What's plan B?

By Sudeep Neelakantan
Published: Tuesday 31 March 2009

Mosquitoes are getting better at evading pest control

for long mosquitoes have been resisting pest control. They are only getting better at it. From 1977 to 1997, India spent more than a quarter of its health budget on malaria control. Yet it remains a formidable public health challenge in the country.

A Nigeria-based study by scientists at the Liverpool School of Tropical Medicine focuses on the mosquito Anopheles gambiae, one of the most important vectors of malaria in Sub-Saharan Africa. The parasite (Plasmodium sp) this vector carries, causes up to 3 million deaths worldwide. To make matters worse, this vector has turned resistant to pyrethroids in more ways than one. Pyrethroids are the chemicals used as insecticides.

Mosquitoes resist pesticides via metabolic resistance or knockdown resistance (kdr). The first occurs when insects develop internal enzymes, such as monooxygenase, to break insecticides down into non-toxic forms. Altered target-site resistance or knockdown resistance (kdr), is brought about by a structural mutation of the targeted site.

Samples of the mosquito were collected from two places in Nigeria: Ipokia, where resistance to pyrethroids has been reported and Alakia, where they are still susceptible. Half of the Ipokia sample was exposed to an enzyme inhibitor and pyrethroids (permethrin or deltamethrin), while the other half was exposed to pyrethroids alone. The Alakia sample was exposed only to pyrethroids.

The Alakia samples were 100 percent susceptible. The Ikopia strain of both halves showed 58 per cent mortality to permethrin and 72 per cent to deltamethrin. This showed how resistant Anopheles gambiae has become to insecticides in Ipokia. On exposure to the enzyme inhibitor, it used the kdr mechanism to save itself. The sample not exposed to the inhibitor revealed high levels of monooxygenase as compared to the Alakia sample. Enhanced enzyme levels led to enhanced resistance. There is much insecticide pressure on mosquitoes from Ipokia than Alakia and this could be the reason for the observed difference, said Awolola Samson, one of the authors of the paper.

This study, published online in the September 2008 issue of Transactions of the Royal Society of Tropical Medicine and Hygiene, calls for strategies to curtail pyrethroid resistance.

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