Is agricultural activity linked to the incidence of human West Nile virus?

Background: West Nile virus (WNV) has spread throughout the contiguous United States. During the 2002-2003 period, there were 14,023 laboratory-confirmed human cases of WNV in 45 states and 541 associated deaths. Factors that affect case distribution are poorly understood. This study assessed the relationship of environmental factors and agricultural activity with the presence of human WNV cases. Methods: County-level data were collected that included 2002 and 2003 WNV surveillance data, temperature, dairy revenue, precipitation, total irrigated acres, and total crop revenue. Logistic regression models were used to determine which risk factors were significantly associated with WNV human cases. Results: Significant independent predictors for counties with human WNV cases were population (odds ratio [OR]=1.20, p<0.0001); higher average daily temperature in April through October (OR=1.19 for each additional degree Fahrenheit, p<0.0001); and total crop sales (OR=1.14 (p<0.001). The ORs for these predictors increased in an analysis of counties with ten or more cases. Conclusions: Higher temperature and farming activity may be strongly associated with the incidence of human WNV infection. Larger studies of more agricultural centers are warranted to determine which environmental factors increase the risk of human infection and how these infections can be prevented.