Yellow Fever

Yellow fever is an infectious disease caused by a virus that is transmitted to humans by mosquitoes.

What Is Yellow Fever?

Yellow fever is a disease caused by yellow fever virus, a member of the flavivirus (FLAY-vih-vy-rus) group of viruses. The disease gets its name because it often causes jaundice ^* , which tints the skin yellow, and a high fever. Yellow fever also can cause kidney failure and uncontrolled bleeding, or hemorrhaging (HEM-rij-ing). Many cases produce only mild illness, but severe cases of yellow fever can be fatal. Once someone has survived the disease, the person will have lifetime immunity ^* against it.

Yellow fever afflicts both humans and monkeys and has been known since at least the 1600s. The disease is not spread by person-to-person contact. It is transmitted by several different species of mosquitoes; a person can contract yellow fever only from the bite of a mosquito that has bitten an infected person or monkey.

The disease once caused epidemics in the Americas, Europe, and the Caribbean, but at the beginning of the twenty-first century the disease occurs almost exclusively in South America and Africa. Each year, outbreaks lead to an estimated 200,000 cases and 30,000 deaths worldwide. Vaccines against the virus were developed in 1928 and 1937, and mosquito-eradication programs have made great progress in controlling the disease. The last recorded outbreak of yellow fever in the United States was in New Orleans in 1905. However, lapses in prevention programs in Africa and South America have allowed yellow fever to once again become a serious public health issue on those continents.

Are There Different Kinds of Yellow Fever?

Yellow fever occurs as three subtypes: epidemic (urban), intermediate, and jungle-acquired. Epidemic yellow fever spreads in densely populated areas of Africa and South America via the bite of Aedes aegypti (a-E-deez eh-JIP-tie) mosquitoes. Intermediate yellow fever occurs in Africa as the result of mosquitoes breeding in humid flat grasslands (savannahs) during rainy seasons, then infecting both monkeys and humans. In dry seasons, the virus can remain alive in unhatched mosquito eggs that are resistant to the heat.

Jungle-acquired yellow fever occurs mainly in South America when mosquitoes pick up the virus from infected forest monkeys and then transmit the disease to humans in jungles and rainforests. People who are regional settlers, soldiers, or agricultural or forestry workers are at greatest risk for this less common form of the disease.

How Do People Know They Have Yellow Fever?

After an incubation ^* period of 3 to 6 days, the yellow fever virus begins to produce symptoms. An early phase of disease occurs, which includes fever, headache, muscle aches, and vomiting. The infected person may have a slower heartbeat than that expected with a high fever. After a few days, most of the symptoms disappear. Many people recover from yellow fever at this point without complications. However, about 15 percent of patients develop a second, toxic phase of the disease, in which fever reappears and the disease becomes more severe. Inflammation of the liver occurs, along with jaundice, stomach pains, and vomiting. The mouth, nose, eyes, and stomach can bleed uncontrollably, with blood present in vomited material and bowel movements. The kidneys may begin to fail, and patients may go into a coma (an unconscious state in which a person cannot be awakened).

How Do Doctors Diagnose and Treat
Yellow Fever?

Early stages of yellow fever can be easily confused with other diseases such as malaria ^* , typhoid fever ^* , and other hemorrhagic (heh-muh-RAH-jik) fevers and types of viral hepatitis ^* . Blood tests can detect whether a patient's body has produced yellow fever antibodies ^* to fight the infection. Doctors also will take a travel history to see if a patient recently has visited a country where yellow fever occurs.

No specific treatment exists for yellow fever. Care is geared toward treating complications of the disease. In serious cases, intensive care in the hospital usually is needed. Patients may be given fluids to prevent dehydration ^* , and blood transfusions ^* may be necessary if bleeding is severe.

Most people who contract yellow fever recover from the early phase of the disease within a week; those who progress to the toxic phase may take several weeks or longer to recover. About half of those who develop toxic phase symptoms die within 2 weeks; the other half may recover without significant long-term problems.

How Can Yellow Fever Be Prevented?

Vaccination against yellow fever is the single most important prevention measure, and it is a must for people traveling to countries where the disease is common. Most countries in which yellow fever occurs require a certificate proving that travelers have been vaccinated before they are allowed into the country. One dose of vaccine provides at least 10 years of immunity.

Doctors recommend that infants under 6 months of age, pregnant women, people allergic to eggs (eggs are used in producing the vaccine), and people with weakened immune systems (such as people who have AIDS ^* or certain cancers) not receive the vaccine; these people are advised to delay visits to countries where yellow fever is endemic ^* .

Avoiding mosquito bites when traveling abroad reduces the risk of contracting yellow fever. To help prevent infection, experts suggest that travelers:

• wear long sleeves and pants
• avoid going outside when mosquitoes are active—at dawn, dusk, and early evening
• use mosquito repellent
• sleep beneath a mosquito net