Etiology and Epidemiology

Dengue viruses belong to the family of flaviviridae, the prefix of the family name being derived from flavus (Latin: "yellow") and referring to the yellow fever virus. The family consists of the genus Flavivirus, which has 65 related species and two possible members - cell fusing agent (CFA) virus and simian hemorrhagic fever virus.

Most flaviviruses are arboviruses, and all are serologically interrelated. They infect a wide range of vertebrate hosts, causing asymptomatic infections and diseases such as yellow fever, dengue, and numerous encephalitides. A. aegypti and A. albopictus are responsible for dengue transmission in Asia. A. albopictus has recently been discovered in the United States and in Brazil, but it has not yet been implicated in the transmission of disease. It is an aggressive, human-biting mosquito with both urban and rural habitats and transmits dengue viruses transovarially (from female mosquitoes to their offspring through infection of the eggs) and from person to person. Populations of A. albopictus found in the United States are capable of overwinter survival in northern latitudes because they are capable of diapause, and thus the spread of this particular mosquito represents a major public health concern. Although recent outbreaks of dengue in Brazil have been atrributed to A. aegypti, the presence of A. albopictus gives rise to serious concern that it may become an important vector for the introduction of flaviviruses into areas previously free of them.

A. albopictus is primarily a sylvan species that has become adapted to the urban environment, although it is not as strongly dependent on humans as is A. aegypti. It breeds in tree holes, bamboo stumps, coconut husks, and other naturally occurring containers in Asia, as well as in tires and discarded water containers. It has apparently been established in Hawaii for many years, although Hawaii seems to be free from dengue.

Dengue outbreaks are particularly likely in endemic areas when there has been a heavy rainfall. Monthly rainfall exceeding 300 mm is associated with a 120 percent increase in the number of cases of dengue, and the lag time between the onset of heavy rain and the outbreak of dengue is between 2 and 3 months. Any condition that increases the number of mosquito breeding sites, or the number of susceptible persons, in dengue endemic areas automatically facilitates the transmission of dengue.

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