Prevalence of twin foaling and blood chimaerism in purebred Spanish horses

Twin foaling is associated with chimaerism in several domestic species and is recognised in horses. In this study, 21,097 purebred Spanish (Pura Raza Español) horse births from the 2015 to 2016 breeding season were investigated for chimaerism. Twin foaled and chimaeric individuals were assessed on the basis of foaling records, short-tandem repeat (STR) parentage test results and a sex-linked STR-based technique. Fourteen twin pregnancies with 23 twin foals born alive were identified (0.066% twin foaling prevalence), including five blood chimaeric cases (21.7%; overall prevalence 0.011%), suggesting that this genetic condition is extremely low in horses. Furthermore, no true chimaeras were detected. This is the first large scale study analysing the occurrence of chimaerism in a horse population and the first assessment of twin foaling in purebred Spanish horses.