Yellow fever, a member of the genus Flavivirus, is an arboviral infection found throughout Africa and South America. It is transmitted primarily by the bite of the Aedes aegypti mosquito and also by Haemogogus mosquitoes in South America.
Though yellow fever caused epidemics in the United States and Europe in earlier centuries, today it exists only in Africa and Central and South America.
There are two main cycles of transmission of yellow fever: the sylvatic, or jungle, cycle; and the urban cycle. In the sylvatic cycle, the infection is maintained between monkeys and mosquitoes. A human entering the jungle environment (e.g., loggers, hunters) is at risk if bitten by an infected mosquito. Urban yellow fever occurs when the virus is introduced into urban centers, for example by migrant laborers arriving from rural regions. The domestic mosquito, A. aegypti, then carries the infection from person to person. In contrast to jungle yellow fever, where only small numbers of individuals are at risk, urban yellow fever epidemics may be quite extensive.
An intermediate cycle has also been described in Africa in areas where there is increased contact between humans, monkeys, and mosquitoes, such as at the edges of forested areas; this is a likely source of larger urban outbreaks.
Following the bite of an infective mosquito, the incubation period is three to six days. Although some cases may be asymptomatic or very mild, most cases are characterized by sudden onset of fever, chills, myalgias, backache, headache, nausea, and vomiting. Relative bradycardia (Faget's sign) is common, as are leukopenia and proteinuria. This early stage lasts three to five days, at which point the majority of patients will recover. Approximately 15 percent will relapse within twenty-four hours and develop a stage of "intoxication" characterized by a reocurrence and worsening of the above symptoms. Jaundice appears (hence the name "yellow fever"), and patients develop a bleeding tendency marked by blood in the vomit and
As the clinical presentation of yellow fever is similar to that of other viral hemorrhagic fevers, the diagnosis should be confirmed in a laboratory. Diagnosis can be made by culture of the virus or by finding viral antigen in blood or liver tissue. It is also possible to identify virus-specific antibodies in blood.
A live, attenuated vaccine against yellow fever is over 95 percent effective and confers protection for ten years. As it is a live vaccine, it is contraindicated in infants under the age of six months, in pregnant women, and in immunocompromised individuals. It should be used with caution in anyone with a history of egg allergy.
The best method for control of yellow fever is mass vaccination of susceptible populations. Although the World Health Organization advocates including the yellow fever vaccine in the Expanded Programme of Immunization (EPI) for children, most countries use the vaccine only in outbreak situations, a strategy that has not proven to be very effective in controlling the disease.
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