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Case Control Data
The investigators are in the final stages off completing a retrospective case-control study intended to examine risk factors for disease. Two hundred and fifty EPM cases admitted to The OSU Veterinary Medical Center between 1992 and 1995 were enrolled in the study, as were 2 control series. One control series consisted of horses diagnosed with neurologic diseases other than EPM (neurologic control series, n=250), and the other control series were horses that were admitted for reasons other than neurologic disease (non-neurologic control series, n=250). Information regarding individual horse characteristics were obtained from hospital records and differences in rates among EPM cases and controls have been analyzed. Information regarding environmental exposures prior to admission is still being collected by telephone follow-up, as is information regarding survival and response to therapy among EPMcases.
Among horses with neurologic disease, those that are seropositive are 30 times more likely to have detectable antibody in CSF when compared to seronegative horses. Preliminary analysis of data from this retrospective study suggests that there is a significant difference between EPM cases and controls with regard to age, breed, and season at the time of admission.
Age was categorized into 10 discrete levels and examined as a risk factor for EPM utilizing the non-neurologic control horses as the comparison series. There was a significant linear trend of decreasing risk from youngest to older horses. When compared with horses less than 1 year old, the risk for EPM in 3 year olds was 22 times greater; the risk for EPM in 4 year olds was 18 times greater; and the risk among yearlings was 16 times greater. This is consistent with other infectious diseases in which the risk of disease is greatest among animals that have not acquired protective immunity through exposure.
We then examined the association between age and the occurrence of EPM using the neurologic horses as the control series. In this comparison there was a significant linear trend; this time risk increased from youngest to older horses. When compared to the horses <1 year of age, the risk for EPM in horses >14 years-of-age was 9 times greater; the risk among horses 10 to 13 years-of-age was 7 times higher; and the risk among yearlings was 5 times higher. Horses with EPM were significantly older than horses in the neurologic control group. This suggests that other common causes of neurologic disease are not manifest among older horses in the hospital population while EPM is seen in a broader range of ages.
We also examined the association between breed and the risk of EPM. When using the non-neurologic control series and the small breeds such as ponies, and miniature horses as the reference group, the risk for EPM was 7 times greater in Warmbloods, Thoroughbreds, and in Standardbreds. The risk for EPM among Quarter Horses was only 4 times higher than the reference group. There was not an association between breed and EPM when using the neurologic controls as the comparison series.
The association between season and the risk of EPM was examined categorizing January to March as Winter, April to June as Spring, July to September as Summer, and October to December as Fall. Using horses admitted during Winter as the reference group, horses admitted in the Fall were 4 times more likely to have EPM, and horses were twice as likely to have EPM if admitted in the Spring or Summer. This seasonal difference in admission rates was similar using both control series in comparisons. Assuming most cases have a short incubation period as has been indicated from experimental infection with S. falcatula, this suggests that there may be greater exposure to S. neurona sporocysts during warm weather.
Further information needed to finalize this study is currently being gathered by the investigators. However, preliminary results suggest that risk factors for EPM include age, breed, and season. In contrast to the seroprevalence where the rate increases with age, it appears that young horses have a much greater risk of EPM than do older horses, although this may be related to the number of horses in a particular age group presented to a veterinary hospital. The highest risk of presentation for EPM was seen among 3 year olds and may be partially related to athletic activity. Young horses represent the predominant proportion of those used in the racing and show horse industry.
Seasonal variations in the incidence of EPM has not been previously reported. The validity of this finding is supported by findings of our previous investigation that seroprevalence is affected by climactic conditions. If there is less exposure to the parasite in winter, this may explain the lower incidence of the disease at that time. Perhaps the organism numbers increase during the summer weather conditions which leads to higher exposure levels and greater risk for disease. Also, horses are often less stressed in the winter, which may explain part of this difference. The highest risk of disease was seen in the fall of the year, which may have been related to the stress of athletic performance at major competitions or year ending races.
Finally, it appears that there are risk factors for EPM not previously reported. It also is apparent there are likely several risk factors that have yet to be elucidated. The retrospective study will be completed by early 1997 and will provide information crucial to the proper conduct of a prospective study. Further work needs to be performed such as that proposed herein to improve our understanding the epidemiology of this devastating disease. This will hopefully lead to the development of control measures for prevention of EPM.