Malaria is a pernicious public health problem in many areas of the world. Sub-Saharan Africa, where cases recorded represent over 90% of the world total, is particularly badly hit. Modelling by IRD scientists has revealed a core feature in the life-cycle of Plasmodium falciparum, the parasite responsible for the disease. Its gametocytes, the pre-gamete sexual forms, aggregate in clusters in human blood capillaries and, once ingested, keep this form until they reach the ideal breeding ground the mosquitos stomach provides. Conditions there favour encounters, and hence binding, between male and female gametocytes, thereby enhancing the parasites fertilization and reproduction capacity. Investigation of this behaviour should yield important information on both the parasites transmission to humans and on the way the disease develops.
Malaria, which infects 600 million people in the world and leads annually to 2 million deaths, is the most widespread of infectious diseases. The pathological agent is a microscopic parasite of the Plasmodium genus which develops inside the hosts erythrocytes. Plasmodia go through a series of asexual reproduction cycles before a transition takes place from asexual stages to production of sexual cells, the gametocytes or pre-gametes, in the host blood. The females of Anopheles, the mosquito vector, ingest blood and gametocytes during a nocturnal feed on human skin. The meal reaches the mosquitos stomach where Plasmodium sexual reproduction takes place. An encounter and subsequent binding between a male and a female gametocyte produces a zygote which will give rise to infectious forms. These migrate up to the mosquito salivary glands. From there they are transmitted to humans during a second blood meal.
Experimental gametocyte counts in the blood ingested by mosquitoes that had bitten volunteers naturally infected with Plasmodium falciparum showed that these sexual forms are overdispersed, in other words they have a heterogeneous distribution in the mosquito stomach. Their numbers vary between the different blood meals taken on the same volunteer, a feature previously observed in the case of large parasites (macroparasites), such as microfilariae (250 microns).
Bénédicte Robert | EurekAlert!
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