Yellow Fever - Epidemiology

Epidemiology

Yellow fever is endemic in tropical and subtropical areas of South America and Africa. Even though the main vector Aedes aegypti also occurs in Asia, in the Pacific, and in the Middle East, yellow fever does not occur in these areas; the reason for this is unknown. Worldwide there are about 600 million people living in endemic areas. WHO officially estimates that there are 200,000 cases of disease and 30,000 deaths a year; the number of officially reported cases is far lower. An estimated 90% of the infections occur on the African continent. In 2008, the largest number of recorded cases were in Togo.

Phylogenetic analysis identified seven genotypes of yellow fever viruses, and it is assumed that they are differently adapted to humans and to the vector Aedes aegypti. Five genotypes (Angola, Central/East Africa, East Africa, West Africa I, and West Africa II) occur solely in Africa. West Africa genotype I is found in Nigeria and the surrounding areas. This appears to be especially virulent or infectious as this type is often associated with major outbreaks. The three genotypes in East and Central Africa occur in areas where outbreaks are rare. Two recent outbreaks in Kenya (1992–1993) and Sudan (2003 and 2005) involved the East African genotype, which had remained unknown until these outbreaks occurred.

In South America, two genotypes have been identified (South American genotype I and II). Based on phylogenetic analysis these two genotypes appear to have originated in West Africa and was introduced first into Brazil. The date of introduction into South America appears to be 1822 (95% confidence interval 1701 to 1911). The historical record shows that there was an outbreak of yellow fever in Recife, Brazil between 1685 and 1690. The disease seems to have disappeared, with the next outbreak occurring in 1849. It seems likely that it was introduced with the importation of slaves through the slave trade from Africa. Genotype I has been divided into five subclades (A-E).