The family Flaviviridae contains only one genus, Flavivirus. They are somewhat smaller than alphaviruses, being 40 nm in diameter. The name flavivirus refers to the type species, the yellow fever virus ( Flavus, L = Yellow).
The yellow fever virus was first isolated in 1927 by inoculating rhesus monkeys with the blood of an African patient named Asibi. The virus was shown by Theiler (1930) to grow well following intracerebral innoculation in mice. The infected mouse brain was used as a vaccine in former French West Africa (Dakar vaccine) though this was encephalitogenic. It was later replaced by a non-neurotropic (17D) vaccine (Panicker 2007, p.527-30).
Cases are classified as inapparent (< 48 h of fever and headache), mild, moderately severe and malignant. Incubation lasts 3 to 6 days. Prodromal symptoms are usually absent. Onset is sudden, with fever of 39 to 40C (102-104 F). The pulse, usually rapid initially, by the second day becomes slow for the degree of fever present (Faget's Sign). The face is flushed and the eyes are injected; tongue margins are red and the center is furred. Nausea, vomiting, constipation, headache, muscle pain (especially in the neck, back and legs), severe prostration, restlessness and irritability are common symptoms. In mild cases, the fever falls suddenly 2 to 5 days after onset and a remission of several hours or days ensues. The fever recurs but the pulse remains slow. Jaundice, extreme albuminuria, and epigastric tenderness with hematemesis, the characteristic triad, appear. Oliguria or anuria may occur. Petechiae and mucosal hemorrhages are common. The patient is dull, confused, and apathetic. In malignant cases, delirium, convulsions, and coma occur terminally. Moderately severe cases may last 3 days to > 1 week; the period of convalescence is usually short except in the more severe cases. There are no known sequele. (Beers 1999, p.1303-4).
In a report by Weir and Shariqe (2004, 1909), each year about 2, 00,000 become ill with yellow fever around the world though the true incidence is likely to be much higher. This was according to estimates by the World Health Organization (WHO) in 1998. About 90% of the cases occur in Africa and 10% in South America.
Epidemiology of yellow fever
Approximately 2, 00,000 cases of yellow fever occur annually; 90% of them occur in Africa. A dramatic resurgence of yellow fever has occurred since the 1980s in both sub-Saharan Africa and South America. A series of epidemics and smaller outbreaks of yellow fever that occurred in West African countries were primarily responsible for the increased incidence of yellow fever in Africa, but the first epidemic reported in Kenya in>2 decades signaled that a change in the distribution of the disease was also occurring. Transmission in Africa is maintained by a high density of vector mosquito population that is in close proximity to largely unvaccinated human populations. Although some countries have incorporated programs, vaccine coverage is not optimal.
In South America, the rate of transmission of yellow fever is lower than in Africa, in part because high vaccine coverage occurs primarily as part of mass immunization campaign in response to outbreaks of the disease. The largest outbreak of yellow fever in South America since the 1950s occurred in Peru in 1995, and cases were reported in Bolivia, Brazil, Colombia,