Spatially Explicit West Nile Virus Risk Modeling in Santa Clara County, California

Abstract

A geographic information system model designed to identify regions at risk for West Nile virus (WNV) transmission was calibrated and tested with data collected in Santa Clara County, California. American Crows that died from WNV infection in 2005 provided spatial and temporal ground truth. When the model was run with parameters based on Culex tarsalis infected with the NY99 genotype of the virus, it underestimated WNV occurrence in Santa Clara Co. The parameters were calibrated to fit the field data by reducing the number of degree-days necessary to reach the mosquito's extrinsic incubation period from 109 to 76. The calibration raised model efficiency from 61% to 92% accuracy, and the model performed well the following year in Santa Clara Co.

Estimating West Nile Virus Transmission Period in Pennsylvania Using an Optimized Degree-Day Model

“…As seen later, and consistent with the Californian study of Konrad et al (2009), the original degree-day model parameters failed to capture the WNV transmission period in Pennsylvania adequately. There could be many reasons for this inconsistency (e.g., different life history parameters in the field compared with lab estimates, possibly different mosquito or virus strains, effects of acclimation or local adaption, etc.…”

“…Most critically, because the rate of pathogen development within the mosquito varies with temperature and the duration of the incubation period tends to be similar to the average mosquito life span (Kilpatrick et al 2008), temperature strongly affects the proportion of mosquitoes that live long enough to potentially transmit the disease. By taking this relationship into account, simple degree-day models of pathogen development coupled with estimates of mosquito longevity can potentially be used to explore disease risk (at least as determined by the minimum thermal requirements necessary to enable transmission) (Reisen et al 2006, Zou et al 2007, Konrad et al 2009). Here we followed this approach to examine WNV transmission potential in Pennsylvania.…”

“…Using the standard degree-day model and a 12-day average life span for Culex (Dohm et al 2006b), they were able to predict temporal ( -1 week) and spatial variation in the first week of potential WNV transmission in the year (measured as the first positive samples of infected mosquitoes collected from the field) with about 80% accuracy. Konrad et al (2009) also used this approach to explore WNV transmission dynamics in Cx. tarsalis in Santa Barbara, California.…”

“…We then explore various adjustments in model parameters to optimize model performance. The major objective of this study was to evaluate the robustness of previous degree-day approaches (Zou et al 2007, Konrad et al 2009) and provide an optimized degreeday model to predict potential West Nile virus transmission period in different locations in Pennsylvania.…”

Autogeny in Culex pipiens Complex Mosquitoes from the San Francisco Bay Area

“…pipiens group and Culex tarsalis Coquillett mosquitoes in 2003 in anticipation of the introduction of West Nile virus into the county. Working with the California Department of Health Services and the Santa Clara County Department of Public Health, the spread of the virus was carefully documented by the appearance of dead, infected crows 29 and, eventually, infected mosquitoes. These efforts resulted in the opportunity to work more closely with Cx.…”