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People whose blood contains infectious malaria parasites attract more mosquitoes, say researchers.

Uninfected people or those who have the parasite in a form that cannot be passed onto others are less attractive.

The French and Kenyan researchers suggest that the parasites can manipulate sweat, breath or body temperature to make their human hosts more attractive to mosquitoes.

Led by Jacob Koella of the Pierre and Marie Curie University, France, they tested this idea by comparing three groups of 12 children, some of whom had naturally been infected by malaria.

One group had the infectious form of malaria parasite in their blood. The second group also had malaria parasites, but not at the infectious stage, and the third group had not been infected.

In a series of experiments, the researchers asked one child from each group to sleep in a separate tent. All three tents were linked to a central chamber containing mosquitoes.

A fan in the central chamber drew the odours from each child towards the mosquitoes, which could fly towards any of the children but were prevented from feeding on them by traps.

The researchers found that, on average, twice as many mosquitoes were attracted to the children with the infectious parasites than were attracted to children in either of the other two groups.

To make sure that the finding was not caused by differences in the children's odours that were unrelated to the parasite, the researchers then treated all infected children with antimalarial drugs.

Two weeks later the researchers ran their experiment again. They found that the mosquitoes were equally attracted to all three groups of children.

Previously, Koella had looked at how carrying malaria influences the biting behaviour of mosquitoes.

He showed that mosquitoes carrying parasites that have not become infectious tend to bite less.

"Biting is risky, and the parasite wants the mosquito to stay alive," explains Koella.

Mosquitoes carrying parasites that are ready to infect humans, in contrast, tend to bite more often. This makes it more likely that the parasite will be passed on to humans or other mammals, where it can reproduce.

Link to full PLoS Biology paper

Reference: PLoS Biology doi:10.1371/journal.pbio.0030298 (2005)

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