A recent study by researchers at the University of California, Davis (UC Davis), School of Veterinary Medicine confirmed that equine protozoal myeloencephalitis (EPM), caused by two different parasites, is widespread throughout the United States. The single-celled protozoal parasite Sarcocystis neurona, which is shed in the feces of opossums, is the most commonly recognized cause of this neurologic disease in horses. However, this study found evidence that Neospora hughesi, the other EPM-causing agent first identified in California, is now being identified in horses across the United States.

After obtaining a total of 3,123 diagnostic submissions from 49 states, UC Davis researchers determined that horses from 42 states were affected by parasites causing EPM (see map above left). Horses in 24 states tested positive for antibodies against both N. hughesi and S. neurona. Horses from 17 states tested positive for antibodies against S. neurona only, while horses in one state—Idaho—tested positive for antibodies against N. hughesi only. As these results show a widespread distribution of the parasites causing EPM, horse owners and practitioners should test EPM-suspect horses for antibodies against both parasites.

“This study returned positive results from more states than we originally thought,” said Nicola Pusterla, DVM, PhD, Dipl. ACVIM, associate professor of equine internal medicine at UC Davis and lead researcher on the study. “As the recognized geographic spread of Neospora hughesi infections expands, we are encouraging horse owners about the benefits of the advanced tests available at UC Davis to more accurately diagnose the disease. Overall, we had not been satisfied with the standard testing available, so we have spent the past decade developing and successfully validating an improved diagnostic tool for EPM.”

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