Surveillance of Above- and Below-Ground Mosquito Breeding Habitats in a Rural Midwestern Community: Baseline Data for Larvicidal Control Measures Against West Nile Virus Vectors

Mosquitoes present a significant threat to public health because they serve as vectors for numerous infectious agents that are pathogenic for humans and animals. In the United States, West Nile virus (WNV) has emerged as the most prevalent mosquito-borne infection, with over 9,800 human cases documented in 2003.1 In the western hemisphere, WNV is endemic across most of the United States, as well as southern Canada, Mexico, and parts of the Caribbean.2,3 WNV is maintained in nature by enzootic transmission cycles between wild birds and mosquitoes.4 Previous research has shown that virus levels are not amplified to the same degree in all bird species, and mosquito species can vary greatly in their competence as efficient vectors.5–11 In the eastern United States, two closely related mosquito species, Culex pipiens and Culex restuans, have been implicated as being among the most important WNV vectors. Both species have adapted well to urban areas and are strongly ornithophilic (bird loving) in their feeding preferences.8–11 Studies have demonstrated that some populations of Culex pipiens may exhibit a more generalized feeding behavior and may take blood meals from humans as well.12–14 In the western United States, Culex tarsalis is believed to be one of the most important vectors for transmission of WNV.6 In general, humans and other mammals are not thought to contribute substantially to the overall transmission cycle of WNV in nature. However, they can become infected after being bitten by infected mosquitoes. Fortunately, most human infections with WNV are asymptomatic or mild, but severe cases of meningoencephalitis do occur and are associated with significant morbidity and mortality.4,15,16 Since an effective vaccine for WNV in humans is not yet available, current strategies to prevent or limit cases have focused on reducing mosquito numbers in endemic areas. Mosquito control interventions, such as larviciding and adulticiding, are expensive and are most effective in preventing WNV when they target those mosquito species that are abundant, highly competent to transmit virus, and feed on both birds and humans.17,18 There are over 2,500 different mosquito species worldwide, at least 200 species in the United States, and at least 50 species in Wisconsin.19,20 In some communities, there may be a lack of data on the distribution and relative abundance of many mosquito species. In other communities, mosquito control districts have been functioning for a number of years to control pest mosquitoes; in such districts there is often baseline data on mosquito diversity and abundance going back decades. This background information can make it much easier to develop rational strategies to control Culex populations and prevent human exposure to WNV and other mosquito-borne viruses. Unfortunately, mosquito control districts are usually in larger urban areas, and many small- to medium-sized cities and rural areas lack baseline data on Culex and other mosquito habitats. This study was conducted to determine the distribution and breeding cycles of Culex and other mosquitoes in Marshfield, Wisconsin, a community of 18,908 located in central Wisconsin and representative of many rural midwestern cities.