In Africa, 20 per cent of the children get 80 per cent of the bites from malarial mosquitoes, and an understanding of this could be central to controlling the deadly disease.
Researchers have developed a mathematical model that describes the complex relationship between the proportion of people who are infected with Plasmodium falciparum, the parasite that causes malaria, and the rate at which people are bitten by the mosquitoes that carry it.
Some people are bitten more than others because they live where mosquitoes are more common or because the mosquitoes, for various reasons, find them more attractive. Those who are bitten most often play a role in malarial transmission similar to that played by the most sexually active in the transmission of sexually transmitted diseases: they are the ones who spread the disease.
The people who are bitten most become infected and stay infected.
Then the heavy biting continues, so large numbers of mosquitoes acquire the parasite from their blood and can transmit it to others.
So, the total burden of disease is influenced by a small minority of people.
By applying the mathematical model, the researchers found:
Selective biting and susceptibility to infection play an important role in determining the proportion of people infected.
But immunity to infection in early childhood does not. This may be because people who are immune can nevertheless still infect others.
The model also shows that it is difficult to reduce the proportion of people who are infected just by reducing the number bitten:
Cutting the rate of biting in half would reduce the number of people infected by only 4 per cent, the researchers concluded.
Halving it again would reduce the number by 5 per cent more.
Sources: Nicholas Bakalar, Beating Malaria Means Understanding Mosquitoes, New York Times, December 13, 2005; and D.L. Smith, et al., The entomological inoculation rate and Plasmodium falciparum infection in African children, Nature, November 24, 2005.
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