Study: Mosquito sprays reduce WNV risk

Study lead author Dr. Kristina Lopez, of the Department of Entomology at the University of Wisconsin at Madison, checks a mosquito trap where local mosquito populations were monitored after insecticide applications. Photo: Dr. Justin Harbison

The mosquito truck, a vehicle equipped with spraying equipment, has long been common in regions of the U.S. in an attempt to reduce mosquito populations and the diseases they spread in urban and suburban neighborhoods throughout the summer.

But a team from the University of Wisconsin at Madison has gathered data on how well these ultra-low volume (ULV) truck-mounted insecticides work, and revealed a surprising result. In a study published last week in the Journal of Medical Entomology, the researchers found that, while the trucks did not necessarily reduce overall numbers of mosquitoes, they could reduce the proportion of older, blood-fed mosquitoes within the population, thereby lowering the risk of diseases like West Nile virus (WNV).

The research team evaluated how well ULV trucks in the Chicago, Ill., area could reduce mosquito populations and disease risk. Using a range of measurements, they discovered that, after five weekly applications of a pyrethroid derivative common in ULV treatments in two sites in Cook County, Ill., during the summer of 2018, the abundance of Culex mosquitoes (known for being WNV vectors) did not change significantly. In fact, abundance changes after short-term (up to five days) treatment at sites ranged from a 99 percent reduction to 616 percent increase. For long-term treatments, abundance ranged from a 30 percent reduction to 2,009 percent increase.

However, the proportion of “nulliparous” mosquitoes — those that have neither had a bloodmeal nor produced offspring — increased significantly, and WNV infection rates decreased at one site, according to Entomology Today.

The increase of nulliparous insects is important, as their lack of previous feeding means they have not acquired and therefore cannot spread WNV. The virus remains a significant problem in the Chicago area, and Illinois in general. Between 2002 and 2018, 2,634 human infections and 176 deaths were reported. Chicago and its suburbs have been classified as a “hotspot” for the virus in the U.S. Midwest, because of recent outbreaks and consistently high annual infections in some areas.

In Illinois, WNV is spread by two mosquito species: Culex pipiens and C. restuans — commonly known as the common house mosquito and the white-dotted mosquito, respectively. These species feed mostly on birds and occasionally mammals, including humans.

To control the mosquitoes, abatement districts usually employ public information efforts, source reduction, larval surveillance, and removal of larvae from storm water basins. Active controls such as ULV truck-based applications come into play when the risk of infection is high. Few studies, however, have fully evaluated the effectiveness of these active controls, and those that have been conducted have yielded contradictory results — just like this team’s abundance data appeared contradictory. For the study, the team used a number of measures, including abundance, age of mosquitoes and WNV transmission. 

For the study, Dr. Bartholomay’s team used a number of measures, including abundance, age of mosquitoes and West Nile virus transmission.

“We were really impressed that, of all the things we measured, it was the age-structure shift that was most remarkably different between control and treated sites,” says Dr. Lyric Bartholomay, a professor of pathobiological sciences at Wisconsin who led the research team. “This is an ‘invisible’ impact of adulticide use for mosquito control, because it takes some additional effort to dissect the mosquitoes and check the ovaries for evidence of having a previous bloodmeal. And that invisible impact could have real-world consequences for West Nile virus transmission.”

For mosquito control districts, this could help tailor control measures to reduce disease risk. If a large proportion of mosquitoes after treatment are nulliparous, then WNV reduction will more likely appear, even though overall mosquito populations remain high. In this study, older mosquitoes that had produced offspring appeared to be more susceptible to the insecticide.

“We view this as a win for public health because there is less risk of people and animals being exposed to an infected mosquito,” Dr. Bartholomay said in the news release. “A reduction in West Nile virus … would also impact whether or not mosquito control operations would have to implement additional mosquito control measures.”