Drug-resistance

Drug-resistant P. falciparum was first reported in Thailand in 1961. Drug-resistant malaria has become one of the most important problems in malaria control in recent years. Resistance in vivo has been reported to all antimalarial drugs except artemisinin and its derivatives. Drug-resistance necessitates the use of drugs that are more expensive and may have dangerous side effects. In some parts of the world, artemisinin drugs are the first line of treatment, and are used indiscriminately for self-treatment of suspected uncomplicated malaria - so we can expect to see malaria forms resistant to artemisinin within a decade (Greenwood, 1999).

The areas most affected by drug-resistance are the Indo-Chinese peninsula and the Amazon region of South America. The problem of drug-resistance can be attributed primarily to increased selection pressures on P. falciparum in particular, due to indiscriminate and incomplete drug use for self-treatment. In areas such as Thailand and Vietnam, mosquitoes of the Anopheles dirus and A. minimus species spread the drug-resistant parasites. These mosquitoes adapt their feeding activity to human behaviour patterns, and maintain intense transmission capacity (Greenwood, 1999).

In man, the problem of resistance to the common antimalarial drugs such as chloroquine and pyrimethamine, and the decreasing effectiveness of quinine is mainly limited to P. falciparum infection; chloroquine remains the treatment of choice for P. vivax. Several mechanisms can account for changes in drug sensitivity in the malaria parasites, for example, physiological adaptations due to non-genetic changes, selection of previously existing drug-resistant cells from a mixed population under drug pressure, spontaneous mutations, mutations of extranuclear genes, or the existence of plasmid-like factors (Krogstad et al, 1987; Cowman, 1991; Wellems et al, 1990).