Increasing Insecticide Resistance Challenges Malaria Fighters

January 23, 2003
By Michelle Vachon
The Cambodia Daily

After half a century of using insecticides, malaria-carrying mosquitoes have developed resistance to so many products in India that the country is facing a major problem to control the disease.

Insecticide-resistant mosquitoes have become a threat in all malaria-prone countries using insecticide in their control programs.

In Cambodia, the National Malaria Center has been monitoring resistance to bednet insecticide in Ratanakkiri province. Results last year indicated a slight increase in mosquitoes' tolerance to the product.

Six species of mosquitoes are responsible for most cases of malaria in India, and each of them has its own resistance pattern, reported K Raghavendra and SK Subbarao of the Malaria Research Center in Delhi during the Mekong Malaria Symposium, which took place in December in Siem Reap.

The An Culicifacies species, which accounts for 60 percent to 70 percent of the cases, they said, "has developed resistance to all groups of insecticides used so far in public health."

Resistance seems to have followed a six- to seven-year cycle. For example in the early 1980s, the government sprayed malathion indoors to combat a malaria outbreak in the Sonipat district of Haryana state. This was successful.

However by 1989, mosquitoes started reappearing they now could survive that insecticide, wrote Raghavendra and Subbarao in their conference summary.

"The continuation of malathion spray [after the malaria epidemic had been curbed] rendered malathion ineffective for future use," they said. "Ideally, DDT should have replaced malathion after the control of the outbreak, and malathion could have been a reserve insecticide for future use."

Insecticides for malaria control started in the 1940s in India, when DDT was sprayed in military camps and some civilian areas. Results were so spectacular that insecticide became part of the country's malaria control program in 1953, said Raghavendra and Subbarao. Farmers also began using pesticides.

The combined effect of agricultural and public-health spraying made insecticides ineffective. When mosquitoes built resistance to one product, Indian health authorities tried another, which would solve the problem for a few years. But this led mosquitoes to develop resistance to an array of products, Raghavendra and Subbarao said.

So far, India's approach has been to react rather than prevent insecticide resistance, they said. The experience of the last decades clearly shows that insecticide use must be carefully planned.

"Efforts should be made to design strategies which delay or restrict the onset of resistance, and this is only possible by decreasing the continued selection pressure of a given insecticide," Raghavendra and Subbarao said. In addition, different formulas of insecticides should be used on a rotation basis, they said.