WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - Yellow fever
Background of the disease
Yellow fever is a viral disease transmitted by infected mosquitos that has caused large epidemics in Africa and the Americas. It can be recognized from historic texts stretching back 400 years. Infection causes a wide spectrum of disease, from mild symptoms to severe illness and death. The "yellow" in the name is explained by the jaundice that affects some patients. The number of epidemics, and the number of people infected with yellow fever have increased over the last two decades, and yellow fever is now a serious public health problem again (Fig. 2.1). Case fatality rates for reported cases are in the order of 15 to 50%.
An important reason for the re-emergence of yellow fever is the lapse of yellow fever immunization programmes in areas where they had been implemented in the past. Although a safe and effective vaccine has been available for 60 years, there are now large susceptible populations living in high-risk areas. Other factors contributing to the spread of yellow fever include increased urbanization, because mosquitos in urban areas increase the potential for explosive large urban outbreaks, increase in the distribution and density of mosquitos that transmit yellow fever and increased intrusion of people into forested areas.
The yellow fever virus is constantly present in mosquitos and non-human primates in some tropical areas of Africa and the Americas. Certain species of mosquitos are the reservoir of yellow fever virus; thus eradication of yellow fever is not feasible. This viral presence sometimes amplifies into regular epidemics. At present, 33 countries, with a combined population of 468 million, are at risk in Africa. These lie within a band ranging from 15°N to 10°S of the equator. In the Americas, yellow fever is endemic in ten South American countries and in several Caribbean islands. Bolivia, Brazil, Colombia, Ecuador and Peru are considered at greatest risk (see Map 2.1).
Fig. 2.1 Number of reported cases of yellow fever per decade, 1950-1998
Map 2.1 Countries at risk of yellow fever and countries that have reported at least one outbreak of yellow fever, 1985-1999