A horse that has been infected with equine protozoal myeloencephalitis (EPM) has inflammation and tissue damage to the brain and spinal cord which may cause transient lameness or incoordination. Infected horses may also be weak, lethargic, or have gait changes.
Some horses may show mild signs of infection, while others become severely compromised. EPM can be confused with other equine issues, so a clear diagnosis is imperative to ensure the horse is being treated for the proper condition.
EPM is primarily caused by Sarcocystis neurona, but it can also be caused by a similar organism, Neospora hughesi. Most horses with EPM respond well to treatment, which includes antiprotozoal or antiparasitic medications like ponazuril, diclazuril, or sulfadiazine and pyrimethamine, reports EQUUS magazine.
However, neurologic signs can return months or years after the original diagnosis and treatment, and treatment may be again required. Unlike viruses and bacteria, to which a horse can develop lifetime immunity, the parasites that cause EPM are skilled at surviving even a concerted medicinal treatment plan.
Making complete clearing of the parasite even more complicated is the fact that the protozoa crosses into the horse's central nervous system, an area of the body that allows the protozoa to “hide” from treatment.
How often horses experience EPM relapses is not known. In one study, 8 percent of horses being treated for EPM relapsed within 90 days after the EPM treatment was stopped. Multiple factors are most likely related to EPM relapses in horses, including the type of drug used for treatment, its dosage and the duration of treatment, as well as the individual horse's immune system function.
There may also be variations in EPM strains, though this has not been investigated.
If a horse experiences an EPM relapse, the treating veterinarian may recommend an additional round of treatment or that the owner treat the horse with ponazuril or diclazuril and lengthen treatment duration.
Read more at EQUUS magazine.
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