The mosquito's ability to spot malaria pathogen

The mosquito's ability to spot malaria pathogen
The American Naturalist carried a study by Joel G Kingsolver, University of Washington, USA, in December 1987 that stated infected and non-infected mice showed clear preference of the mosquito for the infected host, even showing that the preference was high when the infection was at the most infective stage. Similar preference has also been shown by mosquitoes for lambs infected with the Rift Valley Fever, which is spread by mosquito bite, or by tsetse flies for the trypanosome parasite.

In the case of malaria, the journal, PLoS-Biology (September 2005) published an experiment with western Kenyan children in 12 groups of three children each, and the falciparum strain of malaria. One child in each group was uninfected, the second had the infection in the early, non-infective stage, and the third in the most infective stage. The children harbouring the infective stage of the parasite attracted twice the mosquito bites compared to  other children. The trial was repeated after the infected children had been cleared of the infection. It was found that the mosquitoes showed no preference at all, not even for the children who earlier had the parasite at the infective stage. Apart from bare preference, studies have also shown that the presence of infection in the host hastens the location and draining of a blood source. In trials with malaria infected and healthy mice, it was found that the time of feeding was at least a minute shorter in malaria infected subjects.

Theoretical analysis of the feeding
behaviour and the mechanics of transfer of blood from the body of the host to the mosquito suggests that the process would benefit from a reduction of the red blood cell count. Malaria infection is known to have this effect on the blood, a condition known as haematocritis. The mosquito would hence enjoy foraging a malaria-infected person rather than one who wasn’t. It is only the female mosquito that bites humans or some other mammals for blood, as she needs the protein for her reproductive function.

Calling out the mosquitoes
The question that arises is how does the mosquito come to know which person is  infected with malaria, and the infective stage? Or, what is the mechanism by which mosquito bite infected people more often? The answer appears to lie in an evolutionary adaptation by the malaria pathogen, to induce a signal by the host to attract the mosquito.

There are other instances of such adaptation by viruses, to bring about plant features that attract insects, and promote spread of the virus. The features could be odours or even changes in the structure of leaves or petals, to signal insects using light waves.

In the same way, it appears that the malaria pathogen has adapted to induce, in the host, the features that benefit the mosquito. It also gives a signal, which may be the emission of a vapour, that tells the mosquitoes that there are advantages in feeding where the signals come from.

It may be the same kind of adaptation used by the Rift Valley virus or the trypanosome pathogen. In the case of malaria, it is evident that the signal to attract mosquitoes cannot be too strong, as the mosquito would then bite none but infected persons and that would soon be the end of the pathogen, and possibly, of the mosquito too.

While the ratio of the preference of mosquitoes for infected and susceptible hosts is one factor that affects the rate at which malaria spreads in a community, there are other factors, including the speed with which the malaria pathogen reproduces within the body of the mosquito within her lifetime.

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