The first FDA-approved treatment for equine protozoal myeloencephalitis (EPM), Marquis, was launched Aug. 23 at Bayer's headquarters in Kansas City, Mo. Robert MacKay, BVSc, PhD, Dipl. ACVIM, of the University of Florida College of Veterinary Medicine, discussed the knowledge, treatment, and prevention of EPM, which is the leading cause of neurologic problems in horses.

(For more on EPM, see

The Testing of Marquis

The new equine protozoal myeloencephalitis (EPM) treatment made by Bayer is a 15% ponazuril paste that kills several stages of Sarcocystis neurona, the causative parasite of the neurological disease. Ponazuril was proven to cross the blood-brain barrier and reach the horse’s central nervous system (spinal cord and brain) where the parasite resides. To complete its life cycle, the parasite must make its intermediate host neurologically unhealthy, thus causing it to become potential food for the opossum, where it completes its life cycle.

The horse is a dead-end host, meaning that the parasite can’t finish its life cycle in the horse. However, S. neurona can replicate (multiply) in the horse, thus increasing the number of parasites that cause damage. Marquis targets the energy metabolism of the parasite and disrupts its ability to reproduce, thus killing the multiplying stages of the parasite.

Marquis was shown to have a wide margin of safety in clinical trials. Horses given twice, six times, or 10 times the label dose had few adverse effects (such as loose feces, lack of appetite, slight decrease in body weight, and mild colic in one horse). The trial included 102 horses given a dose of 5 mg/kg of body weight orally once a day for 28 days. In this trial, less than 2% developed hives. (The use of this product in horses for breeding purposes, during pregnancy, and in lactating mares has not been evaluated.)

Marquis can be stored at room temperature, and Bayer demonstrated that there is no adverse effect on the stability of Marquis at temperatures as high as 104°F or as low as freezing. There were no complaints from owners regarding ease of treatment.

The field trials were conducted at six referral clinics under the guidance of seven veterinarians. Entry criteria was extremely demanding, and records were kept by the attending veterinarian and the owner. Horses had to improve at least one grade and maintain that improvement through 118 days of treatment and follow-up and/or test Western blot negative for treatment to be considered successful. Also, all horses were videotaped before and after treatment—those videotapes were used without identifying horses to help determine whether the horses actually had improved.

Tom Kennedy, PhD, of Bayer noted that prognosis depends on severity of the disease and duration of infection. Marquis cannot repair existing central nervous system damage, so early treatment is extremely important.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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