FIGURE 2 Incidence of DHF since its first description as a nosologic entity in 1954(modified from ref. 4). A dramatic increase in incidence has occurred during the 1980s, reflecting both a real increase and an apparent increase due to improved reporting.

TRANSMISSION CYCLE, DISEASE, AND PATHOGENESIS

Dengue is caused by four antigenically distinct single-strand positive-polarity RNA viruses, designated dengue types 1–4, belonging to the family Flaviviridae (5). Virus transmission in its simplest form involves the ingestion of viremic blood by mosquitoes and passage to a second susceptible human host. An extrinsic incubation period of 8–10 days is required after feeding on a viremic human for viral replication and internal dissemination in the mosquito before virus appears in the saliva and transmission on refeeding can occur. As the blood meal stimulates oviposition by the female mosquito, which undergoes at least one, and often more, reproductive cycles during the extrinsic incubation period, there is an opportunity for virus to enter the egg and be passed to the next generation of mosquitoes.

In tropical Asia and West Africa, dengue viruses are also transmitted between nonhuman primates and tree-hole-breeding mosquitoes (6), but it is uncertain what relationship, if any, exists between the forest cycle and the circulation of virus between humans and A. aegypti. However, the existence of a completely silent zoonotic transmission



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