Brussels, 10 Aug 2005
Mosquitoes are more attracted to people infected with the transmissible stage of malaria, according to a new study by French and Kenyan researchers. And it appears to be the malaria parasite itself that is responsible for this increased level of attractiveness.
The malaria parasite's ability to manipulate the behaviour of mosquitoes is already well documented. In the late 1990s, Jacob Koella from the Pierre and Marie Curie University in Paris showed that when mosquitoes first take up plasmodium (the cause of malaria), their motivation to bite humans is decreased. Dr Koella points out that at this stage in its lifecycle, the parasite needs time to develop in the mosquito before it is ready to move on to a human.
As biting is risky for a mosquito, such a change in behaviour increases both its and the parasites' chances of survival. Once the plasmodium in a mosquito becomes infective to humans, the biting rate of the insect becomes higher than normal, and researchers believe that it is the parasite itself that is responsible for orchestrating this change. 'Before the parasite is transmitted to a human, its only goal is to survive, and to help the mosquito survive,' according to Dr Koella.
In his latest study, funded by the French Ministry of Education and Research and published in the September edition of the open-access PLoS Biology journal, Dr Koella presents evidence for another way in which the malaria parasite manipulates its host for its own purposes - this time in humans. Scientists have often tried to assess whether humans infected with malaria could be more attractive to mosquitoes, but the results have been ambiguous. 'I think the main problem with the previous studies is that they couldn't really tease apart the effect of infection and the intrinsic differences in attractiveness among people,' explains Dr Koella.
To get around this, he and fellow scientists from the University of Nairobi in Kenya set up a chamber of uninfected mosquitoes and surrounded it with three tents. In one of the tents they placed a child infected with the transmissible stage of malaria, in another was a child in the non-transmissible stage, and in the third they placed an uninfected child.
They found that twice as many mosquitoes targeted the child in the transmissible stage of malaria. What's more, in order to establish that this result was not simply due to one child's innate attractiveness to mosquitoes, the scientists repeated the experiment with the same children after treating them with anti-malarial drugs. In this case, they found that none of the children was more attractive to the mosquitoes. 'The attractiveness was coming from the parasite, and not something intrinsic to the children,' explained Dr Koella.
Again, the scientists believe that the observed difference in attractiveness of the children to mosquitoes was being caused by the malaria parasite. In its paper in PLoS Biology, the team states: 'The mechanism underlying this manipulation is unknown, but it is likely that the parasites change the infected individuals' breath or body odour, as these are involved in attracting mosquitoes at the distances involved in our experiment.'
Some experts have given a cautious welcome to the findings, stressing that Dr Koella and his colleagues drew their conclusions from the results of only 12 such experiments. But while it is too early to say whether the results will lead to new approaches to combating malaria, Dr Koella believes that the experiment has enhanced our understanding of a disease that claims more than one million lives each year. 'Scientists used to see the mosquito as a syringe that moves the parasite from one human to the other. The fact that the parasite manipulates the mosquito to this extent can help to explain the incredibly intense transmission of malaria,' he concluded. To download a copy of the full PLoS Biology paper, please click here