have occurred in Paraguay, Bolivia, Peru, Ecuador, Colombia, and Venezuela. The sequential introduction of dengue type 1 followed by dengue 2 resulted in outbreaks of DHF in Caracas (1989) and Rio de Janeiro (1990). Three of the four dengue serotypes are currently endemic in the region, and it is probable that dengue type 3 virus will be reintroduced in the near future. It is clear that the factors that led to the emergence of DHF as a major public health problem in Asia now exist in the American region.
An interesting illustration of the relationship between human and vector ecology is provided by the introduction and spread of another dengue vector, A. albopictus, from Asia to the Americas in the 1980s. This mosquito is responsible for endemic transmission of dengue in Asia and for epidemic spread in circumstances where A. aegypti is absent or in low density. In 1985, the mosquito was discovered in Houston, Texas, and a year later in Rio de Janeiro, representing separate introductions from different areas of Asia (19, 20). Human commerce in used truck tires imported for the purpose of recapping was responsible for the importation of eggs or larvae of the mosquito (21). These tires were stored in the open prior to export, where they collected rainwater and were used as oviposition sites by A. albopictus. Over a million tires per year were imported into the United States from Asia, and approximately 20% of these, unfit for recapping purposes, were discarded in the environment. The A. albopictus invasion rapidly spread, probably in large part by the tire trade, extending the range of this winter-hardy species throughout the eastern United States. A similar expansion has occurred in Brazil. Dissemination of the mosquito is continuing, and in 1993 it invaded the Dominican Republic (C. Peña, personal communication). It was predicted that this aggressive and adaptable species would become implicated in the transmission of indigenous viruses, and this has in fact occurred; five different agents, including two human pathogens (eastern equine encephalitis and dengue viruses) have now been recovered from the species (ref. 22; P. Reiter, personal communication). The full dynamics of this unique episode have yet to play out, but it will be surprising if a significant public health problem does not emerge in the future. The accumulation of vast numbers of nonbiodegradable transportable man-made mosquito-breeding devices lies at the root of the dengue vector problem. Municipal landfills reject these objects, which are thus often dumped illegally in the environment, and no recycling has proven cost effective.