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Find information on animal health topics, written for the veterinary professional.

Congenital and Inherited Disorders of the Reproductive System in Horses

By Robert O. Gilbert, BVSc, MMedVet, DACT, MRCVS, Professor, Reproductive Medicine, Department of Clinical Sciences, College of Veterinary Medicine, Cornell University ; Fabio Del Piero, DVM, DACVP, PhD, Professor, Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University ; Ronald J. Erskine, DVM, PhD, Professor, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University ; Paul Nicoletti, DVM, MS, DACVPM (Deceased), Professor Emeritus, College of Veterinary Medicine, University of Florida ; Jerome C. Nietfield, DVM, PhD, DACVP, Professor, Department of Diagnostic Medicine/Pathobiology, College of Veterinary Medicine, Kansas State University ; Donald Peter, DVM, MS, DACT, Veterinarian/Owner, Frontier Genetics, Hermiston, OR ; Patricia L. Sertich, MS, VMD, DACT, Associate Professor of Reproduction-Clinician Educator, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania ; Katrin Hinrichs, DVM, PhD, DACT, Professor and Patsy Link Chair in Mare Reproduction, Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University ; Brad E. Seguin, DVM, MS, PhD DACT, Professor Emeritus, Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota

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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.