Ponazuril and EPM: An Ounce of Prevention

Researchers at the University of Florida studying equine protozoal myeloencephalitis (EPM) recently found that administering ponazuril once every seven days might prevent EPM caused by Sarcocystis neurona.

Equine protozoal myeloencephalitis is an important neurological disease of horses. It is typically caused by the parasite S. neurona, a protozoan parasite that penetrates the central nervous system and causes diverse neurologic signs, including ataxia and muscle atrophy.

Ponazuril, known commercially as Marquis, is one of several antiprotozoal medications available to treat EPM.

"While only a very small number of adult horses develop EPM, it is estimated that almost 50% of horses have antibodies to S. neurona (suggesting that they have been exposed to the parasite)," said Robert MacKay, BVSc, PhD, professor and chief of the Large Animal Section at the University of Florida's College of Veterinary Medicine.

In this study, 15 horses were divided into three groups and administered S. neurona sporocytes orally. Horses in one group received ponazuril once every seven days. Another group received the drug once every 14 days, and the third group was not treated. An additional five horses were included as controls and did not receive sporocytes or treatment.

Researchers first identified antibodies (IgG) to S. neurona in the horses' blood 56 days after exposure. IgG antibodies were identified in the cerebrospinal fluid (CSF) in only two out of five (40%) of the horses treated every seven days with ponazuril after administration of S. neurona. In contrast, all of the horses that were exposed to the parasite but received no treatment developed antibodies within 84 days of exposure.

Ponazuril might prevent or reduce equine central nervous system invasion of S. neurona
"Finding IgG antibodies against S. neurona in the CSF of horses is an indication that a transient infection in the central nervous system (CNS) has occurred," explained MacKay. "Since less than half of the horses treated with ponazuril had antibodies in their CSF, our results suggest that the once-weekly administration of ponazuril may prevent infection following exposure to S. neurona."

In summary, ponazuril might prevent or reduce equine central nervous system invasion of S. neurona and, therefore, decrease the chance of neurological disease. Research is ongoing to further evaluate the clinical use of ponazuril in horses to prevent EPM.

This study was funded by Bayer Animal Health, manufacturers of ponazuril.

"Effect of intermittent oral administration of ponazuril on experimental Sarcocystis neurona infection of horses," was published in March 2008 in the American Journal of Veterinary Research.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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