Researchers know that chromosomal anomalies are one of the principle genetic causes of infertility in horses. However, a great proportion of these cases still go undiagnosed, probably due to the fact that their clinical signs are nonspecific and diagnosis is complex. This is the case with chimerism, which is associated with twin births in domestic animals. Chimerism’s prevalence and reproductive effects are still unknown in horses.

Chimerism is a genetic disorder in which two zygotes combine after fertilization to form one single zygote that develops normally. The resulting fetus, therefore, has two types of different cells, each with a different genetic constitution. In the majority of documented cases, the cells of different organs or different parts of the body have different DNA, as if two beings were present in one body.

In general, this disease is produced either by the exchange of hematopoietic stem cells via placental circulation between dizygotic twins (blood chimerism) or by the fusion of two embryos in the first stages of development (true chimerism).

Recently, researchers from the University of Sevilla and the University of Córdoba, both in Spain, published the first study to evaluate the prevalence of twin births and chimerism in a large population of Pura Raza Española horses (PRE, or pure Spanish horses). The researchers analyzed a total of 14 twin births (with 23 live twins, a 0.066% twin foaling prevalence) and five cases of blood chimerism (21.7% of twins and an overall prevalence 0.011%) from a population of 21,097 PRE horses born during the 2015-2016 breeding season