Short-term effect of weather variables on West Nile virus infection in Equids in Spain: A space-time stratified case-crossover design.

West Nile virus (WNV) is an arthropod-borne virus primarily transmitted by Culex mosquitoes, among birds, but occasionally infecting humans and equids. Understanding environmental factors on the disease occurrence is crucial for planning effective prevention and control strategies. Our study estimates the short-term effects of ambient temperature, relative humidity and precipitation on weekly cases of WNV in equids in Spain, using a space-time stratified case-crossover design. Because WNV transmission occurs sometime after the weather conditions become favorable for the vector, we aimed to estimate the lagged effect of the three weather variables on the incidence of WNV cases in equids. Therefore, we applied a conditional Poisson regression within the framework of lag-distributed models. These models are particularly effective for examining the delayed effects of exposures. The incidence of WNV confirmed cases was associated with temperatures registered within the prior two and eight weeks. The highest incidence was observed at lag four, where the incident rate ratio (IRR) increased to 11.96 (95 % CI: 9.84-50.40) with mean temperature at 33 °C, likewise relative humidity effect peaked also at lag four (IRR=3.99; 95 % CI: 1.43-11.16) at 93 % of relative humidity. On the other hand, the confidence intervals for precipitation were very wide, making it difficult to discern a clear change in the risk of WNV infection associated with total precipitation. This study sheds light on the complex relationships between meteorological variables and the incidence of WNV cases. Considering the zoonotic nature of the disease, these results could provide valuable information for surveillance and early warning of the disease.