two Colombian workers (Young and Moore, 1961) and has been partly to blame for the resurgence of malaria. Soon after this account, similar reports were received from Thailand, Vietnam, Cambodia, Malaya, and Brazil. By 1970, nearly 20 percent of all malarious regions had recorded cases of chloroquine-resistant P. falciparum malaria (Lepes, 1981). Today, there are only a few areas in the world where chloroquine is effective against this parasite.
Resistance to the newer antimalarials, such as mefloquine, is also occurring. In Thailand, for example, there is evidence of considerable P. falciparum resistance to the drug (Peters, 1990; Institute of Medicine, 1991a). This is of great concern, both because this drug has not been on the market for long (demonstrating the rapidity with which these parasites can adapt to the presence of the drug) and because the development of new antimalarial drugs to which the parasites are susceptible is not a rapid process; mefloquine was originally synthesized in the late 1960s and underwent 17 years of testing to demonstrate efficacy before being licensed (Institute of Medicine, 1991a).
Although P. falciparum causes the most severe type of malaria and is increasing in prevalence, P. vivax is responsible for the majority of malaria morbidity worldwide. It is of considerable concern, therefore, that P. vivax has also developed resistance to chloroquine, which was first reported in 1989 in Papua New Guinea (Rieckmann et al., 1989). Chloroquine-resistant P. vivax malaria has also been diagnosed in Indonesia (Schwartz et al., 1991). No one knows for certain how quickly and how far this resistance will spread. Because of the problem of resistance to chloroquine, travelers should seek the latest guidance on prophylaxis before going into malariaendemic areas.
Given the distribution of malaria cases throughout the world, drug resistance is of much greater concern outside the United States than within it. Still, the potential for the reemergence of malaria in this country, and the role of drug resistance in such a scenario, cannot be overlooked. In the United States, as recently as the early 1900s, up to 500,000 cases of malaria occurred each year, most of them in the South (Institute of Medicine, 1991a). Currently, some 1,200 total cases of malaria both drug susceptible and drug resistant are reported each year in this country, almost all of which occur in individuals who have been infected in other parts of the world (socalled imported malaria). Small outbreaks of nonimported malaria, the result of mosquito transmission from imported cases, have also been reported (Maldonado et al., 1990; Centers for Disease Control, 1990c, 1991f). Thus far, the outbreaks have been quickly and easily contained. A continued increase in drug-resistant malaria throughout the world, however, may increase the number of cases of imported malaria, thereby improving the chances for malaria to regain a foothold in the United States.