Spatial Risk Assessment of West Nile Virus in Montana Based Upon Temperature Effects on Cx. tarsalis
Spatial and seasonal West Nile virus (WNV) transmission risk was assessed throughout Montana with Geographic Information System (GIS) models based on the temperature threshold below which virus development will not proceed in Cx. tarsalis. The model used maximum, minimum, and average daily temperature data; a degree day modeling derivation that produced West Nile virus development units (WDU); and varying time-scales throughout June, July, August, and September. A temperature of 12.6 °C maintained for 107.7 degree days (dd) was utilized as the point of zero virus development and was based on the extrinsic incubation period (EIP) or the average amount of time between a female Cx. tarsalis imbibing an infectious bloodmeal and transmitting the virus. The models were produced to evaluate transmission risk on monthly scales and multi-month additive scales that utilize accumulated monthly WDUs. Overall, the models based on average temperature data showed higher correlation to Cx. tarsalis prevalence and West Nile virus human infection than their corresponding maximum or minimum temperature based WDU models. The models also showed an increasing risk of virus transmission throughout Montana from west to east and an absence of transmission in areas of cooler daily temperatures. By the end of September about 82% of the state was determined to be viable for West Nile virus transmission while approximately 87% of the state was determined to be at risk of virus transmission by the end of august when peak risk occurs. Further improvements to the present models may arise from utilizing additional Cx. tarsalis trapping data from varying years or by incorporating population and travel density throughout state counties in order to better depict county counts of West Nile virus human infection. In brief, this study assesses the thermal and temporal effects on WNV transmission throughout Montana and provides a guide for WNV prevention.