Every year 30–100 cases of dengue are reported in the United States in persons who have traveled to tropical countries. Many such cases undoubtedly go unreported. A similar incidence has been recorded among Swedish tourists, and it probably occurs elsewhere in Europe as well. Unlike most of Europe, parts of the United States have a resident fauna of efficient vectors (A. aegypti and Aedes albopictus; see below), and there is a risk of secondary spread. On two occasions (1980 and 1986) small outbreaks followed the introduction of dengue from Mexico into southern Texas. The introduction and spread of dengue outbreaks in the southern United States remains a potential threat, particularly in cities along the Gulf of Mexico, where A. aegypti and A. albopictus are abundant and major pest problems. As would be expected from the requirement for sequential infections, DHF has been rare in travelers and does not currently pose a threat to the developed world.
Underlying the emergence of DHF in the Western Hemisphere are changes in human and mosquito ecologies that affect the rate and geographic range of virus transmission. The principal vector, A. aegypti, has made extraordinary evolutionary adjustments to coexist with human