EPM-Causing Organisms Widespread in U.S. Horses (AAEP 2012)

The neurologic disease equine protozoal myeloencephtalitis (EPM) is caused by two protozoal agents, Sarcocystis neurona and, less commonly, Neospora hughesi. While researchers have long understood S. neurona's life cycle and transmission, their understanding of N. hughesi is less concrete. To compare the two organisms, a research team from California analyzed prevalence factors associated with suspected EPM in horses that tested seropositive or negative to N. hughesi and/or S. neurona.

Nicola Pusterla, DVM, PhD, Dipl. ACVIM, a professor in the Department of Medicine and Epidemiology at the University of California, Davis, presented the team's results at the 2012 American Association of Equine Practitioners convention, held Dec. 1-5 in Anaheim, Calif.

The researchers evaluated the history of horses with serum samples submitted to the Immunology Laboratory at the UC Davis School of Veterinary Medicine. The samples were collected from Dec. 1, 2010, to Nov. 30, 2011, and analyzed for antibodes to N. hughesi and S. neurona using an indirect immunofluorescent antibody test (IFAT). They evaluated a total of 3,123 submissions with horses ranging in age from 1-37 years. Horses from 25 states tested seropositive for N. hughesi while horses originating from 41 states tested seropositive for S. neurona.

The team then divided horses into four groups according to the serological results: N. hughesi-positive only, S. neurona-positive only, N. hughesi- and S. neurona-positive, and N. hughesi- and S. neurona- negative. Although male horses were overrepresented, Pusterla and his colleagues found no statistical correlation between gender and group. They did note, however, correlations between certain breeds and the serological groups: There were more N. hughesi-positive, S. neurona-positive, and dual positive EPM suspected horses among Quarter Horses than other horse breeds. There were fewer N. hughesi-positive results in the Thoroughbreds than in the other breeds. Warmbloods fared best, showing the fewest positive results for each type of protozoa and having the highest representation in the negative group.

While most neurologic signs did not seem to be diagnostic for either type of infection, the researchers did note that horses with a head tilt were more likely to be in the dually positive group, and ataxia (a wobbly gait) correlated with a positive S. neurona titer.

N. hughesi and S. neurona cause neurologic disease in the horse, and both agents have a wider geographic distribution in the United States than commonly thought. It is therefore important to test for both protozoal pathogens when investigating the cause of neurologic deficits in a horse suspected of having EPM.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.