Disease causing ataxia caused by a protozoa called sarcocystis neurona. The organism cannot finish its life cycle in the nervous system of the horse. Equine Protozoal Myelitis is an inflammatory process created by the horse’s immune system as the result of the presence of a protoza (parasite). Inflammation in the nervous system looks the same no matter what the cause. According to Dr. Siobhan Ellison, identifying and treating specific inflammatory pathways is an overlooked branch of EPM research. The history of identifying the sarcocystis organism (parasite) in the central nervous system is rare. The definitive diagnosis must come from a dead horse.
Sarcocystis neurona has a preference to reside in the muscle tissues of it’s natural host. They have toxins associated with the cysts created in those muscles. Protozoa adapt to the host by changing their appearance to avoid detection by the immune system. Antibodies are produced by the immune systemin response to specific markers on the surface of this protozoa (parasite). These markers are multiple and changing through the protozoa’s life cycle. Protozoa can initiate antibody formation by the horse to it’s own body know as auto immune disease. Inflammation is the first line of defense in an infection apart from antibodies formed against a specific organism. Inflammatory reactions have many start/stop mechanisms.
Many horses are exposed to sarcocystis neurona and they make antibodies when infected. Specifically, the S. Neurona type antibodies are tested for at Pathogenes as some antibodies the horse makes are cross reactive with other types of sarcocystis protozoa. The notion that cerebrospinal fluid has specific antibody to S. Neurona only is compromised by a leaky blood-brain barrier during inflammation and blood (serum) contamination of the cerebrospinal fluid sample. Many normal animals have antibodies to sarcocystis including non-S. Neurona that do not cause disease. Most horses can eliminate the S. Neurona infections and not experience symptoms. The antibody detection method must be specific for S.Neurona in a horse showing symptoms. The antibody serum testing must incude those of SAG 1, 5 or 6.
The parasites rarely get into the CNS (central nervous system) of the horse. Sapcocystis have predelection for muscle tissue. The inflammatory molecules set off by sarcocystis, not the parasite itself, are capable of causing the symptoms associated with equine protozoal myelitis (EPM). Serum antibodies indicate that the cause of neuroinflammation and it’s symptoms are from sarcocystis neurona. As the cause (one of the multiple possible causes).
The most common cause of EPM worldwide in the horse is sarcocystis neurona with the opossum as the host to complete it’s reproduction cycle and shed the organism in it’s stool. Thus opossum population control or lack of that control can contribute to reinfections of the horse. The opposum eats infected muscle tissue from intermediate hosts like racoons, skunks, armadillos, cats, etc. The parasites sexual cycle is started in the gut of the opossum. Theaoocysts are shed in the opossum stool and they are picked up in food, hay, or water by the horse and of course true intermediate hosts as mentioned above. After replicating in the gut cells, the parasite is released into the blood stream and have muscle as the preferred target. Quite likely, the parasite is carried inside white blood cells (leukocytes) in the muscle cells the parasite forms cysts (sarcocysts). The cycle begins again when cyst-infected muscle is eaten by the opossum. Aberrant intermediate hosts like the horse do not form muscle cysts. The protozoa has deftly avoided the immune system of a normal intermediate host mechanized with cyst formation in muscle. Unfortunately, the aberrant host like the horse initiates an immune response resulting in inflammation. The inflammatory response can lead to disease. Remember, the inflammatory process has many start/stop steps in it’s chain.
**Abba Vet Supply strongly encourages you to consult your veterinarian regarding specific questions about your horse’s health. This information is not intended to diagnose or treat any disease, and is purely educational.**
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