Yellow Fever

What is yellow fever, how is it spread, and how is it treated?

Epidemics of yellow fever were recorded as long ago as 1648. The yellow fever virus, a member of the flaviviridae ( flavus , L. "yellow"), is spread by the Aedes aeypti mosquito, as demonstrated by Dr. Walter Reed in 1900. Two researchers at the Rockefeller Foundation in New York developed a vaccine that was tested in human volunteers in 1936 and found it to be effective. Indeed, the vaccine, along with mosquito control measures, were so effective that by the end of World War II yellow fever was considered a medical curiosity. In the early 1980s about 200 cases were reported annually to the World Health Organization from the African continent, but by the late 1980s as many as 5,000 cases were reported from this region in a single year. Because the yellow fever is easily mistaken for other diseases, it is likely that only a small fraction of the actual cases are reported.

Although the mosquitoes that spread the virus live in many warm climates, yellow fever occurs only in sub-Saharan Africa and the northern half of South America. Unlike the rural mosquitoes that spread Japanese encephalitis, the yellow fever mosquitoes live in the city or the jungle.

Most persons infected with the yellow fever virus have no symptoms, but those who develop symptoms begin to do so within three to six days of the mosquito bite. Symptoms include headache, fever, photophobia (extreme sensitivity to and fear of light), pain in the lower spine and extremities, loss of appetite, upper abdominal tenderness, and vomiting.

As many as 15 percent of persons who become infected develop moderate or life-threatening symptoms: jaundice (liver distress that causes yellowing of the skin and eyes-hence the name "yellow fever"), bloody vomit and stools, decreased urine (a sign of kidney distress), and coma. The only treatment is supportive care. Many of the persons with severe yellow fever do not recover. The case fatality rate in Africa from 1986 through 1995 was 24 percent.

To minimize your risk of mosquito bites, follow these tips: The urban mosquitoes that spread yellow fever primarily feed during the day, so stay in air-conditioned or well-screened quarters. If you go outside, wear a long-sleeved shirt and long pants and apply an insect repellent containing permethrin to your clothes. Wear mosquito repellent containing DEET on exposed skin. In rural areas, sleep under permethrin-impregnated mosquito netting or in a screened room. Also use insecticidal space sprays.

How effective is yellow fever vaccine, and who should get it?

A single dose begins to protect the recipient in 7 to 10 days. A booster is recommended subsequently every 10 years if exposure persists. The vaccine is effective in essentially all recipients.

Some countries require proof of yellow fever vaccination on an International Certificate of Vaccination (or an official letter of exemption) whether you are traveling to them or just passing through them. At least 10 days prior to arrival, the vaccine must be administered at an approved Yellow Fever Vaccination Center. Contact your state or local health department to locate such a center in your area and to learn the most current requirements.

Irrespective of border requirements, the vaccine is recommended for persons living or traveling in yellow fever-infected areas. This includes urban and non-urban areas where yellow fever actually is being reported, and non-urban areas anywhere within the yellow fever zone (sub-Saharan African and the northern part of South America).

Who should not get the vaccine?

• Persons with a history of a severe allergic or systemic reaction following a prior dose of yellow fever vaccine or any of its components (such as eggs) should not take yellow fever vaccine. There are no antibiotics or preservatives in the vaccine.

• Persons who have a moderate or severe acute illness should postpone vaccination.

• The American Academy of Pediatrics states that yellow fever vaccine should not be given to children less than 4 months old and that the decision to give it to children 4 to 9 months old should be based upon the risk of disease exposure.

• Pregnant women should only receive this vaccine if travel to a high-risk area is unavoidable.

• The ACIP recommends that immuno-compromised patients who cannot avoid travel to yellow fever-infected areas should be advised of the risk, informed on how to avoid the mosquitoes that spread the disease, and given a waiver.

What are the vaccine risks and side effects?

The results of studies done on the side effects of yellow fever vary a lot. In general, few persons develop side effects, and when they do the reactions are mild (such as pain and redness at the injection site shortly after vaccination or fever, mild headache, or muscle aches about a week after vaccination), and these side effects do not last long. Severe allergic reactions and other severe reactions occur rarely (fewer than 1 out of 1 million persons). The persons affected are usually allergic to eggs or are younger than 9 months old.