Tuesday, August 09, 2005

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.


Blogger Bill Hooker said...

But, but--! We can already clear the parasite load from an infected person, so there is clear benefit to someone who receives a transmission blocking treatment: they won't get RE-infected. That's frequently, if not usually, the case with parasitic diseases. The problem is not so much treatment as that treated patients are promptly re-infected. (I used to work on anti-schistosome vaccines... now those might be a waste of time, but that's another story.)

Besides, the analogy with vaccine freeloaders is a good one, and I don't think that the widespread propensity to freeload is usually considered an argument against vaccination. You're right that it's a consideration, but I think it can be overcome because the benefit to recipients (of the putative malaria transmission blocker) is easy to demonstrate.

8/09/2005 04:33:00 PM  
Blogger Andrew said...

I didn't mean to say that we shouldn't develop the transmission blocker, I was just worrying that it would be hard to implement from a public health perspective...

Re-infection: That's a good point, but it seems to me that the benefit is diffuse and low rather than direct and high. That is, if the infected person gets a dose of transmission blocker, it's not that he is prevented from getting re-infected, but that his chance of getting re-infected goes down very slightly insofar as there are now fewer infected mosquitoes flying around. So the benefit to himself is the same size as the benefit to other people - a tiny decrease in the chance of infection. It only works if everyone gets the blocker... hence susceptible to the freeloader problem (and worse than usual vaccines, which do actually prevent you from getting the disease). Am I missing something here?

8/10/2005 06:02:00 AM  
Blogger Bill Hooker said...

I think it depends on the efficacy of the putative malaria transmission blocker. It's not going to lower risk only by decreasing the number of infected mosquitoes, but also by lowering one's likelihood of being bitten by any mosquito, infected or not. (Unless all it does is eliminate the increase in attractiveness that goes with being infected -- but that seems unlikely to me.) So there is a direct as well as a diffuse (herd) benefit.

It seems clear, though, that something which works by decreasing one's attractiveness to the vector is not going to be as effective as a traditional vaccine (like the anti-sporozoite one -- if that had, you know, worked). So yes, the freeloader problem is worse -- but still not insurmountable, I think.

Actually, it's just occurred to me that there are other complications that traditional vaccines don't face. What happens if everyone dutifully gets the anti-attractiveness treatment? If all humans are less attractive than they were, by an equal measure, do the mosquitoes go back to random biting? Or do they start to prefer other meals, like livestock?

8/10/2005 07:21:00 PM  
Blogger Andrew said...

Oh right, of course. Thanks for pointing that out, and sorry for not taking your meaning at first!

Your last point is also a great one. My guess would be shifting to livestock, which also suggests that we'd have to be careful that the anti-attractiveness treatment also drives away infected mosquitoes, as I wouldn't be surprised if infected mosquitoes were programmed to prefer humans over other animals even more than they already do.

8/10/2005 07:56:00 PM  
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