Equine protozoal myeloencephalitis (EPM) is an extremely debilitating disease that is common on horse farms and facilities in the Americas, although only a small percentage of horses that are exposed to the causative organism will develop clinical disease.

In the 1960’s and 1970s, a syndrome referred to as “segmental myelitis” was described based on studies involving groups of horses in the eastern United States. Protozoa were first reported in lesions from horses by researchers in 1974. The syndrome was well-described, but the parasite involved was misidentified as Toxoplasma gondii; later, it became clear that the protozoan causing EPM was in fact Sarcocystis neurona.¹ One researcher called the syndrome “equine protozoan encephalomyelitis,”² which was later standardized to equine protozoal myeloencephalitis, or EPM. 

The S. neurona organism infects horses when they ingest the organism in contaminated feed or water. The definitive host of this organism is the opossum, which passes the organism in its feces.² Other hosts of S. neurona are believed to include armadillos, skunks and domestic cats. 

Once ingested, S. neurona sporocysts migrate from the intestinal tract into the bloodstream and cross the blood/brain barrier. There, they begin to attack the horse's central nervous system. The onset of the disease may be slow or sudden and, if left undiagnosed and untreated, EPM can cause lasting neurological damage.

The clinical signs of EPM can vary, and are usually asymmetrical, appearing on one side of the horse or the other. Symptoms can depend on the severity and location of the lesions that develop in the brain, brain stem or spinal cord.²These may include:

• Incoordination
• Weakness
• Muscle atrophy
• Drooping eyes, ears or lips
• Difficulty swallowing
• Seizures or collapse
• Abnormal sweating
• Loss of feeling along the face, neck or body
• Head tilt with poor balance
• Splay-footed stance, leaning

Diagnosis of EPM can be difficult to make, as there is no specific assay for it and because clinical signs of EPM can mimic other neurological diseases. Veterinarians typically conduct a thorough physical examination of the horse and may conduct blood and cerebrospinal fluid (CSF) analysis.

Coccidiosis is caused by coccidia, which are single-celled obligate intracellular protozoan parasites. The genera Eimeriaand Isospora contain many species that infect a variety of birds, mammals and reptiles, but almost all are species host-specific. Infection is via ingestion of infective oocysts from a contaminated environment. When clinical signs occur, they range from diarrhea and decreased growth in mild cases to dysentery and dehydration in more serious cases.¹

Where to buy Toltrazuril

Toltrazuril is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. 

Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Merck Veterinary Manual.

²Reed, S M et al. Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment, and Prevention. Journal of veterinary internal medicine vol. 30,2 (2016): 491-502. doi:10.1111/jvim.13834.

³Dubey, J.P. et. al. (2001). A review of Sarcocystis neurona and equine protozoal myeloencephalitis (EPM). Veterinary parasitology. 95. 89-131. 10.1016/S0304-4017(00)00384-8.