Irrigation linked to a greater incidence of human and veterinary West Nile virus cases in the United States from 2004 to 2006.

Identifying risk factors for West Nile virus transmission is vital for future public health control measures. This study investigated the effect of irrigation, expressed as percent irrigated land, and population density on the incidence of human and veterinary West Nile virus (WNV) cases using a sample of 2827 counties from the United States over a 3-year period. Zero-inflated Poisson regression models were selected because of the large number of counties reporting zero cases. The mean percentage of irrigated land for the entire sample was approximately 0.029, while counties reporting both human and veterinary cases had an average of 0.054. As the level of irrigation rose by 0.1% of the total county land area, the incidence rate of WNV cases increased by 50% in the human model (RR: 1.50, 95% CI: 1.46-1.53) and 63% in the veterinary model (RR: 1.63, 95% CI: 1.57-1.68). Higher equine and human population densities were associated with a slightly decreased incidence of WNV cases. In the logistic portion of the Zero-inflated model, the presence of irrigation significantly decreased the odds of a county reporting zero cases. Future research is warranted to determine the exact nature of the relationship between irrigation and increased disease incidence in human and veterinary populations.