chimerism in PRE horses
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.

The findings suggest that the prevalence of chimerism is extremely low in horses and the condition is not closely connected to infertility, the researchers said.

“To detect this disease, we recommend that (breeders) use a molecular detection tool, prepared by our research team,” says Mercedes Valera, DVM, professor of the Higher Technical School of Agricultural Engineering at the University of Seville. “This fast, cheap, and early detection method is an excellent diagnostic tool which can be incorporated into the veterinary care of horses, as it allows for the detection of chromosomal anomalies connected with infertility and subfertility.”

The study, “Prevalence of twin foaling and blood chimaerism in purebred Spanish horses,” was published in The Veterinary Journal.