Persistent Müllerian duct syndrome (PMDS) is an inherited disorder of sexual development affecting male dogs. In early, in utero development all canine fetuses have precursors of the uterus, fallopian tubes and upper vagina called Müllerian ducts. In normal male fetuses, the Müllerian ducts regress as sexual differentiation occurs in utero allowing for development of male sexual anatomy. In affected male dogs the female reproductive organs fail to regress in utero. Approximately half of the affected male dogs have externally normal testes and are fertile, but internally have remnants of female reproductive organs, including the uterus. The other 50% of affected male dogs have cryptorchidism in addition to remnants of female reproductive structures. Cryptorchidism predisposes dogs to infertility and testicular tumors. Affected dogs may also have small testes. Male dogs may need to have a hysterectomy if the uterus becomes infected. This disorder is limited to expression in males. Females with this condition have normal internal and external female anatomy.

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