Insecticide resistance compromising dengue control programmes in Bangladesh: Study
Insecticide resistance is severely undermining efforts to control the mosquito Aedes aegypti – the primary vector of dengue fever in Bangladesh, according to a recent study.
A study published in Pest Management Science on Tuesday (18 July), conducted by an international team of scientists, highlights the urgent need for sustainable and effective strategies to combat the growing dengue burden in the country, reports the Phys.org.
However, despite implementing insecticide-based interventions, the country witnessed its worst dengue outbreak in 2019, with over 101,000 cases of hospitalisation. The study suggests the presence of high levels of insecticide resistance among Ae. aegypti populations may have contributed to the escalating dengue burden.
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The research team, led by Hasan Mohammad Al-Amin, a PhD student under supervision of Associate Professor Gregor Devine at the Mosquito Control Laboratory, QIMR Berghofer Medical Research Institute, Australia, examined the intensity and mechanisms of resistance to commonly deployed insecticides in Dhaka, the capital city of Bangladesh.
According to their findings, Dhaka Aedes aegypti colonies exhibited extremely high levels of resistance to pyrethroids, a class of insecticides commonly used to target the mosquito species. The research forms part of Al-Amin's PhD project.
Regarding implications of this resistance for dengue control, Dr Devine said, "The use of pyrethroids in Dhaka is no longer effective, and the control programme needs to switch to a different approach."
According to the "Guidance" published in 2016 by the World Health Organization (WHO), where resistance to an insecticide is confirmed, the intensity of the resistance should be determined. Acting upon this recommendation, the team exposed samples of mosquitoes to increasingly higher doses of the insecticides and monitored the mortality rate, using a technique known as a |CDC bottle bioassay".
"These doses do not correspond with recommended field rates. However, they give a comparative measure of the strength of the resistance and help determine whether the efficacy of the insecticide in the field is likely to be compromised," Al-Amin explained.
To assess the real-world implications, the researchers exposed free-flying mosquitoes from Dhaka colonies to standard applications of pyrethroid aerosols at WHO recommended rates in an experimental free-flight room where mosquitoes can exhibit their natural behaviours. A large proportion (up to 74%) of the mosquitoes survived the exposure.
Dr. Devine explained, "Due to the mutations, insecticides cannot bind to the target site and the mosquitoes remain protected." The team also found a metabolic mechanism of resistance where certain enzymes detoxify insecticides in the mosquitoes.
However, Al-Amin notes that new strategies will take time to implement:
"Understandably, a considerable time will be required to adopt a different intervention. During this interim time, we recommend using a non-pyrethroid insecticide class for mosquito control."
Possible alternatives include the insecticides malathion and bendiocarb, which are qualified for application as outdoor space spray and indoor residual spray, respectively. Ultimately, a non-insecticidal approach will be needed to control Aedes-borne diseases in Bangladesh, he said.
