Aedes spp. Photo courtesy of Ary Farajollahi, Bugwood.org.

By John P. Roche

Insect repellents are a $3.6 billion industry globally, and demand for repellents is increasing as the ranges of mosquitoes that vector disease expand in response to climate change. Repellents are crucial to public health because many mosquito-borne diseases are dangerous and pose daunting health burdens to society. Malaria alone causes about 400,000 deaths per year, and dengue sickens an estimated 100 million people and kills more than 20,000 each year.

To explore the efficacy of repellents, Associate Professor Marc J. Lajeunesse and students from the University of South Florida conducted a meta-analysis asking: Do infected and non-infected mosquitoes show different responses to insect repellents? To examine this question, they screened 2,316 studies and identified 13 that had data that could be used for analysis.

In their study, which was published in the Journal of Medical Entomology, they looked at seven repellents, six mosquito species, and various infectious agents, including malaria, dengue, Zika virus, Sindbis virus, and microsporidia. They pooled the data from the 13 studies, performed an analysis, and discovered that overall, infected mosquitoes were less responsive to repellents than non-infected mosquitoes. They also found that older mosquitoes were less sensitive to repellents than younger mosquitoes.

Infected vs. Non-Infected Mosquitoes

This study was a class project from Marc Lajeunesse’s Fall 2018 Medical & Applied Entomology course. All 36 undergraduate students are co-authors. This photo shows Lajeunesse (left) and some of the undergraduates from the class capturing specimens for the students’ preserved insect collections. Photo credit: T.-A. Phan.

Interestingly, mosquitoes infected with the malaria parasite, and mosquitoes infected with dengue, did not show differences in response to repellents when compared to uninfected mosquitoes. However, data from the dengue-infected mosquitoes might have been confounded because the virus was introduced to test mosquitoes by injection, which could have been disruptive to the mosquitoes.

“A key conclusion of our synthesis,” Lajeunesse said, “was that infection status needs more attention when developing repellency protocols, since there is a risk that repellents may have altered effects on infected mosquitoes.”

When asked about the constraints of the study, Lajeunesse said, “As in most research syntheses, we were limited by what has been published. In our synthesis, there were very few studies in the literature, and there was large variability in their outcomes. However, this is where the power of meta-analysis shines: individually, studies on non-model species or repellents may not have enough power to detect alternations in repellency effects, but in aggregate, they can reveal important effects.”

Additional constraints faced in the individual studies analyzed included variability among the techniques with which infection was introduced into mosquitoes, the age of mosquitoes when first tested, and the behaviors measured.

This study yielded data important to medical entomology, but it also had a valuable educational role. It was done with undergraduates as part of a medical entomology course at the University of South Florida, and thus provided students with valuable active-engagement science experience. All 36 students in the course became coauthors on the study.

Future Research Opportunities

Lajeunesse and his students propose that to ensure results that are meaningful in real-world situations, tests of repellents should be made with infected mosquitoes. They list two questions that still need to be answered: (1) How does mosquito age and infection status interact to affect efficacy of repellents? (2) Can old, uninfected mosquitoes that have already reproduced be used as test subjects to develop improved protection?

“Few studies test altered behavior to repellents due to infections,” Lajeunesse said, “but there are even fewer that test for age effects on mosquito behavior to repellents.” To investigate if age and infection status interact to affect repellent efficacy, Lajeunesse hopes that future studies will test all combinations of key factors—that is, infected, not infected, young, and old.

Another question that remains to be answered is how infection reduces response to repellents in mosquitoes. The most common repellent, and one of the most effective, is DEET, which was developed by the U.S. Army in the 1940s. DEET is thought to work by blocking mosquitoes’ ability to smell a human host, or perhaps because mosquitoes are repelled by the taste through their feet. By what mechanism does infection interfere with this process? More insight into proximate mechanisms may offer routes for further improving repellent effectiveness—including effectiveness for infected mosquitoes, which are the most important mosquitoes to repel.

“We hope that our findings kindle more research on this topic,” Lajeunesse said, “and, in particular, replicate the findings of the individual studies synthesized. Replication is key to convincingly validate the negative effects of infections on mosquito behavior.”

John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions™ and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.