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Description

Yellow fever is a mosquito-borne viral disease. Illness ranges in severity from an influenza-like syndrome to severe hepatitis and hemorrhagic fever. Yellow fever is caused by a zoonotic virus that is maintained in nature by transmission between nonhuman primates and mosquito vectors. In some situations, humans may serve as the primary host in the transmission cycle (“urban yellow fever”).

Occurence

The responsible for the disease, and it is in Africa where most cases are reported. The case-fatality rate is >20%, and infants and children are at greatest risk for infection. In South America, cases occur most frequently in young men who have occupational exposure to mosquito vectors in forested or transitional areas of Bolivia, Brazil, Colombia, Ecuador, Venezuela, and Peru.

Risk for travellers

The risk of a traveler's acquiring yellow fever is determined by immunization status, geographic location, season, duration of exposure, occupational and recreational activities while traveling, and the rate of yellow fever virus transmission at the time. Although reported cases of human disease are the principal guide to the level of transmission, they may be absent (because of a high level of immunity in the population) or not detected as a result of poor surveillance. Only a small proportion of yellow fever cases are officially notified, because of the occurrence of the disease in remote areas and lack of specific diagnostic facilities. Indeed, the majority of cases during outbreaks in Africa are missed despite an extraordinarily high incidence of infection and disease.

During interepidemic periods, the incidence of overt disease is below the threshold of detection by existing surveillance. Such interepidemic conditions may last years or even decades in certain countries or regions. This “epidemiologic silence” may provide a sense of false security and lead to travel without the benefit of vaccination. Surveys in rural West Africa during “silent” periods indicate that the incidence of yellow fever illness is 1.1–2.4 cases per 1,000 persons and that the incidence of death due to yellow fever is 0.2–0.5 deaths per 1,000 persons; both these ranges are less than the threshold of detection of the existing means of surveillance.

The incidence of yellow fever in South America is lower than that in Africa because virus transmission between monkeys and mosquitoes occurs in the canopy of the forest, isolated from human contact, and because immunity in the indigenous human population is high. The risks of illness and death are probably 10 times lower in South America than they are in rural West Africa, but the risks vary greatly according to specific location and season. In West Africa, the most dangerous time of year is during the late rainy and early dry seasons (July–October). Virus transmission is highest during the rainy season (January–March) in Brazil.

The low incidence of yellow fever, generally a few hundred cases per year, has led to complacency among travelers. In Brazil, for example, where the majority of the population lives in coastal regions outside the endemic zone, unvaccinated recreational or vocational travelers to the interior are the usual victims of yellow fever. Four of the five cases from the United States and Europe in 1996–2002 were among travelers who were exposed in South America. All five cases were fatal. Although not as dramatic as the situation in Africa, the 1990s represented a period of increased enzootic and epizootic yellow fever transmission in South America. Brazil and Peru are currently experiencing an expansion of yellow fever virus activity, and the risk to travelers is higher than usual.

The risks of illness and of death due to yellow fever in an unvaccinated traveler are estimated to be 1:1,000 and 1:5,000 per month, respectively. (For a 2-week journey, the risks of illness and death are 1:2,000 and 1:10,000, respectively.) These estimates, which are based on risk to indigenous populations, may overestimate the risk to travelers, who may have a different immunity profile, take precautions against getting bitten by mosquitoes, and have less outdoor exposure than do indigenous residents. Based on data for U.S. travelers, the risk for illness in a traveler due to yellow fever has been estimated to be 0.4–4.3 cases per million travelers to yellow fever-endemic areas.

Prevention

Personal protection measures

In addition to vaccination, travelers should be advised to take precautions against exposure to mosquitoes when traveling in areas with yellow fever transmission. Staying in air-conditioned or well-screened quarters and wearing long-sleeved shirts and long pants will help to prevent mosquito bites. Insect repellents containing DEET should be used on exposed skin only. Permethrin-containing repellents should be applied to clothing. Travelers to rural areas should bring mosquito nets and aerosol insecticides or mosquito coils. (For further prevention information, see Protection against Mosquitoes and Other Arthropod Vectors.)

Vaccine

Yellow fever is preventable by a relatively safe, effective vaccine (see Table 3–20 for dosage information). International regulations require proof of vaccination for travel to and from certain countries. For purposes of international travel, vaccines produced by different manufacturers worldwide must be approved by the World Health Organization and administered at an approved yellow fever vaccination center. State and territorial health departments have authority to designate nonfederal vaccination centers; these can be identified by contacting state or local health departments. Vaccinees should receive a completed International Certificate of Vaccination, signed and validated with the center's stamp where the vaccine was given. This certificate is valid 10 days after vaccination and for a subsequent period of 10 years.

A number of countries require a certificate from travelers arriving from infected areas or from countries with infected areas. Some countries in Africa require evidence of vaccination from all entering travelers; others may waive the requirements for travelers coming from noninfected areas and staying in the country <2 weeks. (Yellow Fever Vaccine Requirements and Information on Malaria Risk and Prophylaxis, by Country) Vaccination is also recommended for travel to countries that do not officially report the disease but lie in the yellow fever-endemic zone. The actual areas of yellow fever virus activity can extend beyond the officially reported infected zones.

Some countries require a traveler, even if only in transit, to have a valid international certificate of vaccination if he or she has visited any country either known or thought to have yellow fever virus. Such requirements may be strictly enforced, particularly for persons traveling from Africa or South America to Asia. Travelers with a specific contraindication to yellow fever vaccine should be advised to obtain a waiver before traveling to countries requiring vaccination. (See “Vaccination Certificate Requirements.”)