July 2018 West Nile Virus Update Back »

Written collaboratively by Adam Varenhorst, Amanda Bachmann, Philip Rozeboom, and Patrick Wagner.

There are over 20 species of mosquitoes that call South Dakota home. In an effort to keep track of mosquito population densities and the incidence of West Nile Virus, mosquitoes are closely monitored through trapping efforts. From the historical trap data, it is evident that only two mosquito species account for the majority of observations. The species are Aedes vexans (Figure 1) and Culex tarsalis (Figure 2). While both of these mosquito species are capable of transmitting West Nile Virus to humans, C. tarsalis poses the greater threat. This is due to its habit of feeding on birds, which are the primary vertebrate hosts of West Nile Virus. A. vexans demonstrates a higher preference for humans and rarely feeds on bird hosts.

Brown mosquito.
Figure 1. Adult Aedes vexans. Courtesy: Michael Hildreth

Brown mosquito with white band on mouthparts.
Figure 2. Adult Culex tarsalis. Courtesy: Michael Hildreth

In a July 20, 2018 press release the South Dakota Department of Health indicated that West Nile Virus positive mosquitoes were detected in Beadle, Brookings, Brown, Minnehaha, and Stanley counties in South Dakota. So far in 2018, no human cases of West Nile Virus have been reported, however we are entering peak WNV transmission season.

To reduce the chances of contracting West Nile Virus, it is important to understand the behavior of the mosquito vectors. A. vexans is known to search for human hosts both day and night; however, the majority of its activity occurs at night. C. tarsalis is a nocturnal feeder with the greatest risk of bites occurring between dusk and dawn. Therefore, the risk of West Nile Virus infection can be greatly reduced by limiting outdoor activities between dusk and dawn. If you are outside after dark, make sure to cover as much of your skin as possible by wearing long sleeves and pants. We also recommend using insect repellants that contain DEET or similar active ingredients to provide further protection. This link provides a helpful tool for choosing a repellant that will provide the best protection based on the desired activities.


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