New Test for Horses with Retained Testicles (AAEP 2011)

Ridgling, crypt, cryptorchid. Call it what you want, but a horse with one or two testes that have not descended into the scrotum can present a diagnostic challenge. Anthony Claes, DVM, Dipl. ACT, a graduate student at the University of Kentucky's Gluck Equine Research Center, discussed a new way to diagnose cryptorchidism during the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas.

Many stallions can be challenging and even dangerous to handle on a daily basis, especially in the presence of mares. "A horse that displays stallion-like behavior could be a bilateral cryptorchid, a cryptorchid that has its descended testis removed, or a gelding with behavior problems," explained Claes. "Therefore, (horses) thought to be 'geldings' that display stallion-like behavior might not actually be true geldings after all. Approximately 3-8% of the male equine population is cryptorchid."

According to Claes, the most common method veterinarians use to diagnose retained testicular tissue in a horse that displays stallion-like behavior involves measuring basal or human chorionic gonadotropin (hCG) stimulated circulating testosterone levels and/or estrone sulphate levels. In previous research Claes and Gluck colleagues had shown that a protein called anti-Müllerian hormone (AMH) is expressed by certain (Sertoli) cells in the testes and can be measured in the blood. They took the research a step further and, in the current study, measured AMH in blood samples from 48 geldings, 44 cryptorchids, and 15 stallions and found:

  • AMH levels were higher in cryptorchids than in either stallions or geldings; and
  • AMH levels were higher in stallions than in geldings.

Other benefits to the new test include "only a single blood sample is required and can potentially be used in cases where testosterone is inconclusive, however more research needs to be done in this area," Claes said.

"This data revealed that AMH can be used successfully to diagnose cryptorchidism in the horse, although the test is not yet commercially available in the United States," concluded Claes.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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