Prevalence and genetic parameters for cryptorchidism in Swedish-born Icelandic horses

• We estimated 9% prevalence of cryptorchidism in one-year-old Icelandic horses.
• Inguinal, unilateral cryptorchidism was the dominating form of the condition.
• Estimated heritabilities were 0.12–0.32 (SE 0.08–0.12) on the observable scale.

Cryptorchidism is a defect in males where one or both testes fail to descend normally into the scrotum. If both testes are retained, the stallion will be infertile. There are few studies of the prevalence of the condition in horses based on field data, and heritability estimates for equine cryptorchidism are lacking. The objective of this study was to provide data on prevalence and heritability of cryptorchidism in Icelandic horses born in Sweden. A questionnaire was sent to breeders, asking whether or not stallions born at the farm had both testes present in the scrotum at the age of 1, 6 and 12 months. Fixed effects were analysed using logistic regression and genetic parameters were estimated using linear animal models. Information about 595 yearling stallions born 1997–2011 was used in the analyses. Close to 9% of the yearlings did not have both testes in the scrotum. Probability of cryptorchidism in yearlings was significantly influenced by farm and time period of birth. Heritability estimates for cryptorchidism ranged from 0.12 to 0.32 (SE 0.08–0.12) on the observable scale, and from 0.35 to 0.96 (SE 0.24–0.40) when transformed to the underlying continuous scale. The results support that equine cryptorchidism is heritable. Further studies with more individuals included would be needed to confirm this.