Cryptorchidism is a failure of one or both testicles to descend into the scrotum and is seen in all domestic animals. It is the most common disorder of sexual development in horses. It is thought to be a genetic disorder inherited from either parent. If both testicles are retained in the abdomen, the horse will be sterile. Because a retained testicle still produces male hormones, cryptorchid animals have normal mating behavior. If only 1 testicle is retained (unilateral cryptorchidism), the horse may still be fertile because the normal testicle produces normal sperm. However, it is highly recommended that you not breed a cryptorchid horse because the condition is hereditary and will likely be passed on to any offspring.

Mares that are missing one of the sex chromosomes (designated as XO) have hereditary underdevelopment of the ovaries and are sterile. Affected mares may be smaller than average and may not have an estrous cycle because the ovaries do not have any eggs. The ovaries are smooth and firm and have no follicles. There is no treatment for this condition.

Last full review/revision July 2011 by Robert O. Gilbert, BVSc, MMedVet, DACT, MRCVS; Fabio Del Piero, DVM, DACVP, PhD; R. J. Erskine, DVM, PhD; Paul Nicoletti, DVM, MS; Jerome C. Nietfield, DVM, PhD, DACVP; Donald Peter, DVM, MS, DACT; Patricia L. Sertich, MS, VMD, DACT; Katrin Hinrichs, DVM, PhD, DACT; Brad E. Seguin, DVM, MS, PhD DACT