Malaria

Classification

The Centers for Disease Control and Prevention (CDC) classifies malaria as a biosafety level three—it is considered a serious or potentially deadly disease requiring specific safety equipment and procedures. Malaria is listed as ICD-9 084. Malaria is caused by five Plasmodium species, which include Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale, and Plasmodium knowlesi (monkey malaria, rare in humans). Plasmodium falciparum is the most severe and lethal of the species, while Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale cause mild disease and rarely death. The most common species are P. falciparum and P. vivax.

Transmission and Incubation

Malaria is principally transmitted by bites from female Anopheles mosquitoes. There are more than 480 species of Anopheles, and every continent, except Antarctica, has at least one species. Approximately 100 of the Anopheles mosquitoes are able to transmit malaria, but only about 40 commonly do. Anopheles gambiae, which lives mostly in sub-Saharan Africa, is the best known mosquito that transmits the parasite P. falciparum, which causes severe illness in humans.

Anopheles species need a blood meal from a vertebrate to nourish their offspring. The preferred meal is from humans (anthropophily) or animals (zoophily). Anopheles mosquitoes that are anthropophilic, such as A. gambiae and A. funestus, are better able to transmit the malaria parasites from human to human. Anopheles mosquitoes also feed indoors (endophagic) and at night (nocturnal) between 11 p.m. and 2 a.m., with peak feeding time being around midnight. Because the vector is a mosquito, the transmission is contingent on conditions being favorable for the mosquito, which are adequate rainfall, temperature, and humidity.

Transmission of the malaria parasite from the mosquito (vector) to human (host) and vice versa is necessary for the complicated life cycle of the parasite. All five Plasmodium species have a similar life cycle, with slight variation. For example, an infected female Anopheles mosquito injects [Page 586]Plasmodium sp. sporozoites into a human while feeding on them, which travel to the liver, attack liver cells, undergo asexual reproduction, and develop into schizonts. These schizonts mature after about 5 to 15 days and release thousands of merozoites, which exit the liver and enter the bloodstream. The merozoites attack red blood cells (RBCs) and undergo a second asexual reproduction stage, and after maturing, the infected RBCs (after 1–3 days) rupture repeatedly, causing more merozoites to be released into the bloodstream, which causes another asexual reproduction and multiplication. Some of the merozoites discontinue replicating and develop into gametocytes (i.e., sexual forms [male and female] of the parasite) and circulate in the bloodstream. When a mosquito feeds on an infected human, it takes in the blood meal (needed for reproduction) and the gametocytes. The gametocytes travel to the mosquito gut, where the human blood cells rupture, releasing the gametocytes, and they become male and female gametes. These gametes combine into diploid zygotes and develop into ookinetes. The ookinetes embed into the midgut of the mosquito and become oocytes and produce thousands of sporozoites; after 8 to 15 days, they mature and rupture, releasing the Plasmodium sp. sporozoites into her body cavity, where they travel to her salivary glands. The female mosquito can inject into a human or restart the parasites’ life cycle.

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