The challenge of insecticide resistance
Insecticides are an essential tool in the battle against malaria. There are four classes of insecticide that are used in malaria vector control. Pyrethroids are the most commonly used since these are the only insecticides recommended by WHO for use on insecticide treated bed-nets. Organophosphates, carbamates and the organochlorine DDT are also used in other vector control interventions, most commonly indoor residual spraying. Mosquito populations can become resistant to these insecticides through a number of mechanisms:
- Changes in how well or how quickly the mosquito detoxifies the insecticide – ‘metabolic resistance’.
- Changes to the insecticide’s intended target site within the mosquito, which mean the insecticide can’t bind as effectively – ‘target site resistance’.
- Changes to the insect’s cuticle or digestive tract, which mean the insecticide can’t penetrate as easily – ‘penetrative resistance’.
- Changes in the behaviour of the mosquito population leading to insecticide avoidance, For example an early biting strain of a vector species might assume dominance over a late biting strain because it’s behaviour allows it to avoid contact with lethal insecticide treated bed-nets.– ‘behavioural resistance’.
Resistance to one insecticide in a class generally confers some level of resistance to the rest of that class of insecticides. Cross-resistance across classes also exists. Depending on the type of resistance, mosquitoes resistant to pyrethroids may also be resistant to DDT, and vice versa; or to organophosphates, and vice versa.
Resistance comes about when mosquito populations are exposed to considerable levels of insecticide, giving a strong competitive advantage to any mosquitoes which are not killed off. Following this, resistance can rapidly spread. Use of insecticides is often widespread in agriculture and in the past it has often been agricultural use of insecticides that has led to insecticide resistance in mosquito vectors. Limiting agricultural use of insecticides in the interest of insecticide use for public health can be useful; however large scale public health use of insecticides can also lead to resistance.
Various strategies have been developed to limit the development of insecticide resistance. They include: rotations of insecticides, use of interventions in combination and mosaic spraying. The use of insecticide mixtures is also being investigated.
ADDITIONAL RESOURCES
on the prevention of malaria