Antibodies are formed in response to infections whether the horse gets the symptoms of EPM (neuro inflammation) or not. These antibodies appear at day 17 and persist for months, even after the infection is gone. Inflammation is the way offending organisms are removed by the host. Lasting inflammation after removal of the offending protozoa can mimic active protozoan disease. If opossums are around the horse can also be reinfected. Since chronic inflammation can persist after removal of the organism (infection), the neuro inflammation must be treated.
Immune reactions to sarcocystis neurona start immediately after 3 days the disease is chronic, with horses rarely showing symptoms within the first 3 days. If an untreated infection continues, antibodies will form and can be detected in a couple weeks. The longer an active infection, the higher the antibody titre. Antibody levels will decline with resolution of the infection, unless plenty of opossum cause reinfection through their shedding protozoal ooctysts in feces. The horse’s own immune responses can eliminate the parasite. The majority of horses have no EPM disease but still have serum antibodies. Thus, horses are efficient at parasite elimination since they are aberrant hosts. We do not know why some horses succumb to EPM and it’s neuroinflammation, while most horses do not! Some EPM serum antibody testing is not specific enough creating as many as 80% of horse EPM Horses in a population to test positive.
If a horse has clinical signs and a rise in antibody levels, treatment for EPM should begin. If the antibody levels fall, this indicates that the protozoa (parasite) has been eliminated. Yet, the horse can remain symptomatic and unresponsive to anti-protozoal therapy. Hence, very specific anti-inflammatory therapy is necessary.
Sarcocystis Fayer: Is a protozoal infection where the horse is the true intermediate host with cysts formed in the horse’s muscles. These cysts do product toxins. These cysts normally produce no reaction in the horse unless debilitated causing inflammation and disease. Dying S. Fayeri cysts may show signs consistent with EPM.
Protozoal proteins associated with release from cysts or merozoites can be toxic and cause intestinal inflammation (ileitis). The sarcocysts typically do not induce inflammation. If the cysts die or merozoites are circulating inflammatin is induced and the inflammation leads to clinical signs. Remember, finding parasites in the nervous tissues of the sick horse is very rare.
The inflammation (antibody formation), however, is used to identify EPM horses. These inflammatory pathways elicited by the parasite do not respond to non-steroidal anti-inflammatory drugs (NSAID’s). Steroid anti-inflammatories suppress the horse’s immune system favoring parasite growth, and should not be used.