New research (Lawson et al., 2024) from the Royal Veterinary College (RVC), in collaboration with Cornell University, has found that inbreeding is a contributing factor to mid- and late-term pregnancy loss (MLPL), but not early pregnancy loss (EPL) in Thoroughbreds. This is the first study which explores the effect of genomic inbreeding levels on late-term pregnancy loss in the horse and will help inform mating choices to minimise the risk of miscarriages in Thoroughbreds.
Inbreeding is a common practice in the livestock industry because individuals with desirable traits are highly prized for breeding. However, excessive inbreeding is known to increase the probability of uncovering homozygous recessive genotypes, which have previously been associated with an increased risk of retained placenta and lower semen quality in horses.
Approximately 5 to 10 percent of equine pregnancies end in early pregnancy loss and a further 7 percent of pregnancies are lost between Day 70 of gestation and 24 hours post parturition (MLPL). 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 a previous PhD student at the RVC, Charlotte Shilton, analysed DNA samples from 189 individuals, including a control group. The study found that Thoroughbred pregnancies lost in mid and late gestation (MLPL) had significantly higher inbreeding metrics than UK Thoroughbred adults. However, pregnancies lost early in gestation (EPLs) showed no significant difference in inbreeding metrics compared to the adults.
While Thoroughbred breeders carefully select their mating and breeding choices, this research shows the critical importance of cognisance in mating decisions in the industry. These findings also highlight the need for further research to identify and characterise genomic changes that may be lethal to pregnancy.
Dr Jessica Lawson, Alborada Trust research fellow at the RVC, said:
“This research provides critical evidence showing that mating highly related individuals do have a tangible effect on our breeding operations, as 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 takeaway from our work should be to carefully consider breeding choices that involve mating highly related individuals as, ultimately, this may increase the chance of the foal inheriting mutations that 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 RVC’s Paul Mellon Trust for Equine Research.
The RVC provides equine reproductive services and neonatal support at the equine hospital and practice in South Hertfordshire.
The article is available to read here.
