By Mark Andrews

Research from the Royal Veterinary College (RVC) has demonstrated that a chromosomal defect is responsible for a significant proportion of horse pregnancies that fail within the first two months of development.

These findings will pave the way for new diagnostic tests for what could be one of the most common causes of pregnancy loss in mares.

Pregnancy loss is a notoriously difficult condition for veterinary surgeons to manage. Up to 10 percent of confirmed mare pregnancies end in the first two months, and in over 80 percent of those cases the cause is unknown.

In this study, researchers found that aneuploid pregnancies (in which a copy of a whole chromosome is either duplicated or missing) are a key cause of equine pregnancy loss.

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The research team, led by Dr. Mandi de Mestre, Reader in Reproductive Immunology and Head of the Equine Pregnancy Laboratory at the RVC, collaborated with seven different veterinary practices to gain access to samples from across the UK and Ireland. They found that around 20 percent of the pregnancies lost were aneuploid. Charlotte Shilton, RVC PhD student who performed the analysis, applied three different genetic approaches to confirm the results.

Work is now underway to identify the underlying cause of these aneuploid pregnancies, with early data from this study suggesting it is most commonly introduced via the egg or sperm. Until now, chromosomal defects such as aneuploidy have only been reported as a rare condition in young horses with developmental disorders.

This study explains why the condition is so rare in horses: most embryos and fetuses possessing this genetic change die very early in development, as also occurs in humans. The study highlights the need to reconsider this genetic condition both in pregnancy loss but also for early developmental disorders.

“Early pregnancy loss remains a very frustrating condition for clinicians to treat as the underlying cause is unknown in around 80 percent of cases,” says Dr. de Mestre. “These findings will allow researchers to develop new diagnostic tests for pregnancy losses, which would offer hope to thousands of owners of breeding mares that suffer this condition.

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“A diagnostic test would allow them to make informed decisions on treatment strategies and to advise on whether they should invest in further attempts to breed their mare, benefiting both horses and their breeders alike in the future.

“I would like to thank both the Thoroughbred Breeders Association, and our collaborators at Texas A&M University, and the participating veterinary surgeons for their support on this project.” 

For more details, see: Whole genome analysis reveals aneuploidies in early pregnancy loss in the horse.

Published with the kind permission of Mark Andrews, Equine Science Update.

Photo: Shutterstock/Pfluegler-photo