Rising insecticide resistance challenges mosquito control

Aedes species collected in El Oro Province, Ecuador
Aedes species collected in El Oro Province, Ecuador

Mosquitoes are developing high levels of resistance to all major classes of chemical insecticides. UF medical geographers investigate how resistance can fluctuate across seasons and geography, revealing useful data for real-time adaptive strategies to mitigate mosquito-borne diseases in people.

Efforts to control disease-carrying mosquitoes with insecticides are encountering the same challenges with resistance that doctors face with declining efficacy of antibiotics. A new study focused on southern Ecuador’s El Oro Province reveals high levels of insecticide resistance, but in a new twist researchers documented fluctuating levels of resistance across cities and between seasons. The rapid change in resistance status reveals a corresponding necessity for health agencies to continually monitor mosquito populations in order to employ effective control measures in real time.

Stephanie Mundis, a UF PhD student in medical geography, studying under EPI investigator Sadie Ryan co-authored the study which was recently published in PLOS-Neglected Tropical Diseases. Another of Ryan’s doctoral students, Cat Lippi, worked with Mundis on authoring the paper and analyzing statistics. Ryan and her colleague Anna Stewart-Ibarra, an assistant professor of medicine at SUNY Upstate Medical University, collaborated on the study development and with contacts at the Ecuador Ministry of Health. Postdoctoral fellow Rachel Sippy, who is affiliated with Ryan’s research group as well as SUNY Upstate Medical University, also contributed to the study.

The research team focused on Aedes aegypti mosquitoes, which prefer urban habitats and primarily feed on people. This mosquito is the main vector in the Americas for yellow fever, dengue, chikungunya and Zika viruses. Of these, dengue is the most prominent disease in Ecuador, sickening more than 13,612 people in 2016.

“We tested for insecticide resistance in four cities with different histories for endemic dengue transmission, across three different seasons,” Mundis says. “Across the board, we found quite a bit of resistance in all four populations. It seems there have been strong selective pressures happening where these mosquitoes are really resistant, in particular to malathion.”

The team tested for resistance to three insecticides commonly used by the Ecuadorian Ministry of Health: malathion, deltamethrin, and alpha-cypermethrin. Malathion is an organophosphate, whereas the latter two are pyrethroids. They used phenotypic tests as well as genetic screening to identify two alleles associated with resistance to pyrethroids. The two different lines of evidence ensured they were not missing resistance characteristics which may be either genetic in origin, or be physical in nature.

Because of the high rates of illness associated with disease carried by Ae. aegypti, it makes sense to understand more deeply the drivers and patterns of insecticide resistance so that health agencies can respond in real time.

Two coastal study sites, Machala and Huaquillas were compared to two cities located further inland at higher elevations, Zaruma and Portovelo. The team found varying levels of resistance at all sites, and across three mosquito “seasons.” Season one corresponds to peak dengue transmission from February to April, season two reflects when transmission levels decline between May and June, and season three captures the drier months of July through August when transmission is at its lowest levels.

The port city of Machala had the highest levels of resistance across all three seasons, with almost 92 percent of collected mosquitoes showing resistance. Huaquillas showed relatively high levels of resistance too, with 68.92 percent showing resistance. Portovelo and Zaruma showed comparatively lower levels with about 55 percent of sampled Ae. aegypti showing resistance.

Mundis points out that many homeowners use chemical control methods, in addition to the insecticide campaigns carried out by the Ecuadorian Ministry of Health. Varying levels of insecticide use across neighborhoods and cities, and even between cities, results in uneven selective pressures over time and across space. This variability in selective pressures, combined with a mosquito’s short generational time span of one month, likely leads to the fluctuating levels of insecticide resistance the team found in El Oro. Mundis theorizes that there are likely fitness costs of insecticide resistance in mosquitoes once the selective pressure of insecticide applications are removed, since recent research has shown populations can lose resistance in just 10 generations, which is a mere 10 months.

Even so, Mundis found the results eye opening. “I was surprised at the temporal variation, because from looking at the resistant genotype frequencies from Machala from season one to season two, we saw a decrease in the mutant genotype,” she says. “It’s a question of whether the selective pressure was removed, because they were spraying less in season two. How could the resistance frequencies change that rapidly? It is much more fluid than I had thought was possible.”

Prior to this study, the Ecuadorian Ministry of Health had little information about the seasonal and spatial variation in insecticide resistance in El Oro. But this is not unusual, most governments do not actively monitor for resistance, Mundis says. “It has only been in the past 10 years that resistance has started becoming an alarmingly prevalent thing that we all need to study. This is very new,” she says. “But people are realizing that these chemical treatments are losing their effectiveness, and it’s a big logistical challenge.”

But now that resistance is being studied in more detail, Mundis says the data can empower health agencies to act adaptively and in real time to the challenge of mosquito control. Incorporating insecticide resistance monitoring into mosquito vector control programs would vastly improve how health agencies design their plans of action.

 “A lot of this points to rotating between insecticide choices, and using larvicides,” Mundis says. “But there are nonchemical solutions too, such as doing public health education campaigns that encourage people to empty containers that allow for the breeding of these mosquitoes.” 

EPI investigator Ryan notes that it is resource intensive to identify if insecticide resistance is occurring in a local mosquito population. “It is also logistically and financially challenging for many vector control groups – from city-level to national-level – to allocate the resources to switch insecticides or strategies,” Ryan says. “We need more studies like this, to identify the scale of insecticide resistance, both on the landscape, and through the seasons.”

By: DeLene Beeland