Of all the ways that have been tried to control malaria, one of the easiest could also be among the most effective, a new study suggests. Yet the idea of cutting back on the regular food supply of mosquitoes has been neglected, possibly because it just seemed too simple to work.
Malaria is transmitted when parasite-infected Anopheles mosquitoes drink human blood. However, this isn't the way mosquitoes get most of their energy. Normally, these pesky critters live on plant sugars – indeed, the males always do. The females use blood to provide the nutrients needed for their eggs to develop, but go back to plants the rest of the time.
To fight this devastating disease, we've tried everything from killing the insects with environmentally destructive chemical sprays to preventing them getting at us using bed nets. We've even tried removing the stagnant water in which they breed and vaccinating to stop the parasite while the mozzies go unmolested.
Dr Gunter Muller of the Hebrew University pointed out one possibility that had been ignored – cutting off the mosquitoes' access to food plants to limit numbers. Of course, removing all plants around population centers is highly undesirable, if not impossible. However, Muller noted that for mosquitoes, one plant is not as good as another. In the Bandiagra District of Mali, the mosquitoes appear to be getting most of their non-human energy supply from the nectar of the Prosopis juliflora shrub.
In Malaria Journal, Muller reports on a trial where the shrub was removed from three villages where it had been common. These were compared with three others where the P. juliflora was allowed to remain, and three more where it has never appeared at all.
In the villages where the shrubs were removed, the numbers of older female Anopheles mosquitoes (the ones where the parasite has had time to develop) fell by 69.4 percent. There was also a shift in the type of Anopheles mosquitoes, from multiple thriving species to domination by Anopheles coluzzii.
“This suggests that removal of the flowers could be a new way to shift inherently high malaria transmission areas to low transmission areas, making elimination more feasible,” Muller said in a statement.
P. juliflora is invasive in north Africa, being native to Central and South America, so its removal also contributes to bringing the ecosystem closer to its natural state. The thorny shrub interferes with agriculture and grazing, although it prevents desertification and is a good source of fuel wood.
The trial lasted just 18 days, and was conducted well away from peak malaria season. Nevertheless the lasting effectiveness of removal can be inferred from the fact that older females were also rarer at the villages where the shrub never took hold.
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