Equine Protozoal Myelitis (Sarcocystis neurona infection) is a debilitating neurological disease of horses. It can affect the brain, brainstem, spinal cord or any combination of these three areas of the central nervous system.
Until recently, the predisposing factors that lead to the disease have been something of a mystery. A recent study, however, has enlightened us about some of these predisposing factors. This study showed that certain horse- and farm-related factors increase or decrease the likelihood of being diagnosed with EPM.
Young horses have had less time to be exposed and build up antibodies to S. neurona. They also have an active, stressful, athletic career, which may suppress what immunity they have to S. neurona.
A prior diagnosis of EPM may increase the likelihood that S. neurona or opposums (a carrier animal) can be found in the area. Opossums are the source of S. neurona, so their presence would obviously increase risk. In addition, once a diagnosis of EPM is made on a farm, there's a heightened awareness to signs of EPM in other horses. Therefore, they may be a greater tendency to pursue a diagnosis of EPM on those farms.
Health events (e.g. prior illnesses) in the year that preceded diagnosis of EPM may be associated with increased stress and/or decreased immunity in those horses also predisposing them to EPM.
Horses with a racing or showing career may also be younger, or they may have increased risk because they are stressed due to transport and intense training. Alternatively, they may receive a diagnosis of EPM because it is easier to notice subtle changes in gait in these competitive horses.
This study supports the notion that protecting your feed source from wildlife, especially opossums, is paramount to decrease the risk of EPM.
The reason why proximity to a river decreased risk of EPM could not be explained, unless opossums are less likely to dwell in those microenvironments.
In summary, the contention that opossums are an important reservoir for EPM was supported by this study. Young horses with intense competitive schedules that have had an important health event (i.e. illness) are predisposed to a diagnosis of EPM.
Recently, it has been demonstrated by Dr. JP Dubey that domestic cats can act in the laboratory as carnivore hosts for Sarcocystis neurona, but it is important NOT to interpret cats are carriers. They are not thought to be carriers, only good laboratory hosts. The present thought is that they are more likely to keep the opposums OUT of the barn.
Recent Review on EPM
Dubey, JP; Lindsay, DS; Saville, WJ; Reed, SM; Granstrom, DE; Speer, CA. "A review of Sarcocystis neurona and equine protozoal myeloencephalitis (EPM)" Veterinary Parasitology, 95 (2-4):89-131; 2001.
To learn more about equine protozoal myelitis,please click on Equine Protozoal Myelitis.