Seroprevalence and evaluation of risk factors associated with seropositivity for Borrelia burgdorferi in Ontario horses.

Background: Recently, the blacklegged tick (Ixodes scapularis), which is the vector of Borrelia burgdorferi, has undergone a range expansion from the northeastern and mid-west United States to areas of southeastern Canada, including parts of Ontario. Understanding the seroprevalence of antibodies against B. burgdorferi in horses and risk factors for exposure is important for monitoring and preventing this emerging disease.
Study Design: Cross-sectional study of 551 horses in southern, central, and eastern Ontario, Canada.
Objectives: To assess the seroprevalence of B. burgdorferi in horses in Ontario, Canada; to evaluate risk factors associated with seropositivity; and, to compare the performance of two diagnostic tests.
Methods: Serum samples were obtained from clinically healthy horses in Ontario, Canada, along with completed questionnaires that were used for the risk factor analysis. Sera were tested with a Multiplex ELISA (Animal Health Diagnostic Center, Cornell University) and C6 ELISA (IDEXX SNAP ^® 4Dx ^® Plus test, IDEXX Laboratories).
Results: The seroprevalence of B. burgdorferi on at least one test was 17% (91/551), though only 15 (16%) horses tested positive with both tests. A spatial cluster of cases was detected in Eastern Ontario. The odds of being seropositive for B. burgdorferi on the C6 ELISA were significantly increased when oak trees were present by pastures (OR = 7.3 (1.8-29.2), P = .005), while the odds were significantly decreased when regular tick checks were performed (OR = 0.1 (0.01-0.7), P = 0.02).
Main Limitations: Recruitment focused on known areas with blacklegged ticks as well as areas of higher horse density, which may have led to selection bias.
Conclusions: The expansion of blacklegged tick populations poses an ongoing risk for horses. Assessment of diagnostic testing options and risk factors is important for diagnosis and prevention, and with further investigation this information may be used to propose changes in management.
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