Aedes mosquitos are insecticide-resistant in Dhaka: Study

Aedes mosquitos which spread dengue fever have become resistant to the bottled insecticides that are often used in Dhaka households, and 74 per cent of flying and resting Aedes mosquitos survived exposure to liquid mosquito repellents, according to a study.

The study was conducted on the efficacy of six commercially available aerosol products in Dhaka. Four of these selected insecticides for the free-flight tests with Aedes aegypti were from Bangladesh and one each from India and Australia.

A research article titled “Insecticide resistance compromises the control of Aedes aegypti in Bangladesh” was published in the Pest Management Science journal in the UK in March this year.

Hasan Mohammad Al-Amin, a researcher at Mosquito Control Laboratory, QIMR Berghofer Medical Research Institute, Brisbane, Queensland, Australia, led the study.

Amid a surge of dengue that saw the record highest death, people are increasingly using various methods including liquid insecticides at households to kill the mosquito population.

Mohammad Mushtuq Husain, advisor of Institute of Epidemiology Disease Control and Research (IEDCR), thinks Aedes mosquitos becoming resistant to bottled insecticides is a matter of concern. He said it is necessary to observe the issues related to patients of mosquito-borne diseases, mosquitos, as well as mosquitos turning resistant to insecticides. Components of mosquito repellents must be changed, if needed be. Otherwise, no protection from aedes mosquitos will work, he added.

Eggs of Aedes aegypti were collected from five areas of the capital – Khilgaon, Mirpur, Uttara, Dhanmondi and Bashundhara Residential Area in June 2019. These areas were selected based on dengue infection, population density, and household

Tests were carried out using 3- to 5-day-old, non-blood-fed female mosquitoes in the lab. Aerosols that are mainly pyrethroid class and often used in Dhaka households, was sprayed on the mosquitoes, and if a mosquito did not recover within a stipulated time was considered dead. Tests were also conducted on both free-flying and resting mosquitos with maintaining a household temperature at lab.

Resistance was classified using World Health Organisation guidelines. If 90 per cent of mosquitos recover after exposed to standard mosquito repellents, it means those mosquitos have developed resistance to the insecticide.

Tests were also carried out exposing Australian mosquitos to insecticide, and these mosquitos did not recover after 30 minutes of post-exposure, and died.

On the other hand, 74 per cent of free-flying and resting mosquitoes from Dhaka colonies survived exposure to standard applications of pyrethroid aerosols in an experimental free-flight room, the study found.

This short-term efficacy of bottled insecticide does not work much to kill mosquitos because when these mosquitos bite people after 24 hours they may spread dengue virus, researcher Hasan Mohammad Al-Amin said.

Research has been conducted on efficacy of insecticide on mosquitos in Dhaka and this is not the scenario outside the capital. IEDCR advisor Mohammad Mushtuq Husain said it may happen that mosquitos are not resistant to aerosol in other areas. Yet, this research is very significant.

The Plant Protection Wing of the Department of Agricultural Extension gives approval to various mosquito repellents and insecticides used in households, but these products were sent to IEDCR to test their efficacy.

IEDCR director general professor Tahmina Shirin told Prothom Alo, “We check the efficacy of the products sent by the Plant Protection Wing. It is not our duty to see whether mosquitos become resistance to these products. Whether the manufacturing companies supply products to markets similar to the samples that they provide us, is not monitored properly, therefore, mosquitos may become resistant to these insecticides.”

Tahmina Shirin, however, said if 90 per cent of mosquitos do not become lifeless after using insecticides, then those insecticides are not considered effective.

Plant Protection Wing director Faridul Hasan told Prothom Alo, “We, in fact, approve mosquito repellents, but we send those to IEDCR to test their efficacy. We, however, do not check whether mosquitos have become resistant to those insecticides.”

This report appeared in the print and online edition of Prothom Alo and has been rewritten in English by Hasanul Banna