Equine protozoal myeloencephalitis (EPM) is the most common neurological infectious disease of horses in North America, but it's difficult to diagnose. The disease is caused by two different protozoal parasites called Sarcocystis and Neospora, which affect the nervous system. Symptoms include progressive and asymmetrical incoordination, muscle wasting, and gait abnormalities. The asymmetrical aspect is an important part as it is typical that one side of the horse or one leg is more affected than the other side. There are multiple tests available for EPM but all of them only indicate exposure to the parasites; they do not indicate the cause of the symptoms due to the disease. Different labs promote their blood tests as the best, and different vets have confidence in different blood tests.
A recent study involving over 5,000 horses in 18 states revealed that 78 percent of the horses tested were positive for Sarcocystis and 34 percent were positive for Neospora while 31 percent were positive for both parasites. However, none of these horses had any symptoms of EPM and were normal. It is important to realize that only less than 1 percent of the horses that are found to be positive on blood tests are actually going to show symptoms of the disease. EPM may be the most overdiagnosed disease in America because it has so many varied symptoms and testing is so inaccurate. Testing spinal fluid does increase the accuracy but there can be problems with this test as well as difficulty in obtaining the sample. So if you think your horse might have EPM, just realize that 78 percent of horses that do not have EPM are positive on the blood test, and that even if your horse is positive, it does not indicate your horse has EPM.