Buzzworthy Bites- Human and Mosquito West Nile Virus Surveillance Activity in Nebraska 2003-2013

BACKGROUND:  Since its introduction, the annual incidence of West Nile virus (WNV) in Nebraska is consistently among the highest nationally.  A comprehensive mosquito surveillance program has been employed by the Nebraska Department of Health and Human Services (DHHS) to monitor vector populations and viral activity among them. This study describes human and mosquito surveillance activities in Nebraska from 2003-2013.

METHODS:  Human surveillance data for the study period was obtained from Arbonet and cases assigned to a region by county of residence. Mosquito trapping occurred biweekly across the state from May to October each year. Mosquitos were identified, counted and pooled for testing using PCR. These data were used to calculate vector indicators including count, number of positive mosquito pools, mosquito minimum infection rates (MIR), and vector index (VI). The spatio-temporal relationship between these indicators and epidemiology of human disease were explored.

RESULTS: During 2003-2013, 3,210 confirmed or probable cases of WNV disease were reported (16.3 cases/100,000). 2,739 (85.3%) cases classified as WNV fever (13.9/100,000), 464 (14.5%) as neuroinvasive disease (2.4/100,000), and seven (0.2%) were unclassified.  The central and west regions of the state generally had higher case incidence across years. During the same period, 2,801,274 total mosquitoes were captured, 31.5% of these were from the Culex mosquito genus, with an average of 70 Culex mosquitoes caught per trap. A total of 2,757 positive mosquito pools were detected. Seasonal patterns in mosquito surveillance showed WNV activity typically had a unimodal distribution peaking in mid to late August. Similar temporal trends were observed for reported human disease. For 8 of 11 years, statewide VIs peaked several weeks in advance of peak reported cases of human disease.  High case counts of disease occurred in 2005, 2006, 2012 and 2013 in which the highest average annual vector indices were also observed. Higher Culex mosquito abundances, more positive mosquito pools, and higher MIRs and VIs were detected across the years in the central and west regions of the state.

CONCLUSIONS:  With its agro-ecology and abundance of efficient Culex vectors in Nebraska, it is assumed Nebraska will continue to experience high annual incidence of WNV disease. Active mosquito surveillance has provided data that spatially and temporally correlates with human epidemiologic data. The observed peaks in WNV in mosquitos, weeks in advance of reported increase in human cases, may allow establishment of threshold indicator values that can be used to better focus strategies to prevent mosquito exposure.