By Jill Pease
Spraying insecticide on the household surfaces where mosquitoes that carry chikungunya, dengue and Zika rest after feeding led to a 24% reduction of those illnesses among community members, according to a new study published in the New England Journal of Medicine.
The randomized trial was conducted by an international group of infectious disease experts including Ira Longini, Ph.D., a professor in the Department of Biostatistics in the University of Florida College of Public Health and Health Professions. Researchers tracked households in Mérida in Mexico’s Yucatán Peninsula, from 2021 to 2023, a period which also coincided with a dengue outbreak in that city.
Applying a mosquito insecticide once a season indoors on surfaces such as lower walls and under furniture resulted in a 60% reduction of mosquitoes in treated households compared to untreated households, and one-quarter fewer cases of chikungunya, dengue and Zika at the community level.
“Previous studies have shown control strategies can reduce mosquito densities, but the real question was what impact did that have on human health itself?” said Longini, a member of UF’s Emerging Pathogens Institute and director of the Center for Statistics and Quantitative Infectious Diseases. “This is one of the few studies that actually measures the impact not only on the vector (the mosquito), but on human disease as well.”
Dengue cases are surging in the Americas, with an estimated 10 million suspected cases last year, including in the United States. Zika, which caused widespread illness in the Americas in 2015 and 2016, is once again on the rise. And a quarter million cases of chikungunya have been reported this year in the Americas, China and Europe. The viruses are carried by two species of mosquitoes: Aedes aegypti and Aedes albopictus, both of which regularly cause illness in Mexico, the rest of Latin America, and southeast and southern Asia.
The new study, a cluster randomized trial, involved 50 household clusters made up of five by five city blocks each. Half of the household clusters were randomized to receive targeted indoor residual spraying in July, the start of mosquito season.
A primary study aim tested whether or not indoor spraying led to fewer cases of chikungunya, dengue and Zika among children living in the study clusters. Results, however, did not demonstrate a lower risk of mosquito-borne disease in the children, a somewhat puzzling finding that may be attributed to children being exposed to mosquitoes in locations outside of the home, such as in school, Longini said, adding that future analysis of the data will likely tease apart contributing factors.
“Still, the team’s finding that targeted spraying reduced the number of mosquitoes and mosquito-borne illness among Mérida’s study population as a whole is encouraging and suggests that indoor insecticide application would be beneficial for all residents,” Longini said. “Given the fact that conventional vector control, for the most part, has been a failure all these years, the measured impact and protection against illness in the general population, where the targeted indoor residual spraying was applied, is a big message of this study.”
Because Aedes aegypti and Aedes albopictus breed in urban areas and feed on humans indoors, the conventional intervention of spraying insecticide outdoors has been largely ineffective at preventing disease. The insecticide used in targeted indoor residual spraying is safe for humans and pets, quick and easy to apply, effective for a full season, and because it is only used indoors, does not harm beneficial insects and will not enter the food chain, Longini said.
Targeted indoor residual spraying could be combined with other strategies, such as dengue vaccination and the release of mosquitoes infected with Wolbachia, a bacteria that hinders mosquito reproduction, to successfully limit transmission of chikungunya, dengue and Zika.
“One of the things we’re learning about arbovirus and vector control is it’s best to layer interventions,” Longini said.
In addition to Longini, investigators on the National Institutes of Health-funded study included scientists from Emory University, the University of Washington, Universidad Autónoma de Yucatán, Israel’s Ministry of Health, University of Central Florida, Mexico’s Secretary of Health and National Institute of Public Health, and the U.S. Centers for Disease Control and Prevention.
This year marks four decades of research by Longini on viruses spread by biting insects, also known as arboviruses. His work spans outbreak investigation, epidemiology, vaccine development and vector control. In 1985, he was recruited by a colleague to investigate dengue outbreaks on the Pacific coast of Mexico and to help establish the Mexican Epidemic Intelligence Service, or EIS, in 1989. Next, Longini and colleagues published a nationwide investigation of dengue transmission determinants in Mexico, the first of its kind anywhere in the world.
Longini said the new study on targeted indoor residual spraying will likely lead to several more studies with additional analysis of the data and hopefully, a larger population-level randomized trial.
“I’m quite excited about the possibilities,” he said.