Public health aspects
Yellow fever (YF) is a mosquito-borne, viral haemorrhagic fever that is endemic in tropical areas of Africa and South America, where is has caused outbreaks at irregular intervals for centuries. Like plague and cholera, YF is subject to control measures outlined in the International health regulations. WHO estimates that a total of 200 000 cases of YF occur each year, with about 30 000 deaths. More than 90% of YF cases occur in Africa, where over 500 million people live in the YF at-risk zone. Furthermore, YF is a significant risk to more than 3 million travellers who visit areas affected with YF each year.
Exposure of susceptible persons to bites from infected mosquitoes is the only significant mode of YF transmission. An urban and a jungle (forest, sylvatic) form of YF can be distinguished by differences in their respective transmission cycles. Urban YF, which frequently occurs as large outbreaks, is transmitted from infected to susceptible humans by "Ae. aegypti", a mosquito species that breeds in the proximity of human habitats. The urban form of transmission is found mainly in Africa. The jungle form of YF is primarily an enzootic viral disease of non-human primates, but the various mosquito vectors involved may occasionally cause individual cases or small outbreaks of YF among humans in the forested savanna of Africa and in jungle areas of South America. As a result of high vector density, the risk of contracting YF is usually greatest towards the end of the rainy season and at the beginning of the dry season, particularly with the jungle type of transmission. The YF virus is maintained during the dry season by trans-overial transmission in mosquitoes.
Since the beginning of the 1980s, the incidence of YF has increased dramatically, particularly in Africa. In Nigeria, more than 20 000 cases, notably children, were reported in successive outbreaks between 1986 and 1994. The Nigerian outbreaks were mainly caused by urban-type YF, but small outbreaks due to the jungle transmission cycle are believed to cause thousands of YF cases in Africa each year. During outbreak periods in Africa, about 20-40% of the population in affected areas show serological evidence of YF infection. The ratio of infection to clinical illness was found to be 3.8: and 7.4:1 respectively, in two separate epidemics.