Mosquito productivity and surveillance for St. Louis encephalitis virus in Chicago during 1993

The City of Chicago's Department of Health monitors weekly deposition of egg rafts of Culex species, prevalence of St. Louis encephalitis (SLE) virus-specific antibodies in feral birds, and prevalence of the virus in mosquito pools. The total number of Culex egg rafts collected in 1993 (4,623) was 2-fold greater than for the 1992 mosquito season. Virtually all of the early summer egg rafts were identified as Culex restuans. After the week of July 18, Culex pipiens accounted for 20-70% of the total rafts collected weekly. The prevalences for SLE viral antibodies (avian) and RNA (mosquitoes) were 0.2% and 0.02%, respectively. Both values were about 25-fold less than normally occur in epidemic years. It is important for practical considerations to continue this surveillance in order to recommend time- and site-specific mosquito abatement.