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Volume 17, Number 8–August 2011
Research
Early Warning System for West Nile Virus Risk Areas, California, USA

Ryan M. Carney,1 Comments to Author Sean C. Ahearn, Alan McConchie,2 Carol Glaser, Cynthia Jean, Chris Barker, Bborie Park, Kerry Padgett, Erin Parker, Ervic Aquino, and Vicki Kramer
Author affiliations: California Department of Public Health, Richmond, California, USA (R.M. Carney, C. Glaser, C. Jean, K. Padgett, E. Parker, E. Aquino); City University of New York, New York, New York, USA (S.C. Ahearn, A. McConchie); University of California, Davis, California, USA (C. Barker, B. Park); and California Department of Public Health, Sacramento, California, USA (V. Kramer)

Suggested citation for this article

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.

West Nile virus (WNV; family Flaviviridae, genus Flavivirus) is a mosquito-borne pathogen that has led to ≈30,000 reported (>325,000 estimated) human cases and 1,172 reported deaths in the United States since it was first detected in New York, New York, in 1999 (1). The virus was first detected in California in a pool of Culex tarsalis mosquitoes in July 2003 (2), and in 2004 and 2005 the state had the highest number of reported human cases (779 and 880, respectively) and deaths (29 and 19, respectively) in the United States (3). Humans are incidental, dead-end hosts of WNV and generally become infected after intense viral amplification and spillover from local avian populations (4). Birds are the natural reservoir and amplification hosts of WNV and infections can cause death rates up to 100% among avian species (5,6). Beginning in 2000, bird carcasses in California were submitted by local agencies to the WNV Dead Bird Surveillance Program (DBSP) at the California Department of Public Health (CDPH; previously known as the California Department of Health Services) as part of the California Mosquito-Borne Virus Surveillance and Response Plan (7,8). A toll-free telephone hotline and website for recording public reports of dead birds was established in 2002.

Previous efforts for the early detection and monitoring of WNV activity have used dead bird density or spatial scan statistic as a proxy for transmission risk for humans (9–13). However, aggregation of reports over nonuniform spatial units (i.e., counties and census tracts) may fail to detect WNV amplification clusters that span regional boundaries or that are contained within large areas. In addition, temporal aspects of the WNV transmission cycle should be considered to avoid false-positive identifications in circumstances in which sustained but slow transmission leads to an accumulation of dead bird reports above the designated risk threshold but does not result in spillover to the human population.

Another approach is the DYCAST system (14,15), implemented in New York, New York, in 2001 and Chicago, Illinois, in 2002. This system detects statistically significant spatiotemporal clustering of dead bird reports by modeling the WNV amplification cycle using biological parameters; it also includes a statistical method for evaluating effectiveness of human case predictions in space and time. Results indicated that clusters of dead bird reports and human cases of WNV were significantly associated in space and time (15). This association suggests that this procedure may be useful for predicting areas at high risk for WNV transmission to humans. Because there is no drug prophylaxis, human vaccine, or treatment available for WNV, integrated pest management and personal mosquito protection remain the only options for reducing human illness and death, and early warning of high-risk areas allows for these measures to be implemented in a timely and effective manner. The objective of the present study was to evaluate implementation of DYCAST as an early warning system in California to target public education campaigns, surveillance, and mosquito control efforts during an anticipated statewide outbreak of WNV.

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Suggested Citation for this Article

Carney RM, Ahearn SC, McConchie A, Glaser C, Jean C, Barker C, et al. Early warning system for West Nile virus risk areas, California, USA. Emerg Infect Dis [serial on the Internet]. 2011 Aug [date cited]. http://www.cdc.gov/EID/content/17/8/100411.htm

DOI: 10.3201/eid1708.100411



1Current affiliation: Brown University, Providence, Rhode Island, USA.

2Current affiliation: University of British Columbia, Vancouver, British Columbia, Canada.

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Ryan M. Carney, Division of Biology and Medicine, Brown University, PO Box 2684, Providence, RI 02906, USA; email: ryan_carney@brown.edu