Prostaglandins: Reducing Drug Costs and Side Effects

A major use of prostaglandin is to manipulate mares' estrous cycles. A veterinarian's most commonly used prostaglandin treatments are usually Lutalyse (dinoprost tromethamine or PGF2) and the prostaglandin analogue (a similar compound) Estrumate (cloprostenol or CLO). Some side effects observed when mares are treated with labeled doses of these medications include sweating and colic, and as a result owner distaste for using the drugs is growing.

Gary J. Nie, DVM, PhD, consultant for World Wide Veterinary Consultants, LLC in Rochester, Minn., conducted a study while at Auburn University where he compared "micro" doses of both products with standard doses for their ability to affect luteolysis and short-cycle mares.  He also evaluated the ability of microdoses to reduce side effects observed with standard doses.

Nie found that a microdose of CLO is equally as effective at short-cycling mares as standard doses of PGF2 or CLO, yet it virtually eliminates unwanted side effects and reduces drug costs by 1,000-2,000%.  He does not recommend a single microdose of PGF2, however, because of a delayed progesterone drop observed in the treated mares.

Prostaglandin's function in this case is to lyse the progesterone-producing corpus luteum left after ovulation, and return the mare to estrus. Thus, a progesterone drop is indicative of prostaglandin efficacy.

In the study, Nie treated 17 mares (in 57 cycles) intramuscularly with either the standard doses or microdoses of CLO (Nie administered only 5-10% of the standard dose of CLO as the microdose--25 micrograms rather than 250 to 500 micrograms) of PGF2 (0.5 mg instead of 10 mg), or the control (sterile water) on Day 6 after ovulation. Their progesterone levels and ovarian activity were monitored daily.

Nie said, "Their progesterone concentrations were quite interesting. We sampled out to 96 hours after treatment at 24-hour intervals. Our control mares had no change after the sterile water injections. All the prostaglandin-treated mares had significantly lower progesterone levels than the controls.

"All (of the mares' progesterone levels) dropped quickly," he said. "But the microdose of PGF2 didn't drop the progesterone concentrations like the other treatments did." He added that the progesterone levels were "higher (statistically significant) than the other prostaglandin treatments. It's like we injured the corpus luteum, but we didn't completely lyse (cause its regression) it."

When measuring systemic response to the prostaglandin treatments, he found that the microdoses caused significantly fewer and less severe side effects than either standard dose, but were very similar to the response seen in the control mares. "Both standard doses of prostaglandin did cause significant elevations in heart rate," he said. Additionally, rectal temperature of the mares treated with standard doses dropped rather quickly until 90 minutes post-administration, and had not recovered to the level of control mares by 240 minutes. Nie said the drop in rectal temperature  observed probably reflected heat loss through evaporation of sweat.

Nie said he has used the same microdose of CLO in a 900-pound Arabian mare as he has used in a 1,600-pound Warmblood mare with much success in both.

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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