Virginia Maryland Group Works at Solving the EPM Enigma

Researchers have puzzled over how Sarcocystis neurona, the single-celled protozoan parasite notorious for causing equine protozoal myeloencephalitis (EPM), travels from the intestine, through the blood-brain barrier, and into the centra

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Researchers have puzzled over how Sarcocystis neurona, the single-celled protozoan parasite notorious for causing equine protozoal myeloencephalitis (EPM), travels from the intestine, through the blood-brain barrier, and into the central nervous system to cause the neurological signs that we see in horses with EPM. A team at the Virginia Maryland Regional College of Veterinary Medicine (VMRCVM) has been studying the mechanisms by which S. neurona causes disease, trying to find ways to protect horses from EPM, and developing additional tests for diagnosing EPM.


The S. neurona life cycle involves the definitive host (an opossum) that feeds on the muscles of dead intermediate hosts (such as the striped skunk, raccoon, nine-banded armadillo, and cat). The protozoan parasite advances to a specific stage of its life cycle (sarcocyst) in the intermediate host’s muscle before an omnivore such as the opossum eats the muscle, which continues the parasite’s life cycle. The horse contracts EPM by inadvertently consuming infected opossum droppings while grazing or while eating contaminated feed or hay.


It’s a complicated journey from the digestive tract to the central nervous system that scientists have long tried to understand. “Others have demonstrated that S. neurona can infect leukocytes (white blood cells) and endothelial cells (those in the lining of blood vessels),” said Sharon Witonsky, DVM, PhD, Dipl. ACVIM, associate professor in the large animal clinical sciences at Virginia Tech in Blacksburg, Va. “It appears plausible that S. neurona could infect leukocytes in the body. These infected leukocytes could then cross the blood-brain barrier, where S. neurona may be released through some unknown mechanism. Once there, S. neurona could cause encephalitis (inflammation of the brain and spinal cord, as seen in EPM).”


The Horse reported in October about Witonsky’s team’s discovery of S. neurona in specific types of white blood cells, and this could explain the parasite’s ability to cross the blood-brain barrier. Witonsky explained that Siobhan Ellison, DVM, PhD, of Pathogenes Inc., had already been using a leukocyte infection model several years prior to the current study and has published work on the model. “So in our study we wanted to more specifically demonstrate that the cells could be infected and begin to determine the cell types,” she said. “We wanted to determine in a more defined (in vitro, outside the living body in the laboratory) system, whether S. neurona could infect equine peripheral blood leukocytes, and if possible, what cells S. neurona preferentially infects.We (confirmed Ellison’s findings) that S. neurona can infect leukocytes, and based on the study, it appears that S. neurona preferentially infects monocytes.” Monocytes are a particular type of  white blood cell found in the circulation that convert into active macrophages (specialized white blood cells that fight infection) when they enter the tissue

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Stephanie L. Church, Editorial Director, grew up riding and caring for her family’s horses in Central Virginia and received a B.A. in journalism and equestrian studies from Averett University. She joined The Horse in 1999 and has led the editorial team since 2010. A 4-H and Pony Club graduate, she enjoys dressage, eventing, and trail riding with her former graded-stakes-winning Thoroughbred gelding, It Happened Again (“Happy”). Stephanie and Happy are based in Lexington, Kentucky.

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