Yellow fever, caused by a flavivirus, results in an illness of widely varying severity so that the disease is under-reported. It is a disease confined to Africa (90% of cases) and South America between latitudes 15° N and 15° S. For poorly understood reasons, yellow fever has not been reported from Asia, despite the fact that climatic conditions are suitable and the vector, Aedes aegypti, is common. The infection is transmitted in the wild by A. africanus in Africa and the Haemagogus species in South and Central America. Extension of infection to humans (via the mosquito or from monkeys) leads to the occurrence of 'jungle' yellow fever. A. aegypti, a domestic mosquito which lives in close relationship to humans, is responsible for human-to-human transmission in urban areas (urban yellow fever). Once infected, a mosquito remains so for its whole life.

Clinical features

The incubation period is 3-6 days. When the infection is mild, the disease is indistinguishable from other viral fevers such as influenza or dengue.

Three phases in the severe (classical) illness are recognized. Initially the patient presents with a high fever of acute onset, usually 39-40°C, which then returns to normal in 4-5 days. During this time, headache is prominent. Retrobulbar pain, myalgia, arthralgia, a flushed face and suffused conjunctivae are common. Epigastric discomfort and vomiting are present when the illness is severe. Relative bradycardia (Faget's sign) is present from the second day of illness. The patient then makes an apparent recovery and feels well for several days. Following this 'phase of calm' the patient again develops increasing fever, deepening jaundice and hepatomegaly. Ecchymosis, bleeding from the gums, haematemesis and melaena may occur. Coma, which is usually a result of uraemia or haemorrhagic shock, occurs for a few hours preceding death. The mortality rate is up to 40% in severe cases. The pathology of the liver shows mid-zone necrosis, and eosinophilic degeneration of hepatocytes (Councilman bodies).

Diagnosis and treatment

The diagnosis is established by a careful history of travel and vaccination status, and by isolation of the virus (when possible) from blood during the first 3 days of illness. Serodiagnosis is possible, but in endemic areas cross-reactivity with other flaviviruses is a problem.

Treatment is supportive. Bed rest (under mosquito nets), analgesics, and maintenance of fluid and electrolyte balance are important.

Prevention and control

Yellow fever is an internationally notifiable disease. It is easily prevented using the attenuated 17D chick embryo vaccine. Vaccination is not recommended for children under 9 months and immunosuppressed patients unless there are compelling reasons. For the purposes of international certification, immunization is valid for 10 years, but protection lasts much longer than this and probably for life. The WHO Expanded Programme of Immunization includes yellow fever vaccination in endemic areas.