Scientists have shown the protozoan parasite that causes equine protozoal myeloencephalitis (EPM) might enter the horse’s central nervous system hidden in leukocytes (white blood cells) that cross the blood-brain barrier.

“Horses are considered accidental hosts for Sarcocystis neurona (the parasite), and they often develop severe neurological disease when infected with this parasite,” wrote the researchers who performed the study at the Virginia-Maryland Regional College of Veterinary Medicine in Blacksburg, Va. “Schizont stages (the stage of the parasite that reproduces asexually by splitting into daughter cells) develop in the central nervous system (CNS) and cause the neurological lesions associated with equine protozoal myeloencephalitis,” and the investigators wanted to determine if S. neurona merozoites–cells that arise from the asexual division of the protozoa–had the ability of penetrating and developing in equine peripheral leukocytes.

They found that the merozoites penetrated the leukocytes quickly, with up to three merozoites present in an infected cell. In the cell they did not develop to schizont stage. “We postulate that S. neurona merozoites may cross the blood brain barrier hidden inside leukocytes,” the scientists observed. “Once inside the CNS, these merozoites can egress and invade additional cells and cause encephalitis.”

The study appeared in the June 15 issue of Veterinary Parasitology