Malaria transmission occurs in large areas of Central and South America, the Caribbean, sub-Saharan Africa, the Indian subcontinent, Southeast Asia, the Middle East, and Oceania. Certain species may predominate in a given geographic area. 2 For example, P. vivax is more common in the Indian subcontinent and in Central America, whereas P. falciparum is the most prevalent form in Africa, Haiti, and New Guinea.

The risk of contracting malaria, which varies considerably between regions, is largely dependent on the intensity of transmission in both urban and rural areas, and, for travelers, on the itinerary and time and type of travel. From 1990 to 1994, the Centers for Disease Control and Prevention reported 2580 cases of malaria among US civilians.3 Of the more than 500 cases in 1994, a total of 303 (58 percent) were acquired in sub-Saharan Africa, 102 (20 percent) in Asia, 74 (14 percent) in the Caribbean and Central America, 24 (5 percent) in Oceania, and only 9 (2 percent) in South America. Plasmodium vivax accounted for 46 percent of all cases, and P. falciparum for another 46 percent. Mixed infections were uncommon, representing less than 1 percent of all cases. Thus, more than half of all cases of malaria, including the majority of cases due to P. falciparum, were acquired from travels in sub-Saharan Africa. Yet, for every traveler to sub-Saharan Africa, at least 10 travelers visit potential malarious areas of Asia and South America each year. Clearly, the intensity of exposure appears to be much higher in sub-Saharan Africa.

Resistance of P. falciparum to chloroquine continues to spread (Table 142-1) 4 In addition, strains of P. falciparum are resistant to other chemotherapeutic agents, including pyrimethamine-sulfadoxine, quinine, mefloquine, doxycycline, and new agents, including halofantrine and artesunate (the latter two agents are not available in the United States). Recently, strains of P. vivax have been isolated from patients who have failed chloroquine therapy.5 Prior to 1990, no strains of P. vivax, P. ovale, or P. malariae were resistant to chloroquine.

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