Sarcocystis fayeri produces a toxin that may cause weakness in horses
Sarcocystis spp. are prevalent in the muscles of horses in various regions of the United States and Europe, as many as 30% of the population are infected. There are four species that infect the muscles of horses, S. asinus, S. bertrami, S. equicanis, and S. fayeri. All of these species use canids as a definitive host and trans-placental infection can occur. The pathogenicity of the four species is variable.
Sarcocystosis can be mild or severe. Signs include fever, apathy, anorexia, myositis, muscle weakness, autoimmune disease, or hair loss. In chronic disease signs include weight loss, difficulty in chewing and swallowing, depression, ataxia, emaciation, and generalized weakness. Generally, the mild histological lesions of myositis don’t correlate with the profound muscular weakness seen in afflicted horses. The role of toxins associated with bradyzoites from sarcocysts is not defined, but may be associated with lost strength.
The pathway to chronic inflammation in horses is unknown. Evidently there is a prepatent period of greater than 79 days in naive horses because three S. neurona seronegative ponies that were fed S. fayeri oocysts remained clinically normal at day 79 when the experiment was terminated. Sarcocystis fayeri sarcocysts were found in striated muscles of all inoculated ponies. Perhaps degenerating cysts release toxin-producing bradyzoites. This pathway may explain profound muscle weakness in debilitated or chronically infected horses. The toxin presumably sensitizes CD4+ T cells. Upon re-exposure to S. fayeri in the gut inflammation may ensue. Inflammation may be measured by serum C-reactive protein (CRP) levels. Serum antibodies against S. fayeri toxin are detectable in horses.
The S. fayeri toxin, called sarcocystine, was first associated with sarcocysts in 1899. There is occasional mild interest in Sarcocystis toxins. In 1997 sarcocystine drew interest, there was an association between human sarcocystosis and multiple sclerosis. Sarcocysts, identified as S. gigantea and S. cruzi, produce a small, heat labile protein-toxin that causes increased respiration, depression, ascending paralysis, even death, in rats and rabbits. Last year severe food poisoning was reported in people eating raw horse meat.
Food poisoning was documented in up to 78% of people consuming contaminated raw horsemeat. Sarcocystis fayeri caused enteritis in people that ingested the muscle cysts. Sarcocysts in the meat were identified by morphology and completing the parasite lifecycle in dogs. The enterotoxin was isolated and characterized. Toxicity was confirmed in rabbits receiving 15 µg of S. fayeri sarcocystine. Molecular analysis indicated the toxin is 15kDa and has homology with virulence and cell-invasion factors of Toxoplasma gondii.
Toxoplasma-induced inflammatory bowel disease is studied in a pathogen-driven mouse model. Acute, lethal ileitis is induced in susceptible mice by single antigen of T. gondii. The susceptible mouse required antigen specific CD4+ effector cells and these cells could transfer susceptibility to resistant mice. The intestinal pathology induced by T. gondii required the induction of proinflammatory cytokines that included INF-γ and TNF-α. Neutralization of INF-γ or CD4+ T cells prevented necrosis of the ilea and prevented acute mortality. This model demonstrated that disease requires the production of chemokines that elicit a proinflammatory responses from CD4+ cells by transferring disease susceptibility to mice using these cells.
After S. fayeri infection, serum antibodies against S. neurona are detected by IFAT.* Sarcocystis fayeri infections may be misdiagnosed as being S. neurona infections using IFAT tests. Treatment for S. fayeri cysts in muscles may differ from treatments for S. neurona merozoites. It is important to determine the effects EPM treatments on S. fayeri cysts. Toxin-specific assays may associate degenerating S. fayeri sarcocysts to muscle weakness in some horses. We developed a serum ELISA to detect S. fayeri toxin-specific antibodies in equine serum. Unlike IFAT, tests the S. fayeri toxin antibodies do not cross-react on SAG 1, 5, or 6 ELISA tests.
*Saville, Dubey, Oglesbee, Sofaly, Marsh, Elitsur, Vianna, Lindsay, Reed 2004