Apicomplexan parasites are intracellular protozoa that are responsible for a great range of diseases in man and animals. The family Sarcocystidae contain the cyst-forming coccidia, cysts form in muscle tissues of the prey-host. Carnivores (prey) eat the infected muscle tissue and that completes the parasites lifecycle. It is generally thought that muscle cysts (sarcocysts) cause no pathology in the prey-host, except perhaps in debilitated animals. A compromised host may lack a robust immune response that can limit the infection. Exposure to protozoa, and the resulting immunity, builds resistance to infections. It is well known that overuse of antimicrobials and antiparasitic agents was an unwise strategy, it is no different in the fight against Sarcocystis. An unintended consequence of chemical prophylaxis is an animal with no natural immunity.
Sarcocystis fayeri produces sarcocysts in horse muscles and like most Sarcocystis infections, this finding is considered incidental. Early research dismissed S. fayeri as a factor in equine protozoal myeloencephalitis (EPM). Experimentally infecting ponies with S. fayeri and evaluating the immune responses convinced the researchers that infections by two species neurona and fayeri, were indistinguishable using the IFAT test. Important molecular tests identifying S. neurona resulted in 22,076 nucleotide sequences. In contrast, S. fayeri has 15 reported sequences-all are the small subunit ribosomal RNA gene. Variability in the small subunit ribosomal gene is useful to identify Sarcocystis species. Incorrectly, S. neurona and S. falcatula were reported as the same organism based on synonymous regions of this gene.
Our investigations reveal the need for a reassessment of the pathogenesis of S. neurona infections in horses and a need evaluating the role of the immune responses in equine disease. There are reasons that S. fayeri should get a more serious look.
Sarcocystis neurona causes muscle weakness in horses but the parasite isn’t thought to develop cysts in horses. There are four horse-related species that develop cysts in horses: S. asinus, S. bertrami, S. equicanis, and S. fayeri. Canids, including dogs, are the reported definitive hosts for these organisms.
Generally sarcocysts are not associated with inflammation in horse muscles (examined by histopathology), although in some debilitated horses, muscle degeneration is reported. Equine sarcocystosis, considered a mild disease, but can cause fever, apathy, anorexia, myositis, difficulty chewing, muscle weakness, autoimmune disease and sometimes hair loss. Surprisingly the profound muscle weakness exhibited clinically doesn’t correlate with the mild lesions observed by histopathology. And this leads some to hypothesize that there is a toxin associated with muscle infections.
The toxin idea isn’t new. There were reports of Sarcocystis-cyst toxins, called sarcocystine, in 1899. A toxin was isolated from cattle muscle cysts and characterized one hundred years later. Yet the possibility of muscle toxins causing disease in horses hasn’t been evaluated. Sarcocystine causes disease in people. A toxin found in raw horsemeat was associated with human food poisoning. The toxin was isolated from S. fayeri sarcocysts and toxic effects were evaluated in rabbits. The protein toxin, histopathological lesions, animal feeding experiments, rabbit enterotoxin assays, enzymatic digestion experiments, and heat/acid lability assays are similar between S. cruzi and S. fayeri-cyst toxins.
So far, initial research on the enterotoxin from S. fayeri sarcocysts is in humans. It would be interesting to explore a relationship between a S. fayeri sarcocystine and myositis in horses.
There is enough molecular information to investigate sarcocystine as a cause of muscle weakness in horses. The proteomics suggest the S. fayeri sarcocystine is homologous to proteins of Eimeria tenella and Toxoplasma gondii. Protein similarity, if high enough, would indicate conservation of the protein and a role in parasite survival. Protein similarity between organisms would also sink the protein as a good diagnostic to implicate an organism. The best it could be is a diagnostic for protozoal myositis.
It is possible that detecting the sarcocystine would benefit the treatment of sarcocystosis in horses.
Coccidiosis in horses is complicated. It is important to examine many factors before initiating practices that have unintended consequences. It took many animal infection studies to correct the false claim that S. neurona and S. falcatula were synonymous. False assumptions have plagued EPM research for 25 years. This has cost many horses their lives. Studies that examine the effect of equine coccidial infections and the immune response to infection should dominate the conversation. Initially clarifying the definition of EPM and the pathogenesis of disease are important.
Recent attention to daily prophylaxis to reduce antibodies against S. neurona may have unintended consequences in the disease EPM. One must weigh the need for antibody prevention (the consequence of prophylaxis) against the risk of neurologic disease and the consequences to the reduction of natural protective immunity against coccidiosis in the horse. Natural immunity holds S. fayeri in check and probably minimizes the effect of cyst toxins on the infected horse.
Overuse of antimicrobials led to superbugs. The human pharmaceutical industry will spend the many millions of dollars to develop new antimicrobials—if they can. The animal pharmaceutical industry will not spend any dollars on developing new anti-protozoals for the treatment of EPM, especially if there is a resistant superbug. Unintended consequences of prophylaxis may be the release of toxins from S. fayeri cysts. A veterinarian may misdiagnose a toxic event for an active infection—how can one distinguish these cases?