A case-control study of factors associated with development of clinical disease due to West Nile virus, Saskatchewan 2003.

Reason For Performing Study: West Nile virus (WNV) was first diagnosed in Saskatchewan equids in 2002. AWNV epidemic was considered highly likely for 2003, which would provide a unique opportunity to study all aspects of WNV subclinical infection and clinical disease development in a relatively naive population.

Hypothesis: There are individual equid attributes and management risk factors associated with development of clinical disease. Specifically, this study could address the question of vaccine efficacy for the prevention of development of clinical disease.

Methods: A case-control study was conducted in the summer of 2003 during a province-wide outbreak of WNV. Between 5 and 10 equids were sampled from each of 23 case premises with clinical disease and 23 control premises with no apparent or confirmed clinical disease. Data were analysed to identify risk factors for the development of clinical disease.

Results: The proportion of equids serologically positive for natural exposure to West Nile virus was 64% (193/300). Nonvaccinated equids were 23 times (95%CI limits 3.0, 168.5, P = 0.002) more likely to develop clinical disease than those vaccinated. The estimate of vaccine efficacy in this field study was 96% (95%CI limits 67%, 99%).

Conclusions: The study demonstrated that vaccination was strongly associated with the prevention of clinical disease.

Potential Relevance: Vaccination is an effective, practical method of prevention of clinical disease.