The number of hot, humid days that allow mosquitoes to thrive is rising in Missouri and Illinois, according to climate change researchers.
The environmental research nonprofit Climate Central reports that mosquito seasons have become longer in St. Louis, Jefferson City, Quincy and other parts of the Midwest than they were in the 1980s. As a result, people will be at a higher risk for catching West Nile virus and other mosquito-borne diseases, researchers said.
Local public health agencies need to increase their surveillance of mosquitoes and the diseases they spread, said Katie Westby, a staff scientist at Washington University’s Tyson Research Center.
“Especially in St. Louis, we don’t have a rigorous surveillance and monitoring program. We’re not keeping track of when we start seeing [West Nile Virus] in mosquitoes,” Westby said. “Without that information and that information coming in very quickly, we can’t respond and start trying to eliminate mosquitoes and warning the public that [viruses] are circulating.”
After West Nile virus is detected in local mosquitoes, it takes about a month for the virus to spread to people, Westby said.
Climate Central noted that mosquitoes can survive temperatures of between 54 and 95 degrees and in places where there’s at least 42% relative humidity. Based on those parameters, researchers calculated that over the past three decades, St. Louis increased its mosquito season by five days, Quincy by four days and Jefferson City by 17 days. Kansas City's, however, dropped by three days.
The numbers could be higher, said Enbal Shacham, a public health professor at St. Louis University.
“I would argue that we are accelerating more because of human development,” Shacham said. “We’ve put in efforts to reduce climate change, but I don’t know how effective we’ve been at doing that in the St. Louis region.”
Shacham also analyzed data related to the Zika virus outbreak in 2015. Public health departments need to monitor diseases beyond West Nile virus, but they’ve experienced many years of disinvestment and are stretched thin due to the coronavirus pandemic, she said.
“I don’t think [the Zika virus] went away,” Shacham said.
The St. Louis mosquito control program involves spraying insecticides in neighborhoods to reduce mosquitoes and dunking chemicals in standing water to kill mosquito larvae, said a city spokesperson. St. Louis County officials also spray pesticide in residential areas at night and trap mosquitoes to track species in the area. But longer mosquito seasons will pose challenges for the county's vector control program, which receives limited funding, said James Sayers, supervisor of the program.
“If we have longer mosquito seasons, that means we have to bring on manpower a little longer," Sayers said. "We have to hire our on-call summer workers sooner, we have to keep them later in the season. We could be using more [pesticides]. It could pose a lot of issues.”
St. Louis collaborates with St. Louis County on surveillance, said a city spokesperson. County officials have not trapped mosquitoes in the city, Sayers said. But the county does plan to start trapping mosquitoes in the city in the next month, particularly to track Aedes egypti mosquitoes, which spread the Zika virus and yellow fever.
A 2017 report from the National Association of County and City Health Officials showed that local governments in Missouri lack competent mosquito surveillance and control programs.
Some research has suggested that the higher temperatures brought on by climate change could reduce mosquito populations. Mosquitoes cannot survive extremely hot weather, Westby said.
“If we see increases in heat waves and droughts, that could have negative effects on mosquitoes too,” she said.
But there’s no doubt that temperate areas like St. Louis could attract many more mosquitoes in the future and that local government health agencies need to be prepared, Westby said.
“[St. Louis] has got a serious mosquito history, and we’ve got a ton of mosquitoes in our area,” she said. “St. Louis has a mosquito-borne virus named after it, St. Louis encephalitis. We were one of the first urban centers to be infested by [Asian tiger mosquito]. It is alarming that no one is willing to invest the resources into monitoring what’s going on here.”
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