Global Alert and Response (GAR)

WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - Yellow fever


Several different species of the Aedes and Haemogogus (South America only) mosquitos transmit the yellow fever virus. These mosquitos are either domestic (i.e. they breed around houses), wild (they breed in the jungle) or semi-domestic species (they display a mixture of habits). Any region populated with these mosquitos can potentially harbour the disease. There are three types of transmission cycles for yellow fever: sylvatic, intermediate and urban. All three cycles exist in Africa, but in South America, only sylvatic and urban yellow fever occur.

Sylvatic (or jungle) yellow fever
In tropical rainforests, yellow fever occurs in monkeys that are infected by wild mosquitos. The infected monkeys can then pass the virus onto other mosquitos that feed on them. These infected mosquitos bite humans entering the forest resulting in sporadic cases of yellow fever. The majority of cases are young men working in the forest (logging, etc.). On occasion, the virus spreads beyond the affected individual.
Intermediate yellow fever
In humid or semi-humid savannahs of Africa, small-scale epidemics occur. These behave differently from urban epidemics; many separate villages in an area suffer cases simultaneously, but fewer people are infected. Semi-domestic mosquitos infect both monkey and human hosts. This area is often called the "zone of emergence", where increased contact between man and infected mosquitos leads to disease. This is the most common type of outbreak seen in recent decades in Africa. It can shift to a more severe urban-type epidemic if the infection is carried into a suitable environment (with the presence of domestic mosquitos and unvaccinated humans).
Urban yellow fever
Large epidemics can occur when migrants introduce the virus into areas with high human population density. Domestic mosquitos (of one species, Aedes aegypti) carry the virus from person to person; no monkeys are involved in transmission. These outbreaks tend to spread outwards from one source to cover a wide area.

The potential for large-scale urban epidemics exists in many parts of the world. The density and habitats of Aedes aegypti, one of the mosquitos that transmits yellow fever, have expanded in both urban and rural areas. This mosquito is infesting regions where it was previously eradicated. Therefore, although yellow fever has never been reported from Asia, this region is at risk because the appropriate mosquitos and primates are present. In addition, in the past, yellow fever outbreaks also occurred in Europe, the Caribbean islands and Central and North America - they must still be considered at risk for yellow fever epidemics even though the virus is not felt to be present in these areas now.