Manipulative malaria

Via Foreign Dispatches, Carl Zimmer reports a finding published in PLoS Biology that the P. falciparum malaria parasite actually makes infected humans smell more attractive to mosquitoes, thus increasing its chances of propagation.

They set up three tents, each large enough for a person to sleep in. A fan pumped air from the tents into a central chamber swarming with about 100 mosquitoes. Mosquitoes that were attracted to one of the tents would fly toward it, only to become stuck in a trap.

The researchers asked parents in western Kenya to allow them to test their children for malaria. For each round of the experiment, they chose one uninfected child in an early stage of infection and a child who was carrying gametocytes. The children slept for a few hours in the tents, and the scientists checked the traps to measure how many mosquitoes had been attracted to each child.

After studying 12 sets of children, the scientists discovered a striking pattern. "Gametocyte-infected children attracted about twice as many mosquitoes as either uninfected ones or ones infected with nontransmissible stages," Dr. Koella said. "The results really jump out."

Disturbing, yet fascinating. Of course, as a general strategy of infectious agents, this is no aberration - even the common cold virus has a "strategy" of increasing its transmission, by inducing coughing in its victims. Similarly for tuberculosis (coughing), cholera (diarrhea), rabies (aggression), etc.

To counter the optimistic note at the end of Zimmer's article, I have doubts about the practicality of blocking the mosquito-attracting effect. Obviously, the science is fascinating and we should learn all we can about how the plasmodium parasite functions - but even if we discover the mechanism by which the parasite makes people smell attractive to mosquitoes and can interfere with it, this will only stop the transmission to the next victim, not help or cure the presently infected person. Stopping transmission is great, of course, but depending on how risky/invasive the medical intervention would be, you might be hard-pressed to convince someone to undergo medical treatment that would bring zero benefit to them (other than fewer mosquito bites) and only a diffuse benefit to those around them. (This is a problem associated with transmission blocking vaccines in general.) Rather like the reason so many people these days are reluctant to get vaccines when they can just ride off the herd immunity of those around them.

Update, 12 August: In comments, Sennoma points out that there would be a direct benefit to the infected person, because fewer mosquito bites would translate into a lower risk of being reinfected during/after treatment.