Protozoans are widely spread in the world and live in very different environments: seas, oceans, fresh water and soil. Most protozoans have adapted to living in the bodies of plants, animals and humans. More than 1,000 species of pathogenic protozoans have been identified. More than 500 million people are infected with pathogenic protozoans.
Antimalarial Drugs
Malaria is the most widely spread protozoan infection. There are 4 kinds of plasmodia that cause malaria in humans:
• Plasmodium falciparum (causative agent of tropical malaria);
• P. vivax, P. ovale (causative agents of tertian malaria);
• P. malariae (causative agent of quartan malaria).
P. falciparum is the causative agent of the most severeform of malaria. When an Anopheles mosquito bites an infected person, the mosquito ingests Plasmodium gametocytes found in the human blood. Then in the mosquito?s body male and female gametocytes fuse, and further maturation of the resulting zygote takes place. The resulting sporozoites spread out in the mosquito?s organs; particularly, they enter the salivary glands. When the mosquito bites, sporozoites together with the saliva get into the human blood, then within 30 minutes they migrate to the liver, where they replicate with yielding tissue schizonts. Later infected RBCs burst releasing a new generation of merozoites. This ectoglobular stage is symptomless. Usually, after 1-12 weeks after the bite, merozoites start to escape from hepatic cells into the blood stream. Over 30,000 merozoites may emerge from one sporozoite. Merozoites enter RBCs, replicate asexually and form blood schizonts, defining the erythrocytic stage of the disease. Further on, the infected RBCs burst with an efflux of a new generation of merozoites. A part of merozoites matures to the stage of gametocytes, which, getting into the mosquito?s body during the bite, complete the development cycle (fig. 20.2). The symptoms of malaria are primarily defined by lysis of RBCs. At the same time, RBCs infected with P. falciparum change their properties, increasing their adhesive ability in respect of endothelium cells and thrombogenicity. The