Reflexions on two current viral infections: yellow and dengue fevers

Yellow fever and dengue are two current viral diseases induced by flaviviruses and usually transmitted by the same mosquito vector, Aedes aegypri. From 1987 to 1991, 18,753 cases of yellow fever, mainly from Africa, have been notified to WHO, leading to 4,522 deaths. On the other hand, WHO estimates that 2.5 billions individuals living in tropical areas are at risk to contract dengue fevers. In fact, 500,000 patients are hospitalized each year for dengue hemorrhagic fever/dengue shock syndrome and 90% of them are chiIdren. Nevertheless, the control of these two viral diseases would be reached easily in destroying mechanically the mosquito larval resting places. Although superficially similar, the two entities are in fact quite different. Relatively few is known about the pathogenesis of yellow fever whereas: for dengue fevers, it is difficult to integrate so many results accumulated to explain the occurrence of haemorrhagic phenomena according to the two main theories so far proposed which are not exclusive. The immunological one (S.B. Halstead) tries to explain the pathological events by the effect of antidengue enhancing antibodies acquired during a previous exposure to one of the dengue viruses, whereas that of increased virus virulence (L. Rosen) refers to fast passages between individuals during explosive epidemics. The virological diagnosis of both diseases was recently improved following the progresses of immunology and molecular biology of flaviviruses, but the local conditions where it is applied are basically opposed : usually, the African bush for yellow fever and the large underurbanized tropical cities, for dengue. In addition, an excellent vaccine is available to protect man against yellow fever, the 17D live vaccine (1937), while no dengue vaccine is presently available despite extensive efforts.