Editors' ChoiceMalaria

Malaria gives mosquitoes the munchies

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Science Translational Medicine  13 May 2015:
Vol. 7, Issue 287, pp. 287ec76
DOI: 10.1126/scitranslmed.aab3975

The malaria parasite Plasmodium falciparum is transmitted from the mosquito vector to the human host when the female mosquito takes a human blood meal. Plant nectars are also an important mosquito food source, and certain plants are particularly attractive to Anopheles mosquito species because they produce molecules called terpenes, which are sensed by odor receptors on the mosquito’s antennae.

Terpenes give many plants their distinctive smells, such as d-limonene in citrus fruit and pinene in pine resin. In humans, terpenes are vital precursor molecules for several hormones and for cholesterol. Unlike animals, plants can make terpenes in subcellular compartments called plastids. Malaria parasites also contain a plastid, the apicoplast, which an ancestor assimilated from a symbiotic alga in the distant past. The apicoplast is thought to synthesize compounds vital to the malaria parasite’s survival and, in particular, its ability to infect human red blood cells (RBCs).

Given a choice between Plasmodium-infected and uninfected hosts, female Anopheles mosquitoes will bite infected hosts more frequently than uninfected hosts. In a new study, Kelly and colleagues hypothesized that, like nectar-producing plants, the plant-derived apicoplast of P. falciparum may be able to synthesize terpenes that act as chemoattractants for Anopheles mosquitoes. They found that several terpenes, including d-limonene and α-pinene, could be identified in the air around laboratory cultures of parasite-infected human RBCs, but not in the vicinity of uninfected RBCs or empty culture vessels. In addition, α-pinene could be extracted directly from the cultured parasites. Treatment of parasite cultures with fosmidomycin, a drug that interrupts plastid (but not human) terpene synthesis, dramatically reduced the amount of α-pinene that could be extracted, implicating the plant-derived apicoplast in its synthesis. Last, limonene and pinene were able to activate specific Anopheles odor receptors, suggesting that the mosquito antennae could detect the terpenes produced by P. falciparum in culture. The authors propose that the parasite may be mimicking nectar-attracting signals to entice the mosquito vector to preferentially feed on humans with malaria, ensuring transmission of P. falciparum from host to vector.

It is important to note that terpene production by P. falciparum was demonstrated in culture under laboratory conditions and has not yet been confirmed in humans infected with the malaria parasite. However, if the malaria parasite does generate mosquito chemoattractants in vivo, drugs that disrupt apicoplast function may not only render Plasmodium unable to reproduce, but also would reduce malaria transmission by making infected humans not quite as appealing to hungry mosquitoes.

M. Kelly et al., Malaria parasites produce volatile mosquito attractants. mBio 10.1128/mBio.00235-15 (2015). [Full Text]

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