Most malaria affects 'unlucky few'
Some people are more susceptible because of their smell or location.
Most cases of malaria hit a relatively small proportion of a given population, according to an analysis of health data from sub-Sarahan Africa. Identifying these high-risk people could lead to better use of public-health resources, say the authors of the study.
Just 20% of people in a given area account for 80% of all infections, says a research group led by David Smith of the US National Institutes of Health in Bethesda, Maryland. The next problem is to work out just who they are.
There are some obvious factors that make people more susceptible, such as living in poor homes without nets, close to stagnant ponds where mosquitoes tend to breed, Smith says. But other factors, such as the chemicals given off in breath, sweat and dirty linen may also play a role that is not yet well understood.
"People often say, 'I'm one of those people that mosquitoes love to bite'," Smith adds. "Now we've shown that it's worth taking this biting pattern seriously."
The analysis, which uses existing data on malaria incidence in more than 90 populations of African children, shows the extent of this person-to-person variation. And the results, reported in Nature1, could help to direct efforts to combat malaria, Smith argues. "If we can find these people we can target them," he says.
Malaria currently infects some 300 million people a year, mostly in sub-Saharan Africa. It has proven difficult or impossible to distribute enough drugs, vaccines and mosquito-proof bednets to everyone at risk.
Smith argues that identifying the most vulnerable should help to concentrate resources where they are needed. But others argue that the best way to do this is to target efforts at those who present themselves with malaria, rather than searching for a mysterious sub-section of the population who are most at risk.
The patterns spotted by Smith's group might be useful in urban areas, says Charles Delacollette, a coordinator in the World Health Organization's Roll Back Malaria Department. Here, the at-risk 20% will probably be identified as those living in poor housing, close to known sites of mosquito breeding, he suggests.
But in remote villages, he argues, the main problem is simply getting supplies to the villagers at all, let alone determining the 20% within each community who are most at risk. "Those most at risk are those who cannot access health services. This is where the major problem is," he says.
Smith counters that the most effective protections against malaria are expensive. So identifying the people most in need of it will be important in reducing the overall public-health burden of malaria, he argues. "If we get these people right we'll be protecting everyone else too."
- Smith D. L., Dushoff J., Snow R. W.& Hay I.. Nature, 438. 492 - 495 (2005).