Exflagellating O I microgametocyte O

^ Microgamete x^

V rirtlyi

FIGURE 12.30 Life cycle of Plasmodium vivax in human and Anopheles hosts. (Modified from Strickland, 1991.)

cells release saliva into the skin prior to ingesting blood. The period of time between ingestion of gametocytes and infection of the salivary glands with sporozoites is the extrinsic incubation period.

In the human host, sporozoites migrate in the blood to the liver within minutes of entering subdermal capillaries. They invade liver parenchymal cells, where they typically form a primary tissue meront. In the case of P. vivax and P. ovale they may form a hypnozoite, a quiescent or resting stage of the parasite. Inside each meront, merozoites develop through the process of exoerythrocytic merogony. The meront then bursts, releasing merozoites into the bloodstream, where they circulate and invade red blood cells. The merozoites form meronts in the red blood cells, where they produce more merozoites through a process called erythrocytic merogony. An invasive merozoite, once inside a red blood cell, transforms into a trophozoite, which utilizes hemoglobin for nutrients, then into a segmenter form distinguished by dark dots of heme in the red blood cell, and finally into a mature meront, which produces still more merozoites. Merozoites released from infected red blood cells invade new red blood cells, where the process of erythrocytic merogony begins anew. Other invasive merozoites form microgametocytes or macroga-metocytes within the blood cells, which will mature into microgametes and macrogametes if they are ingested by Anopheles mosquitoes during blood feeding.

Clinical Disease

Malaria is characterized by sudden paroxysms of fever and chills, which recur at highly predictable intervals, often in the afternoon. Other acute symptoms include headache, lethargy, fatigue, and profuse sweating after each bout of fever. After infection of erythrocytes by merozoites, erythrocytic merogony leads to the synchronous rupture of the erythrocytes and release of new merozoites, toxins, and heme digestion products. This event in the circulatory system prompts each episode of chills and fever. The next episode occurs in 48 or 72 hr, depending upon the species of Plasmodium.

Malarial infections in humans can result in severe illness and sometimes death. However, the particular symptoms, including timing and severity, vary with the species of Plasmodium. The most severe form of malaria is caused by P. falciparum, whose merozoites invade both young and old red blood cells. Over time, repeated reinvasions and mass destruction of red blood cells may lead to high parasitemia, severe anemia, and anoxia of tissues. In some cases, hemolysis results in a condition of hemoglobinuria, blackwater fever, when the urine contains hemoglobin and turns reddish brown. Toxins from dead red blood cells stimulate macrophages to produce chemicals such as tumor necrosis factor and other cytokines, which cause characteristic malaria symptoms such as fever. In falciparum malaria, infected red blood cells stick to the vascular epithelium of capillaries in organs including the brain, impeding blood flow and causing a serious and sometimes lethal condition called cerebral malaria. Because of this affinity for internal organs, only very young trophozoites and gametocytes are common in peripheral blood. Malaria caused by P. falciparum is called malignant tertian malaria because of the severity of symptoms and because of the typical 48-hr interval between paroxysms. The term "tertian" for a 2-day cycle originated from counting the day when the paroxysm occurs as the first day, so that the next paroxysm occurs on the third. Left untreated, nonfatal infections with P. falciparum last 5 months or more, depending on the immune status of the individual.

P. vivax malaria is called benign tertian malaria because symptoms are less severe than P. falciparum malaria, and death rarely occurs. Paroxysms occur on a 48-hr cycle. In this type of malaria, the merozoites invade only immature red blood cells, called reticulocytes, which typically comprise less than 6% of the total red blood cell count in circulation. Thus vivax malaria, compared to falciparum malaria, has less severe symptoms of anemia and toxemia, making death unlikely. The infected red blood cells do not stick to the epithelial lining of capillaries as they do in falciparum malaria. Vivax malaria can evolve into chronic infection with development of an enlarged spleen, or splenomegaly. However, persons infected with other malarias also may have enlarged spleens, as these organs work to replace red blood cells lost to infection. The hypnozoite stage of P. vivax provides a mechanism for the parasite to overwinter in humans in temperate areas with short transmission seasons. The period between infection and onset of symptoms can last up to 9 months, and untreated infection persists in the body for many months to many years, with relapses recurring at irregular intervals after initial infection and acute onset of disease.

P. ovale is an uncommon tertian malaria with milder symptoms. Its course of infection is similar to that of P. vivax.

P. malariae, which causes quartan malaria, differs from P. vivax and P. falciparum in that the parasites invade only mature erythrocytes. Therefore, symptoms can be more severe than in vivax malaria in the acute phase. However, infections tend to develop more slowly and become chronic. Malaria caused by P. malariae has a 72-hr erythrocytic cycle. The term quartan refers to the 4 days included within one cycle, with a 3-day interval from the beginning of the first paroxysm to the beginning of the next. Recrudescences of P. malariae may occur in individuals up to 50 years after initial infection, owing to low levels of parasitemia that increase under periods of immunosuppression .

After a person is bitten and inoculated with sporozoites and exoerythrocytic merogony commences in the

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