Use of the vector index and geographic information system to prospectively inform West Nile virus interventions.

We sought to estimate West Nile virus (WNV) activity in mosquito populations weekly at the census tract level in Chicago, IL, and to provide this information graphically. Each week we calculated a vector index (VI) for each mosquito trap then generated tract estimates using geographic information systems. During June 29-September 13, 2008, a median of 527 (60%) of 874 possible tracts per week had a VI value. Overall, 94% of the weekly VI tract estimates were 0; among those with a VI estimate greater than 0, the median was 0.33 (range 0.003-3.5). Officials deemed risk levels and weather conditions appropriate for adulticide treatments on 3 occasions, resulting in the treatment of approximately 252 linear kilometers of residential streets and alleys. Our analysis successfully converted complex, raw surveillance data into a format that highlighted areas of elevated WNV activity and facilitated the determination of appropriate response procedures.