areas in the Pacific. Multiple dengue serotypes were geographically shuttled by viremic troops and refugees, and the vector was spread by vehicles, water storage containers, and tires carrying along the ova and larvae of A. aegypti. The dissemination of virus and vector was enhanced after the war by rapid population growth and urbanization. Asian cities were characterized by poor sanitation, the necessity for domestic water storage, and crowded living conditions, creating conditions favoring breeding of A. aegypti. Superimposed on these phenomena was the rapid rise in air travel, providing the means for movement of viremic human beings within the region and beyond. These factors led to the establishment of hyperendemic dengue infection in Southeast Asia, a pattern of annual outbreaks caused by all four dengue serotypes, and an increasing frequency of sequential infections of children (13). It is in this setting that DHF/DSS emerged in 1954 in the Philippines. Over the next 20 years, outbreaks occurred that involved many parts of Asia and the Pacific, with a mean annual incidence of about 30,000 cases. In the 1970s and 1980s, the incidence of DHF rose dramatically, to over 250,000 cases per year (Figure 2). DHF is now the third or fourth leading cause of hospitalization of children in some areas (14).

The emergence of dengue fever and DHF in the American region provides a paradigm for the changing features of dengue epidemiology. Prior to World War II infrequent epidemics occurred at intervals of up to 37 years, probably caused by introduction of a single serotype (12). The opportunities for introduction of new viruses were limited. Outbreaks were rarely sustained for more than a few years because human populations were relatively low and isolated in island situations where immunologically susceptible hosts were rapidly exhausted. Postwar changes in dengue epidemiology in the American region occurred somewhat later than in Asia. During the 1960s, dengue virus types 2 and 3 became established in the region, and in 1977, dengue type 1 was introduced, rapidly spread, and became endemic. The pattern of intermittent epidemics at long intervals and transient circulation of one serotype changed to one of annual outbreaks in multiple locations and persistent cocirculation of multiple dengue serotypes. It was predictable that these events would eventually lead to the emergence of DHF in the Americas (2). In the 1970s, a few DHF cases were identified in Puerto Rico, where dengue was under intense study. The first true outbreak occurred in Cuba in 1981, with 116,000 hospitalized patients, 34,000 documented cases of DHF, and 158 deaths (15). Similar events occurred in Venezuela in 1989–1990 (16), with over 3000 cases of hemorrhagic fever, and in Rio de Janeiro in 1990. During the decade that followed the Cuban epidemic, 11 countries in the Americas have reported DHF (Figure 3).



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