A case-control study of factors associated with development of clinical disease due to West Nile virus, Saskatchewan 2003.
Authors: Epp T, Waldner C, Townsend H G G
Journal: Equine veterinary journal
Summary
During Saskatchewan's 2003 West Nile virus outbreak, researchers conducted a case-control study across 46 premises (23 with clinical cases, 23 controls) to identify risk factors for clinical disease development in a largely naive equine population, sampling 5–10 animals per premise. Serological testing revealed 64% natural exposure to WNV across the sampled horses, yet vaccinated animals proved dramatically protected: unvaccinated equids were 23 times more likely to develop clinical disease than their vaccinated counterparts (95% CI 3.0–168.5), translating to an estimated vaccine efficacy of 96% (95% CI 67–99%). This field-based evidence—gathered during active outbreak conditions rather than controlled trials—demonstrates vaccination's substantial effectiveness in preventing overt clinical signs, even within a population experiencing high environmental viral exposure. For equine practitioners, these findings provide robust justification for WNV vaccination protocols as a cornerstone of disease prevention strategy, particularly in endemic or outbreak-prone regions where natural exposure rates are elevated.
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Practical Takeaways
- •West Nile virus vaccination is highly effective at preventing clinical disease in horses, with 96% efficacy demonstrated in field conditions
- •Unvaccinated horses face substantially higher risk of developing clinical disease during WNV outbreaks—consider vaccination as essential preventive strategy
- •In endemic regions or during outbreak situations, vaccination should be prioritized as a practical and effective disease prevention tool
Key Findings
- •64% of equids were serologically positive for natural West Nile virus exposure during the 2003 Saskatchewan outbreak
- •Unvaccinated equids were 23 times more likely to develop clinical disease than vaccinated equids (95% CI: 3.0-168.5, P=0.002)
- •Vaccine efficacy for prevention of clinical disease was estimated at 96% (95% CI: 67%-99%)
- •Vaccination was strongly associated with prevention of clinical West Nile virus disease