Malaria

Alphonse Laveran, a French Army physician working in North Africa in the 1880s, was the first to observe malarial parasites in human blood. Their mode of transmission was not understood, however, until Ronald Ross, a British medical officer in India, found the organisms within the bodies of Anopheles mosquitoes. Malaria is caused by four species of parasitic protozoa: Plasmodium vivax, P. ovale, P. malariae, and P. falciparum. These organisms have complex life cycles involving several different developmental stages in both human and mosquito hosts. Present as infective sporozoites in the salivary glands of the mosquito, they are transferred by the mosquito's bite to the human blood stream, where they travel to the liver. There, each sporozoite divides into thousands of merozoites, which emerge into the blood once again and begin invading the host's red blood cells. This event triggers the onset of disease symptoms, as the merozoites consume proteins necessary for proper red blood cell function, including hemoglobin. The merozoites mature into the trophozoite phase and reproduce by division. As a result, many more merozoites are released into the blood when the host cell finally ruptures. In P. vivax and P. ovale infection, some sporozoites may delay their development in the liver, lingering in a dormant phase, but emerging later and causing the characteristic recurrence of symptoms.

The cycle of red blood cell invasion and parasite multiplication repeats itself many times during a bout of malaria. If the affected person is bitten by a mosquito, the insect takes up merozoites, which reproduce sexually within its gut. The cycle completes itself as the larval parasites pass through the gut wall and make their way to the mosquito's salivary glands, from whence they may again be transferred to a human host as sporozoites.