Study identifies PBIE risk factors in warmblood mares: full analysis

A newly published study in Equine Veterinary Journal reports that three factors stood out as predictors of persistent breeding-induced endometritis in warmblood mares: advancing age, embryo donor status, and insemination with chilled-transported semen. The retrospective analysis included 769 mares over 1,745 estrous cycles, giving clinicians a relatively large field dataset on a condition that remains a routine challenge in equine reproduction practice. (research-portal.uu.nl)

PBIE is the prolonged inflammatory response that follows breeding in mares that fail to clear semen, debris, bacteria, and inflammatory products within the expected window. Reviews of the condition describe it as a common cause of subfertility, with fluid and inflammation normally resolving within about 48 hours in mares that are not susceptible. Older mares and mares with impaired uterine clearance have long been considered higher risk, and embryo donor mares may face added exposure because they are bred and flushed repeatedly over multiple seasons. (irishvetjournal.biomedcentral.com)

In the new study, the authors divided mares into age groups of 6 years or younger, 7 to 13 years, and 14 years or older, and compared embryo donors with broodmares. PBIE was defined as detection of more than 2 cm of intrauterine fluid on the day after insemination. On multivariable analysis, mares aged 14 years or older had higher odds of PBIE than the youngest mares, embryo donors had higher odds than broodmares, and chilled semen was associated with higher PBIE odds than frozen-thawed semen. At the same time, pregnancy or embryo recovery was lower overall with frozen-thawed semen than with chilled semen, highlighting a clinically relevant tradeoff between inflammation risk and fertility outcomes by semen type. (research-portal.uu.nl)

That semen finding is notable because older literature has often framed frozen semen as a trigger for more post-breeding inflammation, or at least as a management concern in susceptible mares. The new paper doesn’t erase that body of work, but it does suggest that in this warmblood practice population, chilled semen was linked to more PBIE events while frozen-thawed semen still produced lower pregnancy or embryo recovery rates. The difference may reflect case mix, breeding management, timing, semen handling, or how aggressively clinicians monitored and treated mares after insemination. That interpretation is an inference, but it fits with the paper’s retrospective design and the authors’ note that the data came from a multi-veterinarian clinic. (research-portal.uu.nl)

The authors also reported that PBIE itself did not significantly affect pregnancy or embryo recovery in the dataset, despite its association with known risk factors. Their conclusion was that simple treatments usually minimized the fertility impact. That aligns with broader clinical guidance in the literature, which emphasizes early diagnosis and prompt management in high-risk mares. Published work has also explored adjunctive approaches, including platelet-rich plasma and other immunomodulatory strategies, though those remain more specialized than the day-to-day tools used in most breeding sheds. (research-portal.uu.nl)

Why it matters: For veterinary professionals, the practical takeaway is less about a single intervention and more about triage. If a mare is older, serving as an embryo donor, or entering a chilled-semen breeding cycle, the threshold for closer surveillance may need to be lower. This study supports building those variables into pre-breeding risk assessment, client counseling, and post-breeding protocols. It may also help veterinarians explain to pet parents and breeding clients why a mare can be at elevated inflammation risk without necessarily losing the cycle, provided the practice identifies and treats fluid accumulation quickly. (research-portal.uu.nl)

The study’s limitations matter, too. It was retrospective, conducted in a single multi-veterinarian clinic population, and had some missing mare-status and pregnancy data. That means the findings are best read as strong practice-based signals rather than final rules for every breed, semen program, or reproductive center. Still, the sample size and real-world setting make the paper useful for clinicians looking to refine protocols rather than reinvent them. (research-portal.uu.nl)

What to watch: The next question is whether prospective studies can confirm which management changes make the biggest difference in high-risk mares, and whether semen-specific protocols for donor mares and older mares can reduce PBIE without sacrificing pregnancy rates. (research-portal.uu.nl)