Temporal and Spatial Variability of Entomological Risk Indices for West Nile Virus Infection in Northern Colorado: 2006–2013

Abstract

West Nile virus (WNV) is enzootic in northern Colorado. Annual surveillance activities in Fort Collins, CO, include collecting female Culex mosquitoes and testing them for the presence of WNV RNA in order to calculate 1) Culex female abundance, 2) WNV infection rate, and 3) the vector index (VI). These entomological risk indices inform public policy regarding the need for emergency adulticiding. Currently, these are calculated on a citywide basis. In this study, we present descriptive data from historical surveillance records spanning 2006–2013 to discern seasonal and yearly patterns of entomological risk for WNV infection. Also, we retrospectively test the hypothesis that entomological risk is correlated with human transmission risk and is heterogeneous within the City of Fort Collins. Four logistically relevant zones within the city were established and used to test this hypothesis. Zones in the eastern portion of the city consistently had significantly higher Culex abundance and VI compared with zones in the west, leading to higher entomological risk indicators for human WNV infection in the east. Moreover, the relative risk of a reported human case of WNV infection was significantly higher in the eastern zones of the city. Our results suggest that a more spatially targeted WNV management program may better mitigate human risk for WNV infection in Fort Collins, and possibly other cities where transmission is enzootic, while at the same time reducing pesticide use.

Characterizing environmental risk factors for West Nile virus in Quebec, Canada, using clinical data in humans and serology in pet dogs

“…However, the fact that agricultural lands represent the main high-risk environment for WNV spillover to dogs supports the possibility of a significant WNV activity in these areas. Agricultural land was previously reported as a high-risk environment in western North America but not in the north-eastern USA [34–36]. Higher risk in agricultural areas may be explained by the particular characteristics of the study area.…”

Evaluation of a novel West Nile virus transmission control strategy that targets Culex tarsalis with endectocide-containing blood meals

“…Mosquitoes were processed as part of the Fort Collins WNV surveillance program according to established protocols [ 59 ]. Briefly, mosquitoes were collected weekly by Vector Disease Control International using miniature CDC light traps baited with CO 2 .…”

“…Mosquitoes were sorted to species and pooled into groups of typically no more than 50. Mosquito pools were screened at CSU using qRT-PCR using the following primer sequences: forward 5’ 1160-TCAGCGATCTCTCCACCAAAG 3’, reverse 5’ 1209-GGGTCAGCACGTTTGTCATTG 3’, probe 5’ FAM-1186-TGCCCGACCATGGGAGAAGCTC 3’ [ 59 ].…”

Dynamics of West Nile virus evolution in mosquito vectors

“…It may be that the positive correlation between temperature and vector competence also helps maintain viral genetic diversity and thus aids avoidance of the negative consequences of bottlenecks. Additionally, the frequency of spontaneous vesiculovirus mutation doubles when the temperature is raised from just 39 to 39.8 °C [64], suggesting that the WNV mutation rates could profoundly change during the 5–10 °C variations in mean temperature during the transmission season [65]. Ultimately, the numerous virus-vector-environment interactions that determine vector competence (e.g.…”