Plasmodium

Causative agent of malaria

■ Malaria, the most frequent tropical parasitosis, is also of medical significance in central Europe and other regions as a travelers' disease. The infection is caused by plasmodia (Plasmodium vivax, P. ovale, P. malariae, P. falciparum) transmitted by the bite of Anopheles mosquitoes. An infection initially presents in nonspecific symptoms (headache, fatigue, nausea, fever). Untreated malaria tropica (caused by P. falciparum) can quickly develop to a lethal outcome. Therefore, it is important to obtain an etiological diagnosis as quickly as possible by microscopic detection of the parasites in the blood, and to initiate effective treatment. Prophylactic measures are essential for travelers to regions where malaria is endemic (prevention of mosquito bites, chemoprophylaxis). ■

Occurrence. Malaria is one of the most significant infectious diseases of humans. According to the WHO (2000,2004), the disease is currently endemic in more than 100 countries or territories, mainly in sub-Saharan Africa, Asia, Oceania, Central and South America, and in the Caribbean. About 2.4 billion people (40% of the world's population) live in malarious regions. Fig. 9.16 shows the geographic distribution of malaria (WHO, 2000). The annual incidence of malaria worldwide is estimated to be 300-500 million clinical cases, with about 90% of these occurring in sub-Saharan Africa (mostly caused by P. falciparum). Malaria alone or in combination with other diseases kills approximately 1.1-2.7 million people each year, including 1 million children under the age of five years in tropical Africa. About 7000 cases of imported malaria were reported in Europe in the period from 1985 to 1995, whereby the data are incomplete.

Parasites. Four Plasmodium species infect humans and cause different types of malaria:

■ Plasmodium vivax: tertian malaria (malaria tertiana),

■ Plasmodium ovale: tertian malaria (malaria tertiana),

— Distribution of Malaria

— Distribution of Malaria

Geographical Distribution Ovale

Fig. 9.16 Status: 1999. (From: International Travel and Health, Geneva. World Health Organization, 2000.)

I I Malaria-free areas

□ Areas with limited infection risk

□ Areas with high infection risk

Fig. 9.16 Status: 1999. (From: International Travel and Health, Geneva. World Health Organization, 2000.)

■ Plasmodium malariae: quartan malaria (malaria quartana),

■ Plasmodium falciparum: malignant tertian malaria (malaria tropica).

These Plasmodium species can be identified and differentiated from each other by light microscopy in stained blood smears during the erythrocytic phase of the infection in humans (Fig. 9.18, p. 524). A reduced apical complex and other characteristics of apicomplexan protozoa are recognizable in various stages of the organism (sporozoite, merozoite, ookinete) on the electron microscopic level (see Toxoplasma, p. 509).

Life cycle. The life cycle of malaria plasmodia includes phases of asexual multiplication in the human host and sexual reproduction and formation of spor-ozoites in the vector, a female Anopheles mosquito (Fig. 9.17). The developmental cycle within the human host is as follows:

■ Infection and exoerythrocytic development. Humans are infected through the bite of an infected female Anopheles mosquito that inoculates spindleshaped sporozoites (see below) into the bloodstream or deep corium. Only a small number of sporozoites are needed to cause an infection in humans (about 10 P. falciparum). Within about 15-45 minutes of inoculation, the sporozoites of all Plasmodium species reach the liver in the bloodstream and infect hepatocytes, in which asexual multiplication takes place. In this process, the sporozoite develops into a multinuclear, large (30-70 im) schizont (meront) described as a tissue schizont. Following cytoplasmic divi-

sion 2000 (P. malariae) to 30 000 (P. falciparum) merozoites are produced. This development takes six (P. falciparum) to 15 (P. malariae) days. Shortly thereafter, the tissue schizonts release the merozoites, which then infect erythrocytes (see below). In infections with P. vivax and P. ovale, sporozoites develop into tissue schizonts as described above, but some remain dormant as so-called hypnozoites, which may develop into schizonts following activation after months or years. Merozoites released from these schizonts then infect erythrocytes, causing relapses of the disease (see p. 527).

■ Erythrocytic development. The merozoites produced in the liver are released into the bloodstream where they infect erythrocytes, in which they reproduce asexually. The merozoites are small, ovoid forms about 1.5 im in length that attach to receptor molecules on the erythrocyte surface. These receptors are species-specific, which explains why certain Plasmodium species prefer certain cell types: P. malariae infects mainly older erythrocytes, P. vivax and P. ovale prefer reticulocytes, and P. falciparum infects younger and older erythrocytes. Following receptor attachment, merozoites penetrate into the erythrocyte, where they are enclosed in a parasitophorous vacuole.

A Plasmodium that has recently infected an erythrocyte (<12 hours) appears ring-shaped with a thin cytoplasmic rim in a Giemsa-stained blood smear. Also visible are a central food vacuole and the dark-stained nucleus located at the periphery of the parasite. This stage is very similar in all four Plasmodium species (Fig. 9.18, p. 524). The ring forms develop into schizonts, which feed on glucose and hemoglobin. The latter is broken down to a brownish-black pigment (hemozoin)-after the amino acids used by the plasmodia are split off—and deposited in the parasite's food vacuole as "malaria pigment." The schizont undergoes multiple divisions to produce merozoites, in different numbers depending on the Plasmodium species (6-36). The merozoites enter

Fig. 9.17 a In humans: 1 sporozoite from infected Anopheles mosquito; 2 development in the liver; 2a primary tissue schizonts and schizogony in hepatocytes (all Plasmodium species); 2b hypnozoites and delayed schizogony in hepatocytes (P. vivax and P. ovale only); 3 further schizogenic development in erythrocytes; 3a development of sexually differentiated plasmodia (female macrogametocytes and male microgametocytes).

b in the Anopheles mosquito: 4 macrogametocytes and microgametocytes taken up by bloodsucking mosquito; 5 fertilization of macrogametes (round) by micro-gametes (long); 6 fertilized macrogamete (ookinete) in intestinal wall of mosquito; 7 oocyst with sporozoites in intestinal wall; 8 infective sporozoites in salivary gland (according to Peters W. Chemotherapy and Drug Resistance in Malaria. Vol. 1, London: Academic Press; 1987:16). "

Life Cycle Plasmodium Anopheles

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