“Although uncommon in stallions, serum AMH concentrations might be a useful marker for Sertoli cell tumors,” said Ball.

More importantly, however, is AMH’s role as a marker for the common Granulosa cell tumor (GCT) in mares. These account for 2.5% of all tumors in horses and 85% of all reproductive tract tumors, said Ball.

While most are benign, they are associated with stallionlike behavior in mares and anestrus (not cycling) or persistent estrus.

Ball described a retrospective study in which he and colleagues analyzed samples submitted to the University of California, Davis, for GCT diagnosis. Based on elevated AMH concentrations, they were able to accurately predict 95% of GCTs.

He concluded that AMH appears to be a good and sensitive marker for these tumors in mares.

About the Author

Alexandra Beckstett, The Horse Managing Editor

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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