10.2 How does IRS reduce the mosquito population?
In Study Session 5, you learned that mosquitoes enter houses to take blood from humans, mainly at night. Mosquitoes do not fly for long after feeding, as the blood meal they take is more than twice their unfed body weight and they need to spend some time resting. Following a blood meal, the female mosquitoes tend to rest in undisturbed sites for two to three days until their eggs develop and are ready for laying. (Remember that the males do not take blood meals and so are not vectors of malaria.) Understanding the resting habits (the preferred resting places and behaviour) of the malaria vectors is extremely important for IRS programmes.
In drier regions, rooms inside houses are important resting places for mosquitoes because they prefer humid environments and it is usually more humid indoors. In humid forested areas mosquitoes may also rest in vegetation outdoors. However, even species that normally rest outdoors enter houses to feed and will spend some time resting indoors after feeding.
If the inside of a house has been sprayed with insecticide, when mosquitoes rest in the house they come into contact with the residual (long-lasting) insecticide sprayed on walls and furniture, and they die within a few hours.
Parasite development inside a female mosquito takes about 10 days and mosquitoes feed and lay their eggs every two to three days. So they may have to bite people three to four times before the parasite develops fully and they are able to transmit the infection. Every time a mosquito visits a sprayed house to feed on people, it is at risk of coming into contact with the insecticide and dying.
Mosquitoes resting on sprayed walls come into contact with insecticide through their feet and are killed. Some insecticides also irritate mosquitoes and cause them to leave houses before biting. In dry or windy areas, this may also result in death due to lack of suitable outdoor resting places.
Wall-spraying may not prevent biting. Hungry mosquitoes entering a house often bite first and then rest on walls and furniture inside houses. As most anopheline vectors of malaria enter houses to bite and rest, malaria control programmes have focused primarily on the indoor application of residual insecticides to the walls and ceilings of houses.
Indoor residual spraying is one of the primary vector control interventions for reducing and interrupting malaria transmission and one of the most effective methods. The primary effects of IRS towards reducing malaria transmission are:
- It reduces the life span of vector mosquitoes, so that they cannot live long enough to transmit malaria parasites from one person to another.
- It reduces the density/number of the vector mosquitoes.
- Some insecticides used in IRS also repel mosquitoes and by so doing reduce the number of mosquitoes entering the sprayed houses and thus reduce human-vector contact.
However, note that IRS may not provide individual protection – people sleeping in sprayed houses may still be bitten by mosquitoes. Unlike insecticide treated nets (ITNs are the subject of Study Session 11), which provide individual protection from mosquitoes, the aim of IRS is to provide community protection.