Sarcocystis neurona is a trickster. The protozoan parasite, known for causing the neurologic disease equine protozoal myeloencephalitis (EPM), has a complex life cycle that’s notoriously difficult to replicate in a research setting, making EPM studies expensive and time-consuming to perform. But scientists have taken a closer look, literally, and determined that in some cases investigators had successfully replicated S. neurona’s life cycle starting with parasites from an infected horse–something they thought impossible.

“You’ve got to observe carefully, and that’s the trick here,” said Antoinette Marsh, MS, PhD, who studies animal models of disease transmission in her role as associate professor and service head of Veterinary Medical Center Diagnostic Parasitology at The Ohio State University. She and her colleagues worked to create a system whereby they could manipulate the S. neurona life cycle through both in vivo (in the living animal) and in vitro (in the laboratory) means, to get specific stages of the parasite and cryopreserve (freeze) the parasites to be used for later research studies. She presented their results at the Second EPM Society Workshop, held Oct. 25-27, 2017, in Tahoe City, California.

A little background on S. neurona: The opossum is S. neurona’s definitive host, meaning this scavenger is needed to perpetuate the single-celled parasite (S. neurona reproduces in the opossum). Opossums become infected by eating S. neurona-infected muscle of dead intermediate hosts, which essentially act as vectors for the parasite and range from armadillos to raccoons. Horses are not necessary to continue S. neurona’s life cycle. The horse is thought to be an aberrant host, meaning it inadvertently ingests opossum feces—or feed contaminated with opossum feces—containing the parasite sporocysts (eggs). This stage hatches and passes through the horse’s gastrointestinal wall, reaching the circulatory and lymphatic systems and getting into the horse’s cells.

“This is where things go wrong for certain horses,” Marsh told The Horse. “In horses with EPM, the parasite does not receive the correct biological signals to transition from this invasive, rapidly replicating disease-causing parasite to the dormant, non-disease-causing stage. In horses with EPM, the parasite just keeps rapidly dividing, breaking out and invading new cells