Main Mosquito Borne Diseases

1.2.4.1 Malaria

Only Anopheles species are vectors of malaria parasites which affect nearly 50% of the world's inhabitants living in the 109 endemic countries, especially the poor developing countries (see Figure 1.6). The WHO evaluated that there are approximately 250 million cases of malaria annually, with nearly 1 million deaths, 90% in Africa, south of the Sahara which pays a heavy debt to this disease.15

Malaria is due to Protozoan parasites of the genus Plasmodium of which four species are primarily involved: Plasmodium falciparum, P. vivax, P. malariae, and P. ovale.11 Recent reports have suggested a possible fifth species, Plasmodium knowlesi, as an important and common emerging zoonotic pathogen responsible for human infections in Southeast Asia.18 Globally, P. falciparum is the most common cause of malarial infection, responsible for approximately 80% of all cases and 90% of deaths. Plasmodium transmission from the Anopheles vector to humans is accomplished by direct injection of the parasite at the sporozoite stage, contained in salivary gland fluid during blood feeding. Of the 484 recognized species of Anopheles, only about 15% or less are generally involved in malaria parasite transmission. From a biological point of view, the Anopheles mosquito is the definitive host for the parasite, where sexual reproduction between male and female gametes occurs, whereas humans are the intermediate hosts only where asexual multiplication (schizogonic cycle)

Figure 1.6 Malaria distribution around the world (Photo courtesy of John Libbey Eurotext).16

takes place. Anopheles females become infected by ingesting sexually mature gametocytes present in the peripheral blood of the host. In the mosquito midgut fertilization produces the ookinete which traverses the mosquito gut and forms an oocyst under the outermost layer of the gut wall. After repeated multiplication of sporoblasts, each oocyst eventually ruptures, releasing hundreds of sporozoites into the mosquito body cavity, a proportion of which will invade the salivary glands awaiting the opportunity to infect another human upon the next blood feeding by the mosquito. This ''sporogonic phase'' (from gametocytes ingestion to infective sporozoite) takes on average 8-14 days in tropical conditions depending on the ambient temperature for Plasmodium falciparum or P. vivax, and much longer for P. malariae or P. ovale. Sporogonic development does not occur at temperatures below 16 or 18 °C. Infective female mosquitoes will generally remain infectious during their entire life and can therefore inoculate sporozoites at each blood feeding.17

Control: The current recommended treatments for malaria, namely artemi-sinin-combination therapies (ACT), Intermittent Presumptive Treatment (IPT) of pregnant women, together with vector control using primarily insecticide-treated nets (ITN), long-lasting insecticidal nets (LLINs), or indoor residual spraying (IRS) remain as effective methods for controlling malaria when used properly. Larviciding can be used in special situations (well known breeding sites, mainly man-made) and biological or genetical methods are still not operational in spite of great advertising.19

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