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to prevent the development of symptomatic malaria caused by the asexual erythrocytic forms. Also, none of the antimalarials is effective against all liver and red cell stages of the life cycle that may coexist in a patient. Complete cure therefore may require more than one drug.

Class I agents are directed against the asexual erythrocytic forms and are not reliable against primary or latent liver stages or against P. falciparum gametocytes. These drugs will treat, or prevent, clinically symptomatic malaria. When used prophylactically, the class I drugs must be taken for several weeks after exposure until parasites complete the liver phase and become susceptible to therapy. The spectrum is somewhat expanded for the class II agents, which target not only the asexual erythrocytic forms but also the primary liver stages of P falciparum. This additional activity shortens to several days the required period for postexposure prophylaxis. Finally, primaquine is unique in its spectrum of activity, which includes reliable efficacy against primary and latent liver stages as well as gameto-cytes. Primaquine has no place in the treatment of symptomatic malaria but rather is used most commonly to eradicate the hypnozoites of P. vivax and P. ovale, which are responsible for relapsing infections.

The use of drugs for prophylaxis or therapy is dictated by their pharmacokinetics and safety. Thus, quinine and primaquine, which have short half-lives and common toxicities, are reserved for bite from infected

schizonts merozoites

FIGURE 39-1 Life cycle of malaria parasites. See text for details.

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