EPM Treatment Research

More than 50 years ago, horses at The Jockey Club in San Paulo, Brazil, were suffering from a neurologic disease that caused muscle wasting. In 1964, that same disease was seen in North America. Robert MacKay, BVSc, PhD, Dipl. ACVIM, of the University of Florida's College of Veterinary Medicine, gave this timeline as a preamble to his discussion of the knowledge, treatment, and prevention of equine protozoal myeloencephalitis (EPM)--the leading cause of neurologic problems in horses.

MacKay, who has conducted research into the life cycle and potential treatments for EPM for years, spoke to an audience gathered in Kansas City, Mo., for the launch of Bayer Animal Health's Marquis, the first FDA-approved treatment for EPM. He said that 95% of horses with EPM present with signs that could be confused with lameness.

From 1974 until 1995, there was very little research done on EPM. In 1995, University of Kentucky researcher David Granstrom, DVM, Dipl. ACVIM, discovered that the opossum was the definitive host. Since the causative parasite, Sarcocystis neurona, has to have two hosts to complete its life cycle, researchers have since been focused on finding the intermediate host(s).

The University of Ohio found that the cat can be an intermediate host in the laboratory. Then the University of Florida discovered that the nine-banded armadillo is a natural intermediate host, and it is proposed that one or more species of skunk is a natural intermediate host.

"There are three opossum species that fulfill the role as primary hosts in North and South America," said MacKay. "A secondary host in Florida is the armadillo. About 70% of roadkill armadillos are infested with Sarcocystis neurona. In Florida, lots of cats have Sarcocystis species, but not Sarcocystis neurona. There are seven or eight species of skunks that could be involved."

The Testing of Marquis

Marquis is a 15% ponazuril paste that kills several stages of S. neurona. 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.

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 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 administration.

The field trials were conducted at six referral clinics under the guidance of seven veterinarians. 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. Consult with your veterinarian as soon as you suspect problems that could be EPM.

Based on reports from several veterinarinians across the country, cost of the 28-day treatment will range from $800-$1,200 per horse depending on veterinary services involved.

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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