Hopes high for diagnostic test after key cause of early pregnancy loss in mares found

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Scientists who identified a chromosomal defect as an important early cause of pregnancy loss in mares believe their research will pave the way for a new diagnostic test.

Pregnancy loss is a notoriously difficult condition for veterinary surgeons to manage in mares as the underlying cause is often unknown.

In their pioneering study, researchers from the Royal Veterinary College in London identified aneuploid pregnancies — which is when a copy of a whole chromosome is either duplicated or lost (similar to Down’s Syndrome screened for in human pregnancy) — as a key cause of equine pregnancy loss.

It is, they say, the reason why a significant proportion of horse pregnancies fail within the first two months.

The study team, led by Dr Mandi de Mestre, reader in reproductive immunology and head of the Equine Pregnancy Laboratory at the college, collaborated with seven veterinary practices to gain access to samples from across Britain and Ireland.

They found that around 20% of the pregnancies lost were aneuploid.

PhD student Charlotte Shilton, who performed the analysis at the college, applied three different genetic approaches to confirm the results.

Work is now under way 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, with most embryos and fetuses possessing this genetic change dying very early in development, as is also observed in human pregnancy.

The study highlights the need to reconsider this genetic condition both in pregnancy loss but also for early developmental disorders.

De Mestre says early pregnancy loss remains a very frustrating condition for clinicians to treat as the underlying cause is unknown in around 80% of cases.

“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.

“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.”

De Mestre thanked the Thoroughbred Breeders Association and collaborators at Texas A&M University in the United States, as well as participating veterinary surgeons for their support on the project.

James Crabtree, an independent veterinary consultant in equine reproduction and stud medicine at Equine Reproductive Services, said: “This has been an important study to be involved in as the cause of pregnancy failure in any given mare is often without diagnosis and we have to make assumptions in order to best treat and manage our patients after pregnancy failure.”

The findings of the research were reported in the journal Scientific Reports.

Shilton, C.A., Kahler, A., Davis, B.W. et al. Whole genome analysis reveals aneuploidies in early pregnancy loss in the horse. Sci Rep 10, 13314 (2020). https://doi.org/10.1038/s41598-020-69967-z

The study, published under a Creative Commons License, can be read here

Earlier Horsetalk report

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