Defining Relapses Attributed to Equine Protozoal Myeloencephalitis

Disease caused by Sarcocystis spp are associated with a variety of clinical signs including abortion, hemorrhages, hair loss, muscular disease, and neurological dysfunction. In horses, clinical disease is associated with Sarcocystis fayeri and S neurona. The genesis of and clinical progression of disease caused by these protozoans in horses is not well defined but acute, chronic, and relapsing disease is recognized in horses undergoing treatment and recovery from sarcocystosis. In this study, we investigated the seroprevalence of species specific antibodies to S fayeri and S neurona in horses to determine their association with specific aspects of disease progression. We identified three disease presentations associated with chronic sarcocystosis that have been clinically attributed to equine protozoal myeloencephalitis (EPM) caused by S. neurona. Most relapsing remitting disease in this group of horses was due to S fayeri or autoimmune polyneuritis, not S neurona. We offer serological evidence that repeat environmental exposure to S neurona, rather than persistent but unapparent infections, causes disease; S fayeri toxin can be associated with persistent subclinical disease in clinically normal horses; and dysfunctional inflammatory immune responses stimulated by these organisms can result in the development of clinical signs without the presence of parasite stages in tissue.