CALIFORNIA STATE MOSQUITO-BORNE VIRUS SURVEILLANCE AND RESPONSE PLAN: A RETROSPECTIVE EVALUATION USING CONDITIONAL SIMULATIONS* CHRISTOPHER M. BARKER, WILLIAM K. REISEN, AND VICKI L. KRAMER Center for Vector-Borne Diseases, School of Veterinary Medicine, University of California, Davis, California; Vector-Borne Disease Section, Division of Communicable Disease Control, California Department of Health Services, Sacramento, California Abstract. The California Mosquito-Borne Virus Surveillance and Response Plan recently was developed to provide a semi-quantitative means for assessing risk for western equine encephalomyelitis (WEE) or St. Louis encephalitis (SLE) viruses and to provide intervention guidelines for mosquito control and public health agencies during periods of heightened risk for human infection. West Nile virus recently has arrived in California, and the response plan also will provide a baseline for assessing the risk for human and equine infection with this virus. In the response plan, overall risk is calculated by averaging risk due to 1) environmental conditions, 2) adult mosquito vector abundance, 3) vector infection rates, 4) sentinel chicken seroconversion rates, 5) equine cases (for WEE), 6) human cases, and 7) the proximity of virus activity to populated areas. Overall risk is categorized into three levels: normal season, emergency planning, or epidemic conditions. We evaluated this response plan using historical data from years with no, enzootic, and epidemic activity of WEE and SLE in several areas of California to determine whether calculated risk levels approximated actual conditions. Multiple methods of risk calculation were considered for both viruses. Assessed risk based on cumulative temperature, rainfall, and runoff levels over the entire season provided more or equally accurate assessments than biweekly assessments based solely on the previous half-month. For WEE, during years with enzootic activity or early- season periods of years with WEE epidemic activity, combining horse and human cases as a single risk factor improved the model’s ability to forecast pending WEE activity, but separating the two factors allowed a better indication of WEE activity during epidemics and periods with no activity. For SLE, assignment of higher risk to drier conditions as measured by rainfall and runoff yielded the most accurate representation of actual virus activity during all recent study periods. INTRODUCTION The introduction and dispersal of West Nile virus (WN) across North America have heightened interest in arbovirus surveillance and control programs by state and local agencies, resulting in a large volume of data on virus activity. However, clearly defined numerical thresholds to forecast human risk and provide targets for intervention are poorly collated and inconsistent among regions and agencies. This spatial hetero- geneity is expected due to differences in local ecology and productivity that make numerical targets elusive and spatially variable. In addition, encephalitis virus transmission appears to be capable of proceeding at low vector abundance levels. 2 A practical alternative to hard and fast numerical thresh- olds may be a conceptual model to forecast the risk of equine and human cases based on anomalies among environmental factors, mosquito abundance, and enzootic virus activity. The Vector-Borne Disease Section of the California Department of Health Services recently has developed such a model based on a variety of surveillance indicators that interact to produce seasons with low, normal, or epidemic risk of human infec- tion. 3 The model incorporates readily available environmen- tal data along with surveillance data collected by local agen- cies to provide a straightforward way for personnel at these agencies to assess virus risk within their own area. However, before this model and response paradigm can be incorporated into local mosquito control and health planning and interven- tion programs, it seemed prudent to evaluate plan function- ality using retrospective conditional simulations during years with no, enzootic, and epidemic levels of virus activity. California has had a long interest in the epidemiology, sur- veillance, and control of two endemic mosquito-borne en- cephalitis viruses of public and veterinary health interest: western equine encephalomyelitis virus (WEE, family Togaviridae) and St. Louis encephalitis virus (SLE, family Flaviviridae). 4 Both viruses are maintained in an enzootic cycle involving various bird species and mosquitoes in the genus Culex, particularly the primary vector, Culex tarsalis Coquillett. 5 Humans and horses develop insufficient WEE viremias to infect mosquitoes and therefore are tangential, or dead-end, hosts that play no role in virus amplification. Simi- larly, humans are also dead-end hosts for SLE, a virus closely related genetically to WN. 6 Since the formal inception of a state-managed surveillance program in 1969, the activity of the mosquito-borne encephalitides in California has been monitored intensively by vector control agencies, public health personnel, and University of California researchers. Statewide response plans and surveillance guidelines have been drafted to provide guidelines for effective and consistent responses by appropriate organizations during periods of in- creased risk for virus activity. 7,8 These documents are similar to those prepared by the Centers for Disease Control and Prevention. 9 However, these documents do not provide quan- titative or semi-quantitative forecasts of risk or targets for intervention. Recently, the California Mosquito-Borne Virus Surveillance and Response Plan 3 has been revised in prepa- ration for the arrival of WN or other emerging arboviruses. In the current plan, environmental assessments are combined with various measurements of enzootic and epidemic virus activity to determine an overall risk for the occurrence of equine or human clinical cases and to provide escalating re- sponse recommendations to mosquito control agencies for intervention. Because the state’s surveillance program for mosquito-borne viruses has focused on WEE and SLE, our study evaluated the utility of the response plan using condi- tional simulations with historical data for years with varying levels of virus activity. These simulations provided an indica- tion of whether the plan would have prescribed appropriate response levels as the seasonal cascade of surveillance mea- surements unfolded during years with no, enzootic, or epi- demic levels of virus activity. * An abbreviated description of this research was published previ- ously in the Proceedings and Papers of the 70th Annual Conference of the Mosquito and Vector Control Association of California. 1 Am. J. Trop. Med. Hyg., 68(5), 2003, pp. 508–518 Copyright © 2003 by The American Society of Tropical Medicine and Hygiene 508