same time, epidemics of DHF/DSS have spread from Asia to the Americas. In the early 1980s, the disease arrived in Cuba, where it killed 158 Cubans in a major outbreak in 1981. The most recent epidemic took place in Venezuela in 1990; more than 3,100 cases of severe hemorrhagic disease were recorded, as well as 73 deaths (Gubler, 1991). There are currently endemic foci of DHF/DSS in the Caribbean and on the Yucatan Peninsula of Mexico.

Although the disease is concentrated in a small number of areas, worldwide incidence rates for DHF/DSS have skyrocketed. Since its appearance in 1956, there have been an average of 29,803 cases of DHF/DSS reported per year. Between 1986 and 1990, the average number of reported cases per year was more than 267,692 (Gubler, 1991). In Southeast Asia, DHF/DSS ranks as one of the leading causes of hospitalization and death among children.

Although the reasons for the increase in dengue activity and the changing disease pattern are not fully understood, the consequences of increased urbanization, densely populated areas, and poor sanitation play a significant role. These conditions favor the growth of mosquito populations. Dengue virus, which is short-lived in the human host, is best maintained in densely populated areas in which Ae. aegypti is abundant and susceptible individuals are concentrated. The lack of effective mosquito control in many tropical urban centers—a by-product of economic and political problems as well as indifference—has undoubtedly contributed to the dramatic rise in dengue infection worldwide.

The United States experienced dengue fever outbreaks in 1922 and 1945 (Langone, 1990). No cases of DHF/DSS have been reported to date, but imported cases of dengue fever occur annually in U.S. citizens who have returned from travel abroad. In 1990, for instance, 24 confirmed cases of imported dengue were reported to the Centers for Disease Control (CDC) (Centers for Disease Control, 1991c). Although Ae. aegypti and Ae. albopictus (a secondary vector) have become firmly established in much of the southeastern United States, epidemics of DHF/DSS, such as those seen in Cuba and Venezuela, are unlikely. The United States is less vulnerable because its standard of living is higher, houses are more likely to be screened, and discarded tires (see Box 2-3) and other containers that can breed Aedes are much less common than in many cities in the tropics. At present, the only effective way to limit the spread of dengue is to attack its principal vector, Ae. aegypti. Government-supported pesticide application programs, and efforts of private citizens to eliminate mosquito breeding sites (i.e., source reduction) near their homes, have been shown to work. The success of such source reduction efforts rests on public education programs.

Like the yellow fever vaccine, a reasonably priced dengue vaccine will be an important adjunct to vector control in stemming the spread of urban epidemics. Dengue vaccine development, however, has been complicated



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