Mosquitoes Showing Signs of Bednet Insecticide Resistance

September 12, 2002
By Michelle Vachon
The Cambodia Daily

Preliminary results of a malaria-related study show that mosquitoes have started developing a resistance to the insecticide used for bednets in Cambodia.

There is no need to change the insecticide at this point, said Sean Hewitt, malaria control specialist for the European Commission Malaria Control Project in Cambodia. But the situation must be monitored, he said.

Bed nets are impregnated with insecticide to kill mosquitoes and stop them from infecting people in their sleep.

The insecticide used in Cambodia is deltamethrin, a synthetic product based on the natural insecticide found in chrysanthemum and one of the safest insecticides, Hewitt said.

The research, conducted by the National Malaria Center with the support of the EC Malaria Control Project, took place in June and July 2002 in Ratanakkiri province, one of the country's areas most affected by malaria. It consisted of studying mosquitoes collected in two Jarai communities: Leu Kchun village, in which people use bednets, and Leu Touch village, in which people don't.

Catching the mosquitoes was hard work, Hewitt said. The collection team members used themselves as bait, putting a container over mosquitoes that had landed on their skin. They had to work at night, and were given preventative medicine to protect them against malaria.

Researchers ran tests to compare how many mosquitoes from each village would die and how long it took them to do so called "knock down time" when exposed to netting with and without insecticide.

The average knock-down time for insecticide was 15.29 minutes in the village that does not use bednets, and 20.42 minutes in the other one.

"This is the first indication that there might be mosquitoes within the population that can live through encounters with insecticide, and go on producing many insecticide-resistant progeny," Hewitt said. The situation will have to be carefully monitored to make sure that bednets don't lose their effectiveness in the fight against malaria, he said.

Mosquitoes develop immunity to insecticide because of their short life spans, Hewitt said.

A mosquito must live at least 12 days for parasites to turn it into a malaria carrier. The Anopheles dirus mosquito, which lives and breeds in the forests of Cambodia, may live several months, during which it may lay eggs dozens of times depending on the availability of human or monkey blood, Hewitt said.

This is why that specific mosquito is so efficient at transmitting malaria among the estimated 500,000 people who live in forested areas of the country, he said.

Frozen mosquito abdomens have been taken to the Liverpool School of Tropical Medicine in Britain for biochemical analysis. "This should confirm results of our bio-assays," Hewitt said.