Early Warning System for West Nile Virus Risk Areas, California, USA

Ryan M Carney

doi:10.3201/eid1708.100411

Abstract

The Dynamic Continuous-Area Space-Time (DYCAST) system is a biologically based spatiotemporal model that uses public reports of dead birds to identify areas at high risk for West Nile virus (WNV) transmission to humans. In 2005, during a statewide epidemic of WNV (880 cases), the California Department of Public Health prospectively implemented DYCAST over 32,517 km2 in California. Daily risk maps were made available online and used by local agencies to target public education campaigns, surveillance, and mosquito control. DYCAST had 80.8% sensitivity and 90.6% specificity for predicting human cases, and k analysis indicated moderate strength of chance-adjusted agreement for >4 weeks. High-risk grid cells (populations) were identified an average of 37.2 days before onset of human illness; relative risk for disease was >39× higher than for low-risk cells. Although prediction rates declined in subsequent years, results indicate DYCAST was a timely and effective early warning system during the severe 2005 epidemic.

Highlights

West Nile virus (WNV), an arthropod-borne virus in the family Flaviviridae, was first detected in the United States in 1999 [1]
During 1999–2008, 28,961 cases of WNV disease were reported from 1,869 counties in 47 states and the District of Columbia (DC)
WNV human disease cases have been detected in all geographic regions of the contiguous United States, the highest incidence of neuroinvasive disease consistently occurs in the West Central and Mountain regions

Summary

Introduction

West Nile virus (WNV), an arthropod-borne virus (i.e., arbovirus) in the family Flaviviridae, was first detected in the United States in 1999 [1]. WNV is maintained in nature in a mosquito-bird-mosquito transmission cycle primarily involving Culex species mosquitoes, Cx. pipiens, Cx. tarsalis, and Cx. quinquefasciatus [2,3]. Birds are the natural reservoir hosts for WNV. When infected with WNV, many avian species develop transient viremia levels that are high enough to infect feeding mosquitoes [4]. Birds commonly survive their infections and develop permanent immunity, certain species become ill and die [5]. Humans are considered dead-end hosts; they do not develop viremia levels of a sufficient titer to allow virus transmission to feeding mosquitoes [6]

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