Drug resistance in malaria.

Ever since the discovery of the first case of chloroquine resistance along the Thai-Combodian border in the late 1950s, Southeast Asia has played an important role as a focus for the development of drug resistance in Plasmodium falciparum. Although the first case of quinine resistance had been reported much earlier from South America, the onset of chloroquine resistance marked the beginning of a new chapter in the history of malaria in Southeast Asia and by 1973 chloroquine finally had to be replaced by the combination of sulphadoxine and pyrimethamine (SP) as first line drug for the treatment of uncomplicated malaria in Thailand and more than 10 African countries have also switched their first line drug to SP. In 1985, eventually SP was replaced by mefloquine. The rapid development of resistance to this new drug leads to the introduction of artemisinin as a combination drug in the mid-1990s. It is mandatory to mention here that therapeutic regimens for prevention and treatment of chloroquine-resistant P. falciparum are associated with higher costs and side-effects compared to chloroquine. Additionally, some of these alternative treatments are associated with more side-effects, take longer time for cure and are more difficult to comply with than chloroquine. Urgent efforts are needed to identify effective, affordable, alternative antimalarial regimens. Molecular markers for antimalarial resistance have been identified, including pfmdr-1 and pfcrt polymorphisms associated with chloroquine resistance and dhfr and dhps polymorphisms associated with SP resistance. Polymorphisms in pfmdr-1 may also be associated with resistance to chloroquine, mefloquine and artemisinin. Use of such genetic information for the early detection of resistance foci and future monitoring of drug resistant malaria is a potentially useful epidemiological tool, in conjunction with the conventional in vitro and in vivo drug sensitivity assessments. The purpose of this review is to describe the state of knowledge regarding drug resistant malaria and to outline the changing patterns of drug resistance including its determinants, current status in diverse geographical areas, molecular markers and their implications to limit the advent, spread and intensification of drug resistant malaria.