Veterinarians might be able to prevent abortions in some pregnant mares that show premature mammary development, Dietrich H. Volkmann, BVSc, MMedVet (Gyn), Dipl. ACT, told equine veterinarians at the 2010 Western Veterinary Conference, held Feb. 14-18 in Las Vegas, Nev.

In addition to the mammary development, horse owners might also see waxing, which occurs when the balance of hormones changes to signal labor. Before the birth of a normal foal, maternal progestagens increase and then decline sharply, triggering labor.

If there is a problem with the pregnancy and the fetus is stressed, placental progestagen production initially increases and then often declines sharply immediately prior to abortion. Hormone therapy might "trick" the mare's body into keeping the fetus.

"By the time we see premature mammary development, estrogen has decreased and progestagen has increased," Volkmann said. "If I can make the mare 'believe' that these acute fluctuations in progestagen concentrations never happened, and she is carrying a viable foal, we can stop the cascade that would culminate in the abortion of the fetus."

Volkmann has had good success with this approach in mares that have conceived twins. Because the placentas of the two fetuses usually differ in size, the fetus with the smaller placenta often dies during the third trimester of gestation, when the smaller placenta cannot support the fetus.

The initial distress of the malnourished fetus (causing progestagen concentrations to rise), followed by its death (causing progestagen concentrations to fall sharply), triggers the abortion of both fetuses.

"The larger twin is almost always alive at the time that the smaller one aborts," Volkmann noted."If there is still a fetal heart beat, I think there is still a fetus that can be rescued."

Volkmann injects intramuscular progesterone in oil every 12 hours, or will administer oral altrenogest (Regumate) every 12 hours. As long as the udder remains large, but the fetus is still viable, he will continue to administer the same dose of hormones. If the udder becomes smaller, he will reduce the dose by 10-20% every other day.

Some mares only require a few weeks of treatment; others require therapy throughout the rest of the pregnancy.

"You give this treatment as soon as you see the inappropriate mammary development, the earlier the better," he said.

Even with treatment, however, he noted the mare might still abort the fetus or deliver a small or immature foal.

About the Author

Marie Rosenthal, MS

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