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Malaria is big problem in the Solomon Islands and a team of AITHM researchers are working to understand why insecticide treated nets are one of the most effective control strategies.

Stunning vistas, tropical climes and friendly locals are probably what first comes to mind at mention of the Solomon Islands, a small nation in the South Pacific half way between Papua New Guinea and Vanuatu. But the archipelago – which is indeed beautiful, warm and welcoming – also struggles with malaria control and elimination.

Scientists at James Cook University’s Australian Institute of Tropical Health & Medicine (AITHM) are working with local communities to better understand mosquito behaviour to reduce the spread of malaria and improve public health outcomes.

Their research findings are significant: even though mosquitoes have changed their behaviour to avoid human interventions, insecticide treated nets are still one of the most effective strategies for reducing and eliminating malaria.

In the 1960s, a malaria intervention called ‘indoor residual spraying’ – spraying the inside of homes with insecticide to kill mosquitoes – was popular in the Solomon Islands because mosquitoes typically entered homes late at night. But mosquitoes are a challenging foe, and researchers soon realised they had adapted their behaviour to try to avoid the insecticide sprayed inside homes.

“The Solomon Islands is the first country in which behavioural resistance was shown, where the mosquito changes its behaviour to avoid the insecticide,” says Professor Tom Burkot, a vector biologist at AITHM who has worked in the South Pacific nation since 2000. “In this case, the mosquitoes shifted their feeding behaviour from feeding indoors late at night to feeding outdoors early in the evening.”

Thankfully, a more effective intervention was introduced in the mid-1990s that replaced indoor residual spraying in many homes in the Solomon Islands. The big advantage of insecticide treated nets – bed nets treated with insecticide that’s safe for humans and toxic for mosquitoes – is that they protect the families using the nets as well as their communities.

“With the nets, the mosquito lands on the net to try to feed on the person under the net,” says Prof Burkot. “Because the net has an insecticide on it, the net kills the mosquito before the mosquito can take a blood meal.

“This is different from indoor residual spraying because when you spray insecticide on the walls of houses, the mosquito will fly into the house and immediately bite someone before resting on the insecticide sprayed wall and being killed. Essentially, the insecticide on the homeowner’s walls protects the neighbours from malaria mosquitoes but not his family.”

Here’s where it gets particularly interesting. Despite this adaptive strategy of mosquitoes to change their behaviour to try to avoid insecticide on walls and, later, on nets – about 84 per cent of mosquitoes bite people outdoors before 9pm – Prof Burkot says the nets remain an effective strategy for reducing malaria transmission in the Solomon Islands.  

Over the last eight years, researchers from AITHM have been working with local experts to understand why insecticide treated nets are so effective. They conduct ‘mark recapture’ experiments where mosquitoes are marked with different colours depending on where they’re caught – yellow for inside a home and red for outside – so their movements can be tracked.

“The critical thing we found was that the populations of mosquitoes are not comprised of sub-populations – there's not a group of mosquitoes that's dedicated to feeding indoors and another group that's dedicated to feeding outdoors,” says Prof Burkot. “It's all one population.

“This is significant because whenever a mosquito tries to take a blood meal it's a bit random whether it's going to feed indoors or outdoors. While it's more likely that it will feed outdoors and early, our research found that every feeding cycle a small proportion will feed indoors and late, and therefore potentially be exposed to the insecticide in a net.”

As well as reducing the number of mosquitoes, insecticide treated nets change the age structure of the mosquito population. This is important because the malaria parasite takes 10 to 14 days to develop before the mosquito is able to infect humans with malaria.

“Using nets means the mosquitoes flying around are on average of a younger age than if you didn’t use nets,” says Prof Burkot. “If you can keep all of the mosquitoes from not living to be, say, 10 days old, then you'll have essentially have stopped malaria transmission. You don't have to kill them all – you've just got to keep them from surviving through this period of time in which the malaria parasite can be transmitted from a mosquito to a human.”

Since the introduction of insecticide treated nets in the Solomon Islands in the 1990s, the prevalence of malaria has dropped from 442 infected people per 1000 to only 83 infected people per 1000 in 2017.

“There's a tendency to oversimplify and say bed nets won't work because these mosquitoes are ‘outdoor’ mosquitoes, but the reality is bed nets do work and you can see that in the Solomon Islands,” says Prof Burkot.

Professor Tom Burkot


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