Q: I know there are many treatments available for equine protozoal myeloencephalitis (EPM), but are there any that are safe for use in pregnant mares? There seems to be little information on diclazuril (Protazil) or toltrazuril sulfone (ponazuril, or Marquis) in my area. Would sulfadiazine/pyrimethamine (Rebalance) be a safer option over either of these? What would you recommend for use during pregnancy?

Jim Lamb, South Carolina

A: Due to strict Food and Drug Administration (FDA) regulations, it is very difficult to get a drug approved for use in pregnant mares. Of the existing FDA-approved EPM products, none are labeled for this use. That being said, Protazil and Marquis are very safe and have been used in many pregnant mares without side effects. The other option, a sulfadiazine/pyrimethamine combination, should never be administered to pregnant mares. This is the only EPM treatment for which there is published evidence (Toribio RE, 1998) of its toxicity to foals. If these are the only medications available to you for EPM treatment, please use them with great caution and for as short a time period as possible. I have observed abnormalities involving the skin as well as the blood system in foals whose dams were administered this treatment.

Interestingly, the only drug with research studies proving its safety for use in pregnant mares is nitazoxanide (Navigator), which was discontinued in 2009 due to a small percentage of treated horses developing life-threatening diarrhea.

So I would recommend treating your broodmare with either Protazil or Marquis, but never sulfadiazine/pyrimethamine.


About the Author

Stephen Reed, DVM, DACVIM

Stephen M. Reed, DVM, Dipl. ACVIM, earned his veterinary degree at The Ohio State University before completing a residency at Michigan State University. He started his academic teaching career at Washington State University from 1979-1983. He then returned to Ohio State, where he spent 26 years as professor and mentor in the equine medicine department. Reed is an internal medicine specialist and noted author and editor of numerous scientific articles and textbooks. He’s spoken at many state, national, and international meetings. His primary research interests include equine neurologic diseases. He’s currently an internist and shareholder at Rood & Riddle Equine Hospital, in Lexington, Kentucky; an emeritus professor at The Ohio State University; and an adjunct professor at the University of Kentucky.

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