Most equine anthelmintics are safe for use throughout pregnancy, but precautions and contraindications on package inserts should be heeded. Ivermectin and oxibendazole have been used safely in pregnant mares. Cambendazole and organophosphates are contraindicated during the first trimester of pregnancy. In general, anthelmintics should not be administered to mares during the first 60 days of gestation (ie, organogenesis) or during the last few weeks before foaling. Otherwise, mares should be dewormed every 6–8 wk, if fecal egg counts are elevated. A low dose of pyrantel tartrate (1.2 mg/lb) can be administered daily throughout gestation. Anthelmintics effective against bots should be used in the fall.

Mares can be dewormed 0–2 days after parturition to reduce “foal heat” diarrhea in foals, which is actually caused by small strongyles that are ingested when foals are exposed to the mare's feces and/or a contaminated environment. Foals should be administered anthelmintics at 6–8 wk of age on the same day as the dam and again at weaning if fecal egg counts are elevated.

Last full review/revision July 2011 by Patricia L. Sertich, MS, VMD, DACT