Growing evidence of insecticide resistance in malaria

By Henry Neondo, Yaoundé, Cameroon

An extensive study of malaria-carrying mosquito or “vector” populations in different ecological zones of Cameroon has documented widespread and varied resistance to insecticides, part of an alarming trend across Africa that might ultimately jeopardize efforts to control malaria with treated bed nets and indoor spraying.

The study, presented at a special session on insecticide resistance at the Fourth Multilateral Initiative on Malaria (MIM) Pan-African Malaria Conference, currently taking place in Younde, Cameroon, is illustrative of a growing body of research in Africa that is finding increasing mosquito resistance to pyrethroid insecticides, which are used for insecticide-treated bed nets (ITNs), and DDT, which has been used successfully in household spraying programs.

Scientists from the Institute of Medical Research and Study of Medicinal Plants (IMPM) and the Organization for Endemic Disease Control in Central Africa (OCEAC) in Cameroon believe their findings are likely applicable to the other malaria endemic regions of Africa because they emerged from analyzing mosquito populations at 32 different sites located across the country.

Cameroon is often viewed as “Africa in miniature” due to its diversity of climatic and ecological conditions.

The IMPM/OCEAC study illuminates how mosquitoes in different areas of Africa employ a variety of biological weapons to fight off insecticides.

For example, in the northern cotton and rice fields of Cameroon, malaria vectors rapidly detoxify or “metabolize” pyrethroids, effectively neutralizing their killing capability.

Meanwhile, in the country’s western tropical gardens, mosquitoes survived pyrethroids and DDT through what is known as a “knock-down” or KDR mutation, which is a genetic modification of the target site.

This mutation desensitizes the main target of DDT and pyrethroid insecticides—the mosquito’s nervous system. In coastal and inland equatorial areas of Cameroon, mosquitoes sometimes have both resistance mechanisms.

“The range of mechanisms involved in resistance is likely having an impact on the effectiveness of treated bed nets,” said Josiane Etang, the study’s lead author.

“Overall, we need to rethink management strategies, especially in areas where the KDR mutation and detoxification are coupled.”

She said further research is needed to understand which type of KDR mutation is most prevalent in Cameroon, as there are two varieties that have been identified in African mosquito populations. One is more common in the West, the other in the East, but both may be present in Cameroon.

Understanding precisely how mosquitoes in a particular area are resisting insecticides may allow for the selective use of formulations that can overcome these defenses.

“The potential for insecticide resistance to interfere with promising malaria interventions requires urgent attention,” said Andreas Heddini, the MIM Secretariat coordinator.

“But if we have scientists in the field throughout Africa regularly monitoring mosquito populations then we will have the up to date evidence we need to guide an effective response.”