Research Finds Inbreeding a Factor in Pregnancy Loss

New research from the Royal Veterinary College, in collaboration with Cornell University, has found inbreeding to be a contributing factor in mid- and late-term pregnancy loss in Thoroughbreds.

This is the first study that explores the effect of genomic inbreeding levels on late-term pregnancy loss in the horse and will help inform mating choices to minimize the risk of miscarriages in Thoroughbreds.

Inbreeding is a common practice in horse breeding and livestock industries because individuals with desirable traits are highly prized. However, excessive inbreeding is known to increase the probability of uncovering homozygous recessive genotypes that previously have been associated with an increased risk of retained placenta and lower semen quality in horses.

Approximately 5%-10% of equine pregnancies end in early pregnancy loss and a further 7% of pregnancies are lost between Day 70 of gestation and 24 hours post parturition. However, to date, there has been no definitive analysis to investigate the association between genomic inbreeding levels and pregnancy loss in horses.

Therefore, the research team, led by Dr. Jessica Lawson, Alborada Trust Research Fellow at the RVC, and Professor de Mestre at the Baker Institute for Animal Health at Cornell University, together with previous PhD student at the RVC Charlotte Shilton, analyzed DNA samples from 189 individuals, including a control group. The study found that Thoroughbred pregnancies lost in mid- and late-gestation had significantly higher inbreeding metrics than UK Thoroughbred adults. However, pregnancies lost early in gestation showed no significant difference in inbreeding metrics compared to the adults.

While Thoroughbred breeders make careful selection of their mating and breeding choices, this research shows the critical importance of cognizance in mating decisions in the industry. These findings also highlight the need for further research to continue to identify and characterize genomic changes that may negatively impact the pregnancy.

"This research provides critical evidence showing that mating highly related individuals does have a tangible effect on our breeding operations," said Lawson. "There is a real risk of a mare losing her pregnancy late in gestation and failing to produce a foal at all that season.

"The take-home from our work should be to carefully consider breeding choices that involve mating of highly related individuals as, ultimately, this may increase the chance of the foal inheriting mutations which may not be compatible with life. We are already working on the next step, looking to identify these changes so more specific advice can be provided in the future".

This research was funded by the Thoroughbred Breeders Association, Horserace Betting Levy Board, the Alborada Trust, and partial PhD studentship funding from the Royal Veterinary College's Paul Mellon Trust for Equine Research.