Pesticide resistance hurting dengue control activities: Study

Insecticide resistance is severely undermining efforts to control Aedes aegypti – the primary vector of dengue fever in Bangladesh, according to a recent study. However, despite insecticides being ineffective, the two city corporations of Dhaka spent Tk482 crore for mosquito control in the past seven years.

According to the findings of the study titled "Insecticide resistance compromises the control of Aedes aegypti in Bangladesh", Aedes colonies in Dhaka exhibit extremely high levels of resistance to pyrethroids, a class of insecticides commonly used to target mosquito species.

Up to 74% of free-flying and resting mosquitoes in Dhaka colonies survived exposure to standard applications of pyrethroid aerosols in an experimental free-flight room.

The two city corporations in Dhaka have been using pyrethroid for more than 10-12 years to kill Aedes mosquitoes, sources at the Directorate General of Health Services told The Business Standard.

It was only after the biggest dengue outbreak in 2019 that the city corporations moved away from pyrethroid and are no longer using it due to questions about its effectiveness.

The study, conducted by an international team of scientists and its findings published in Pest Management Science on 4 April 2023, highlights the urgent need for sustainable and effective strategies to combat the growing dengue burden in the country.

The research team, led by Hasan Mohammad Al-Amin, a PhD student under the supervision of Associate Professor Gregor Devine at the Mosquito Control Laboratory, QIMR Berghofer Medical Research Institute in Australia, examined the intensity and mechanisms of resistance to commonly deployed insecticides in Dhaka.

Phys.org, an online science, research and technology news aggregator based in the UK, published a report on this study on 18 July.

In the report, Hasan Mohammad Al Amin notes, "Understandably, a considerable period of 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 an 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.

In this study, mosquito eggs were collected during the dengue outbreak in June 2019 from five areas of Dhaka, which were selected on the basis of dengue prevalence, population density and housing type.

With no effective drugs or widely available vaccines, dengue control in Bangladesh is dependent on targeting the primary vector Aedes with insecticides and larval source management. 

Despite these interventions, the dengue burden is increasing in Bangladesh. So far, a total of 155 dengue patients have died and 27,547 were admitted to hospitals this year in the country.

What causes insecticide resistance?

Dr Kabirul Bashar, a professor of zoology at Jahangirnagar University, told The Business Standard, "If an insecticide is used continuously for more than five years, then mosquitoes become resistant to it. Therefore, to get good results, pesticides should be changed on a rotation basis."

MM Aktaruzzaman, a programme manager of Malaria and Aedes Transmitted Disease Control at the DGHS, told TBS that pyrethroid group drugs have been used for a long time to kill mosquitoes. Hence this group of drugs has become resistant. Now non-pyrethroid group drugs should be used.

"Not using effective insecticides increases the risk of dengue outbreak being prolonged. For now, we have to use whatever is available. But for the future, pesticide monitoring is very important. The disease control wing of the health department and the city corporations should conduct research to monitor pesticides regularly," Aktaruzzaman added.

Mosquito control budget rises, but no respite

In the last seven years, Dhaka North and Dhaka South have increased their budget almost every year regarding mosquito control. 

Tk297.59 crore was spent in seven years only to buy mosquito repellents and insecticides.

Dhaka North, with an area of 196.22 sq kilometres, has spent Tk314.54 crore on controlling mosquitoes, while Dhaka South, with an area of 109.25 sq kilometres. has spent Tk167.47 crore.

In fiscal year 2022-23, Dhaka North spent Tk67.50 crore and Dhaka South spent Tk31.10 crore on controlling mosquitoes.

Dhaka South Mayor Sheikh Fazle Noor Taposh told TBS, "Our mosquito medicine is tested in two-three stages and then purchased. We are giving more importance to larvicide than to fogging (adulticide)."

Dhaka North City Chief Health Officer AKM Shafiqur Rahman told TBS, "We sprayed temephos and novaluron larvicide in the morning and Malathion adulticide in the evening. We have been using mosquito control insecticides since the beginning of this year. These are used because they work against Aedes mosquitoes with some steps in testing. I do not know which ones were done earlier."

Dhaka South Chief Health Officer Dr Fazle Shamsul Kabir told TBS, "South City Corporation has been using the same mosquito control insecticides for the last three-four years. We have been using pesticides tested by the Plant Protection Wing and the Institute of Epidemiology, Disease Control and Research (IEDCR). Temephos is used as larvicide and Malathion as adulticide in the South City area. Also, Deltamethrin was used to control mosquitoes but is no longer used."