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FIG. 5. Geographic distribution of dengue virus serotypes in 1970 (A) and in 2006 (B).

The USA and Europe are not immune to the introduction of dengue viruses. Each year for the past 25 years, imported dengue cases to the USA have been documented by the Centers for Disease Control and Prevention (CDC) (Gubler 1996, Rigau-Perez et al 1994, CDC 2004, unpublished data). These cases represent introductions of all four virus serotypes from all tropical regions of the world. Most dengue introductions into the USA come from the American and Asian tropics, and reflect the increased number of Americans travelling to those areas. Overall, from 1976 to 2003, 3697 suspected cases of imported dengue were reported to the CDC (Gubler 1996, Rigau-Perez et al 1994, CDC 2004, unpublished data). Although adequate blood samples were received from only a portion of these patients, 875 (24%) were confirmed as dengue. A similar increase in reported dengue has been seen in Europe in recent years (Wichmann et al 2003).

These cases represent only the tip of the iceberg because most physicians in the United States and Europe have a low index of suspicion for dengue, which is often not included in the differential diagnosis, even if the patient recently travelled to a tropical country (Gubler 1996, Wichmann et al 2003). As a result, many imported dengue cases are never reported. It is important to increase awareness of dengue and DHF among physicians in temperate areas, however, because the disease can be life-threatening. For example, two cases of the severe form of DHF, dengue shock syndrome, were described in Swedish tourists returning from holiday in Asia (Wittesjo et al 1993). In the USA, severe disease also occurs among imported cases of dengue (CDC 1995). It is important, therefore, that physicians in the USA and Europe consider dengue in the differential diagnosis of viral syndrome in all patients with a travel history to any tropical area.

The potential for epidemic dengue transmission in the USA and Europe still exists, since both have infestations of at least one of the principal mosquito vectors. On eight occasions in the past 25 years (in 1980 after an absence of 35 years, and in 1986, 1995, 1997, 1998, 1999, 2001 and 2005), autochthonous transmission occurred in the USA, secondary to importation of the virus in humans. Of interest was the 2001 Hawaii outbreak, which was the first dengue transmission in that state in 56 years (Effler et al 2005) caused by DEN-1 virus introduced from Tahiti where a major epidemic of DHF was occurring. Transmission in Hawaii was sporadic and illness mild; 122 cases were confirmed (Effler et al 2005). Although the outbreaks in the USA have been small, they underscore the potential for dengue transmission in areas where two competent mosquito vectors occur (Gubler & Trent 1994). Ae. aegypti, the most important and efficient epidemic vector of dengue viruses, has been in the USA for over 200 years and has been responsible for transmitting major epidemics in the past (Ehrankramz et al 1971). Currently, this species is found only in the Gulf Coast states from Texas to Florida, although small foci have recently been reported in Arizona. Ae. albopic-tus, another, but less efficient epidemic vector of dengue viruses, was introduced to the continental USA in the early 1980s and has since become widespread in the eastern half of the country. Although CDC has ceased surveillance, at last count it occurred in 1044 counties in 36 of the continental states (C. G. Moore, Colorado State University, 2004, personal communication); this species has also been found in Hawaii for over 90 years. Ae. albopictus has recently been introduced and has become established in several European countries. Both Ae. aegypti and Ae. albopictus can transmit dengue viruses to humans and their presence increases the risk of autochthonous dengue transmission, secondary to imported cases (Gubler 1988, 1996).

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