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Malaria transmission

Has the ignition key been found?

Nature volume 392, pages 227–228 (1998)Cite this article

The events of gametogenesis were classically termed ‘exflagellation’, from the formation of the vigorously motile male gametes out of the previously quiescent male gametocytes. This visually striking phenomenon has fascinated malariologists since the discovery of parasites themselves by Alphonse Laveran while, as it happens, observing the very process of exflagellation in malarious human blood2. Laveran's discovery was possible because exflagellation occurs not only in mosquitoes, but also spontaneously on exposure of gametocyte-infected blood to air, and it is readily seen in a drop of gametocyte-containing blood viewed on a glass slide under a microscope.

Thus, it seemed self-evident that exposure to ambient atmospheric conditions — as when a few microlitres of blood are ingested by a mosquito — must involve all the signals necessary to initiate gametogenesis. Indeed, experiments done in the 1950s and 1970s seemed to show precisely this3,4,5,6. The mechanism uncovered consisted of a bicarbonate-ion-dependent rise in pH, from a non-permissive pH of 7.4 in the host circulation, to a fully permissive pH between 8.0 and 8.3. This rise in pH is due to a fall in the concentration of carbonic acid, achieved by the loss of CO2 from the blood on exposure to air. Temperature was also crucial. Whereas at host body temperature exflagellation was completely suppressed, a fall by as little as 2-3 °C below body temperature was fully permissive7.

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Figure 1: The malaria cycle.

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  1. the Institute of Cell, Animal and Population Biology, University of Edinburgh, EH9 3JTM, Edinburgh, UK

    Richard Carter & Lisa Ranford-Cartwright

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  1. Richard Carter
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  2. Lisa Ranford-Cartwright
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Carter, R., Ranford-Cartwright, L. Has the ignition key been found?. Nature 392, 227–228 (1998). https://doi.org/10.1038/32529

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