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, Dipl. ACVIM

Stephen Reed, DVM, Dipl. ACVIM, joined the Internal Medicine Service department at Rood & Riddle Equine Hospital in Lexington, Kentucky in Jan. 2007, after he retired from Ohio State University as an emeritus professor. His career has been predominantly dedicated to teaching with emphasis on Equine Neurologic Diseases.

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