Risk Factors for Owner-Reported Occurrence of Equine Protozoal Myeloencephalitis in the US Equine Population

Background: Equine protozoal myeloencephalitis (EPM) is a serious and often fatal neurologic disease of horses, but few studies have investigated risk factors. Objectives: To evaluate operation‐ and individual‐level factors associated with likelihood of the occurrence of EPM. Animals: Data were collected as part of a study of the US equine industry from 1,178 operations representing 83.9% of horses and 51.6% of operations with ≥3 horses in 28 states. Methods: Probability‐based sampling was used to enroll representative operations in a cross‐sectional study. Interviews were conducted to collect information regarding health and management of horses. A nested case‐control study was used to investigate risk factors among individual horses. Interview data were combined with climate data, human population density, and opossum regional ecology categories. Data were analyzed using logistic regression to identify risk factors for the occurrence of EPM. Results: Owners reported that 95% of EPM cases included in this study were diagnosed by veterinarians. Variables associated with EPM occurrence on premises included opossum regional ecology, reported exposure to small wildlife, climate, terrain, housing, choice of bedding material, method of storing feeds, equine stocking density, and primary use of horses. Among individual horses, age was most strongly associated with disease risk. Associations also were identified with sex, breed, primary use, and participation in competitions. Conclusions and Clinical Importance: Because the risk of EPM occurrence on operations is closely tied to factors that impact exposure to opossums, their feces, and their environment, controlling these exposures may be important in preventing the occurrence of EPM.