Does inbreeding contribute to pregnancy loss in Thoroughbred horses?
Authors: Lawson Jessica M, Shilton Charlotte A, Lindsay-McGee Victoria, Psifidi Androniki, Wathes D Claire, Raudsepp Terje, de Mestre Amanda M
Journal: Equine veterinary journal
Summary
# Editorial Summary: Inbreeding and Pregnancy Loss in Thoroughbreds Genomic analysis of 189 Thoroughbred pregnancy loss cases and controls reveals that inbreeding contributes significantly to mid and late pregnancy loss (MLPL) but not early pregnancy loss (EPL), with mares carrying pregnancies lost in the second and third trimesters showing substantially higher inbreeding coefficients than controls (median FROH of 0.26 versus 0.20, p<0.05). Using runs of homozygosity (ROH) analysis on SNP genotyping data from fetal and placental samples, researchers found that MLPL cases exhibited a higher median number of ROH segments (188 versus 148 in controls), greater total ROH length (590.8 Mb versus 418.5 Mb), and critically, a significantly elevated proportion of long ROH segments greater than 10 Mb—these longer segments indicate recent inbreeding events rather than ancestral ones. The study identified no unique genetic variants exclusive to pregnancy loss cases, suggesting instead that accumulated homozygosity across the genome may expose lethal recessive alleles affecting placental and fetal viability during later gestation rather than early embryonic development. For breeding programmes and practitioners advising on mating strategies, these findings underline the importance of calculating inbreeding coefficients before breeding decisions, particularly when evaluating mares or stallions with recent inbreeding in their pedigrees, as this genomic approach offers a more nuanced assessment of genetic load than traditional pedigree-based coefficients alone.
Read the full abstract on PubMed
Practical Takeaways
- •Breeding decisions should carefully evaluate pedigree and inbreeding coefficients, as elevated inbreeding is associated with mid and late pregnancy loss but not early loss in Thoroughbreds
- •Consider genetic counselling or genomic testing when planning matings involving closely related horses, particularly when previous pregnancy losses have occurred
- •Early pregnancy loss cannot be attributed to inbreeding levels alone; other management, environmental, or infectious factors should be investigated
Key Findings
- •Mares with mid and late term pregnancy loss had significantly higher inbreeding coefficients (FROH 0.26) compared to controls (p < 0.05)
- •Early pregnancy loss cases showed no significant difference in inbreeding levels compared to controls
- •Mid and late term pregnancy loss cases had significantly higher proportion of long ROH (>10 Mb) at 2.5% versus 1.7% in controls (p = 0.001)
- •No unique runs of homozygosity were identified in pregnancy loss populations, suggesting inbreeding increases risk of homozygosity for existing lethal alleles rather than introducing novel variants