Uterine Inflammatory Response: A Review

One of the biggest challenges in successful equine reproduction is uterine inflammation. Increased inflammation can destroy spermatozoa before they have a chance to fertilize the egg, and it can create a hostile environment that kills an embryo. Temporary inflammation is a natural result of placing semen in the mare, especially frozen semen, but the cause is not fully understood. Nor is it clear whether bacterial contamination plays a significant role. To better understand this phenomenon and its implications for frozen semen use, researchers at the University of Saskatchewan's Western College of Veterinary Medicine in Canada studied uterine inflammation caused by frozen spermatozoa, bacteria, or both.

Veterinary student Barbara Hunter presented the results of the study at the 52nd annual American Association of Equine Practitioners convention, held Dec. 2-6, 2006, in San Antonio, Texas. "The mild or acute uterine inflammatory response that follows breeding is normal and necessary to clear the uterus of debris, bacteria, and excess spermatozoa," she explained. "Uterine inflammation should be completely resolved within 48 hours of breeding. Persistent acute post-breeding uterine inflammation is a leading cause of subfertility in mares, and it has been reported to cause lower pregnancy rates in mares bred with frozen semen."

Bacterial contamination introduced during insemination might contribute as well. "Sperm and bacteria are often found together in the uterus--is there an additive or synergistic relationship between bacteria and sperm that increases inflammation?" she asked the audience.

To answer this question, researchers placed 13 young, reproductively healthy mares in three treatment groups for study. They induced ovulation, then they placed either frozen semen, cultures of Streptococcus equi zooepidemicus bacteria (the most common bacterium isolated from mares with endometritis), or both in their uteri. They evaluated a total of 20 mare cycles with the treatments, monitored the mares with ultrasound, and collected uterine fluid samples at 72 hours to assess bacteria levels, inflammation, percentage of neutrophils (a type of inflammatory cell), and total number of neutrophils.

Hunter reported, perhaps surprisingly, there were no significant differences in percentage or number of inflammatory cells between any of the groups. There were significantly more bacteria recovered from mares receiving bacteria, and the group receiving only bacteria had a significant accumulation of fluid (1 mm) compared to the other two groups.

"It is possible that the presence of spermatozoa in the frozen semen and frozen semen/bacteria groups increased uterine clearance compared with the bacteria group through the stimulation of prostaglandin (hormone that arises from uterine inflammation) secretion, cytokine production, or another mechanism modulating uterine tissue like contractility, fluid secretion, or drainage," Hunter commented.

It's also possible that synergistic/additive inflammatory effects of bacteria might occur earlier than 72 hours post-insemination and have been cleared, or these effects might only occur in susceptible mares that have a harder time clearing their uteri of semen, bacteria, and inflammatory products, she noted.

In short, "No synergistic, additive effects of bacteria and frozen semen together were found in this study," Hunter concluded. "Further research is needed to determine if there is an acute difference in 24 hours (although if this does not affect pregnancy rates, it might not be clinically relevant), and if there is a difference between normal and susceptible mares."

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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