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.

She continued, "Subsequent experiments will determine what subsets (i.e. specific T-lymphocyte subsets (CD4, CD8), B-lymphocytes, and monocytes and/or neutrophils) that S. neurona preferentially infects."

The team is using both equine and rodent models to determine how the horse's body can be protected from S. neurona infection and the best ways to determine infection. "We have determined that a cell-mediated immune response, composed of CD8 cells (specialized white blood cells with CD8 receptors that recognize antigens on the surface of infected cells and bind to the infected cells and kill them) is critical to protection in mice," she said. "We expect there to be similar findings in horses, but this work has not yet been conducted."

They have also determined that "both experimentally and naturally infected horses develop suppressed in vitro proliferation responses to a particular mitogen (a substance that causes cell division)," she said, meaning that the agent suppressed the proliferation of some of the horses' cells under very specific conditions. It is not clear how this correlates to the horse's overall immune response as these are results have been done with cells that have been removed from the horse. One manuscript has been published on this finding, and another is under review. "We are determining the mechanisms of this suppression. Individuals of the group have also been involved in the evaluation of therapeutic agents either in vitro or in vivo (in the live animal)."

Witonsky stresses that the team collaborates with researchers beyond the institutions that make up the VMRCVM. "We have an excellent team of researchers at Virginia Tech, combined with colleagues at other institutions," she said. "The Virginia Maryland Regional College of Veterinary Medicine group consists of myself (immunology, large animal internal medicine), David Lindsay (PhD, parasitology), Robert Gogal Jr. (DVM, immunology), Robert Duncan Jr. (BS, DVM, PhD, Dipl. ACVP, pathology), Yasuhiro Suzuki (PhD, DMSc,  immunology) and Virginia Buechner-Maxwell (DVM, MS, Dipl. ACVIM, large animal internal medicine). Additionally, we have strong collaborations with Ellison and Frank Andrews (internal medicine, University of Tennessee). We have just initiated a collaboration with Dr. Dan Howe (PhD) at Gluck (the University of Kentucky's Gluck Equine Research Center).