Yellow Fever

History

The first descriptions of a disease such as yellow fever can be found in historic texts as early as 400 years ago. It was especially common in American seaports. For instance, Philadelphia experienced an epidemic in 1793, which killed 10% of the city's population, and during which almost half of Philadelphia's residents fled the city. Yellow fever accounted for a significant number of casualties in the American army in the Spanish-American war of 1898: The impact was severe enough to warrant the setting up of the United States Army Yellow Fever Commission, also known as the Reed Commission, in 1900. Carlos Finlay, a Cuban physician, had proposed the mosquito-vector theory in 1881. In collaboration with the Reed Commission and with the help of a few human volunteers, his theory was confirmed. By adopting mosquito control measures, yellow fever was controlled within 6 months in Havana.

At present, yellow fever is considered to be endemic in 33 African countries, 9 South American countries, and several Caribbean islands. Yellow fever has never been reported in Asia.

Transmission

Yellow fever affects mainly humans and monkeys. The virus spreads by horizontal transmission (from one person to another by mosquitoes) or by vertical transmission (transovarially in infected mosquitoes). There are three transmission cycles depending on whether the mosquitoes are domestic (urban yellow fever), wild (jungle or sylvatic yellow fever), or semidomestic (intermediate yellow fever). In South America, only the first two transmission cycles are found. Sylvatic yellow fever usually causes sporadic cases of yellow fever, mainly in young males who work in the forest. Intermediate yellow fever causes small-scale epidemics in African villages. Epidemics of urban yellow fever may occur when a person from an endemic area migrates to a nonendemic area (especially crowded urban areas) and introduces the virus in an unvaccinated population.

Clinical Manifestations

The virus (a flavivirus) enters the body through the bite of a female mosquito. This is followed by an [Page 1104]incubation period of 3 to 6 days during which there are no signs or symptoms. Yellow fever has two phases. The acute phase is characterized by fever, chills, myalgia, headache, nausea, vomiting, anorexia, and general exhaustion. In the majority of the cases, these symptoms subside, and patients improve in 3 to 4 days. A few (about 15%), however, enter a toxic phase within 24 hr of the acute phase. This is characterized by reappearance of fever and deterioration of major organ systems of the body, mainly the liver and the kidneys signaled by jaundice, blood in stools, albuminuria, anuria, and so on. Half of the patients who enter the acute phase die within 10 to 14 days, while the others recover without significant residual organ damage. Yellow fever may be confused with many other diseases, especially in the initial stages, and it can be confirmed by serologic assays and other blood tests. As there is no specific drug for treating yellow fever, treatment is symptomatic.

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