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Little is known about the period of time between exposure to S.neurona and the manifestation of clinical signs in horses. The purpose of another study performed by our group was to follow the clinical course of horses naturally exposed to this parasite. All horses on a research facility were examined for evidence of neurologic disease and serum was analyzed using Western blot to detect antibody to S. neurona. At the initial screening, >80% of the 44 horses were seropositive, therefore only 6 horses could be used for this study.
Exposure to this farm environment prior to initiation of this study varied from 1 to 10 years for these 6 horses,. Horses were examined biweekly and samples of serum and CSF were obtained at this time. All horses developed an antibody response in the serum within 20 days except 1 horse which did not become seropositive until 4 weeks later. All horses were seropositive to S. neurona for greater than 50 weeks, and in 4 of 5 cases for 67 weeks. The presence of S. neurona antibody in serum for the long periods in these horses was likely due to either a protracted antibody response, a high initial antibody titer, persistence of the antigen in this aberrant host, or frequent re-exposures to the parasite. None of the horses developed clinical signs of EPM.
The prevalence of EPM has been estimated to be approximately 1% (D. Granstrom; Personal Communication). However, we know from previous investigations that there is a much higher exposure rate to S. neurona.[Saville, 1996 #142] Horses that are frequently being re-exposed to the parasite may develop a protective immunity which may explain the lack of clinical disease in this group of horses. The evidence for re-exposure includes the variability in exposure time, the high seroprevalence on this farm, the fact that all 6 horses became positive at the same time, and only 1 horse was seronegative after 67 weeks.
Another possibility is that horses in this study had not been exposed to the necessary stresses or risk factors required to produce clinical disease. This study suggests that the incubation period of EPM can be as long as a year or more, although there may be other factors required to trigger the disease process. The results of this study suggest further research into risk factors for EPM is necessary.