Genetic analysis of cryptorchidism in Swedish-born Icelandic horses

Susanne Eriksson¹, Kim Jäderkvist¹, Anne-Marie Dalin², Jeanette Axelsson¹, Gabriella Lindgren¹

¹Dept. of Animal Breeding and Genetics, Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden; ²Dept. of Clinical Sciences, SLU

Cryptorchidism, when one or both testes fail to descend normally into the scrotum, cause fertility problems, increased risk for tumors and costly castration surgery. Moderately high heritabilities have been estimated for cryptorchidism in dogs and pigs, but information on heritability for equine cryptorchidism has been lacking. The aim of this study was to estimate the prevalence and heritability of cryptorchidism in Swedish born Icelandic horses. A questionnaire was sent to 80 of the largest breeding farms. The breeders were asked for each stallion born 1990-2011 if the testes were down at the ages of 1, 6 and 12 months. The answering frequency was 57%, corresponding to 858 horses, from 230 sires and 471 dams. Many breeders did not know the status of the stallions younger than 12 months, some did not check before castration. The data indicated that the testes rather commonly descend later than six months after birth in Icelandic foals. At the age of 12 months, close to 9% of the 655 horses with information did not have both testes in the scrotum. Using logistic regression, probability of cryptorchidism was significantly influenced by breeding value for height at withers, farm and year of birth. Genetic parameters for cryptorchidism at the age of 6 (n=329, mean=0.25), 12 (n=655, mean=0.09) and 12 months or older (n=751, mean=0.06) were estimated using a linear animal model, including fixed effects of farm and birth year. The estimated heritabilities were 0.26 (S.E. 0.19), 0.14 (S.E. 0.12) and 0.08 (S.E. 0.10), respectively, on the visible scale and 0.45, 0.43 and 0.30 when transformed to the underlying continuous scale. The results support that equine cryptorchidism is heritable and could be selected against.