Scientists Uncover New Way to Assess Ovulation

Veterinarians generally assess whether a mare has ovulated and is ready to breed by rectal palpation. However, new research suggests that a mare’s blood plasma sample can tell the veterinarian just as accurately that ovulation has occurred. The Japan Racing Association (JRA)-sponsored study was presented at the 43rd Annual Meeting of Investigation and Research Concerning Racehorses, held Dec. 3, 2001, at the University of Tokyo in Japan.

The study confirms the significance of follicular fluid that is released during ovulation. According to Yasuo Nambo, DVM, PhD, of the JRA’s Equine Research Institute, plasma hormone concentrations before and after ovulation were carefully analyzed. Researchers found that the concentration of inhibin, which inhibits synthesis of follicle-stimulating hormone, and inhibin pro-aC (which is a precursor of inhibin) more than doubled within a few hours after ovulation. They hypothesized that this increase was due to absorption of inhibin (from the follicular fluid released by the ruptured follicle) via the abdominal cavity, rather than an increase in the mare’s secretion of inhibin.

Nambo and his colleagues placed 50 mL (the typical amount a follicle releases) of equine follicular fluid (eFF) in the abdominal cavity. One hour after administration of the fluid, plasma levels of inhibin were measured. “Levels of inhibin were significantly higher in eFF-treated mares than in control mares,” explained Nambo. “The hormone profiles in eFF-treated mares were similar to those in mares with spontaneous or induced ovulations.

“Knowing the exact time of ovulation in both the donor mare and recipient mare is important in making embryo transfer or artificial insemination with frozen semen successful,” said Nambo.

“Plasma concentrations of inhibin pro-aC are determined by human ELISA testing, which requires that the sample incubate for at least three hours, according to our preliminary test,” he added. The researchers believe the test will evolve to yield faster results and thus become more practical for performing artificial insemination with frozen semen.

The only drawback to this method is the cost. “Unfortunately, a human ELISA kit is expensive. Some veterinarians say that diagnosis by rectal palpation is an easy and quick method,” said Nambo. “But we believe that measurement of circulating inhibin is more useful than frequent rectal palpation in making embryo transfer, because it is accurate, convenient, and easy to check a lot of mares by one assay.”

Nambo and his fellow researchers continue to research methods of endocrine diagnosis for equine reproductive disease. “From this study, we also realized that a relatively large amount of follicular fluid might pass into the peritoneal (abdominal) cavity during ovulation. So, we will continue to study the role of oviductal fimbriae (bordering fringe at the ends of the fallopian tubes nearest the ovaries) in capturing ova during ovulation.”

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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