Humoral immune response to equine chorionic gonadotropin in ewes: Association with major histocompatibility complex and interference with subsequent fertility

In dairy ewes, the use of eCG as a convenient hormone for the induction of ovulation is necessary for out-of-season breeding and artificial inseminati on (Al). In this report we show the presence of anti-eCG antibodies in plas ma of treated ewes. The major histocompatibility complex (MHC) was involved in the individual variability of the humoral immune responses to eCG. We f ound significant associations between the anti-eCG response phenotype and s ome MHC class II alleles. The low immune response phenotype was associated with one MHC class II allele only in Lacaune ewes, and the high immune resp onse phenotype was associated with one MHC class II allele both in Manech a nd in Lacaune ewes. In herds, the impact of residual anti-eCG antibodies on subsequent fertility after Al seems minimal because of an indirect elimina tion of high-responder ewes from Al breeding. Therefore, the true magnitude of the association between residual anti-eCG antibody concentration and fe rtility has been underestimated. An additional experiment without any high- responder female elimination showed a significant correlation between high residual antibody concentrations and lower lambing rate after Al at a fixed time, possibly because of a delayed preovulatory LH surge. The results sug gest that anti-eCG antibody concentration is one risk factor for infertilit y after Al.