A recent study completed by French and Brazilian veterinarians showed that administration of ivermectin/praziquantel paste, a common equine dewormer combination, is safe for pregnant mares and their foals. Dewormers of this type have not yet been labeled for safety in pregnant mares and their foals in the United States.

Patrick Mercier, DVM, of the medical department of Virbac Laboratories in Carros Cedex-France, was the principal author in the study that was published in the October issue of the American Journal of Veterinary Research. He said, “The target of this publication was to clearly demonstrate the total safety in sensitive target animals like pregnant mares and their offspring.” A safety study such as this is required in European countries for the registration of any new veterinary product, he added. The protocol design of the European safety study is similar to that required by the U.S. Food and Drug Administration. Similar U.S. studies are under way.

Twenty mares in the study received three times the therapeutic dosage of ivermectin and praziquantel (specifically, Equimax) at 14-day intervals until birth. Another 20 mares received a placebo. Veterinarians who weren’t aware of the treatment status of the horses assessed the general health of the pregnant mares monthly with blood tests, and they performed physical examinations on mares and foals after birth to identify any drug-related effects.

Toxicosis as a result of ivermectin overdose has been noted in past studies–horses given 60 times the recommended dose intramuscularly showed depression, ataxia, mydriasis (long-continued or excessive dilation of the pupil of the eye), lower lip droop, and a decreased respiratory rate. Another study using 10 times the recommended paste dose for two consecutive days reported transient vision impairment, signs of depression, ataxia, and dehydration. However, “At no time during our study were these types of adverse effects observed,” said the scientists in the current study