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