Yellow fever is caused by a virus that is spread by several species of Aedes and Haemogus mosquitoes. Infection causes disease, with symptoms ranging from a mild flu to severe illness, including high fever, severe headache, muscle pain, jaundice, and vomiting of a liquefied black putrid matter. It is the pronounced yellow skin and eye color of severe jaundice that gives rise to the name yellow fever. Historically, epidemics occur when the disease breaks out in previously unexposed populations.
The natural sources of yellow fever are monkeys and mosquitoes, which inhabit the jungles of South America and Africa. Previously unexposed Europeans who traveled to the Caribbean, Central America, and North and South America were common victims for several hundred years. In 1741, 20,000 British soldiers died of the disease, then called “Black Vomit,” during an expedition to capture Peru and Mexico.
Yellow fever was brought to the Americas on slave ships in the 1500s. However, it was United States coastal towns that were particularly vulnerable to the disease in the seventeenth and eighteenth centuries. In 1793, the city of Philadelphia lost nearly 10 percent of its population to an epidemic of yellow fever. Eighty-five years later, in 1878, 20,000 people in the United States died from the disease. The last major United States epidemic occurred in New Orleans in 1905, and the last urban case of the disease occurred in 1942. However, in an all-too-familiar scenario, two cases were reported in 1996 and 1999 from returning foreign travelers.
Yellow fever got its name because of the severe jaundice that is a symptom of the disease—it causes pronounced yellow skin and eye color. At one time it was called “Black Vomit” because of the characteristic black putrid vomit it produces.
There are approximately 200,000 cases of yellow fever, resulting in 30,000 deaths, each year in tropical endemic areas of Africa and the Americas. Thirty-three countries, with a combined population of 508 million, are at risk in Africa. In the Americas, yellow fever is endemic in several Caribbean islands and in nine South American countries, including Bolivia, Brazil, Colombia, Ecuador, and Peru, where sporadic infections occur in forestry and agricultural workers. In Africa, the virus is transmitted in the damp savanna zones of West Africa during the rainy season, and infections occur principally among children. Periodically, large outbreaks occur in cities and villages, resulting in many thousands of cases.
Despite an effective vaccine, the number of yellow fever cases continues to grow worldwide. This is largely due to changes in the world's ecology from deforestation and urbanization, which have increased the mosquito/virus contact.
Everybody who can, is fleeing from the city, and the panic of the country people is likely to add famine to the disease.
—Thomas Jefferson, commenting on the panic caused by the 1793 yellow fever outbreak in Philadelphia
Philadelphia was the most prominent American city in 1793, serving as the seat of the federal government. In August of that year, a deadly epidemic emerged. Numerous individuals shared common symptoms: severe fever, nausea, skin eruptions, black vomit, profound lethargy, rapid but weak pulse, incontinence, and a distinct yellow skin and eye color. Yellow fever was spreading through Philadelphia.
Terror gripped the city as nearly 600 people died of the disease in the first four weeks. Half the population fled in panic. Most business and commerce ceased. All federal, state, and municipal government was suspended, and President George Washington abruptly left the city. By the time the epidemic waned some three months later, 5,000 people, or nearly 10 percent of the population, had succumbed to yellow fever.
Yellow fever is caused by a virus, which infects monkeys as well as humans. The mosquito that carries the virus is most active in the early morning and late afternoon. It breeds in fresh water in both natural and artificial containers in and around human dwellings, including small ponds, gutters, old tires, flowerpots, and water storage containers.
Once someone is infected, the virus remains silent in the body during an incubation period of three to six days. There are then two disease phases. An “acute” phase is characterized by fever, chills, muscle pain, backache, headache, loss of appetite, and nausea. After three to four days, most patients improve and their symptoms disappear.
However, some 15 percent of infected patients enter a second and more deadly “toxic” phase. These patients experience a reappearance of a fever, pronounced jaundice occurs resulting in distinct yellowing of the skin and eyes, abdominal pain, and vomiting. Bleeding may occur from the mouth, nose, and eyes. Bleeding within the stomach produces the so-called “black vomit.” As kidney function deteriorates, patients often die within 10 to 14 days in the “toxic phase.”
Yellow fever is difficult to recognize and in its early stages can be mistaken for malaria, typhoid, rickettsial diseases, other hemorrhagic viral fevers, or viral hepatitis. Specific diagnosis depends on isolation of the virus from blood and/or finding viral antigen in a sample. Liver biopsy, or surgically removing a portion of the liver for testing, has been used to confirm the diagnosis, but it is not recommended given the hemorrhagic properties of the disease.
Humans and monkeys are the primary animals infected by the virus. The virus spreads from one animal to another by the mosquito. Infected mosquito eggs can lie dormant through dry conditions and hatch when the rainy season begins. The transmission cycles for yellow fever include “jungle,” “intermediate,” and “urban.” “Jungle” yellow fever occurs in tropical rainforests where infected monkeys pass the virus on to wild mosquitoes that feed on them. The mosquitoes in turn bite humans entering the forest, resulting in sporadic cases of yellow fever. “Intermediate” yellow fever causes small epidemics in humid or semi-humid savannahs of Africa. “Urban” yellow fever causes epidemics in cities and villages with large numbers of cases.
In 1900, at an experimental station near Havana, Cuba, Dr. Walter Reed and his assistants proved that the Aedes aegypti species of mosquito was the vector for the yellow fever virus. Their work disproved the notion that yellow fever could be spread by direct contact with infected people or contaminated objects. The team made its discovery while they were part of the American occupation force in Cuba in the wake of the Spanish-American War. During the war, yellow fever, malaria, and dysentery accounted for many more deaths than were inflicted by soldiers.
There is no specific treatment for yellow fever. Typically, patients are treated for dehydration and fever with rehydration salts. Intensive care is needed to improve the outcome for seriously ill patients.
Mosquito-control measures to reduce breeding sites are important to prevent virus transmission. Similarly, general precautions to avoid mosquito bites are effective at preventing bites and infection. These methods include the use of insect repellent, protective clothing, and mosquito netting. In many endemic areas, spraying is impractical and vaccine programs are the best way to limit disease outbreaks. The World Health Organization (WHO) strongly recommends routine childhood vaccination to prevent an epidemic in endemic countries.
Vaccination is the single most effective way to prevent yellow fever. The vaccine has been used since the 1930s and is safe and highly effective in preventing yellow fever in adults and children over nine months of age. A single dose of vaccine confers immunity within one week and lasts for 10 years or more. Minor symptoms like mild headache and muscle pain occur within 10 days of vaccination in less than 5 percent of vaccinated individuals.
Vaccination is highly recommended for people traveling to high-risk areas. A vaccination certificate is required for entry to many countries in Asia, Africa, and South America.
More serious side effects are extremely rare but medical help should be sought in the event of seizures, difficulty breathing or swallowing, fast heartbeat, feeling of burning, tingling of skin, severe headache, skin rash or itching, sneezing, stiff neck, throbbing in the ears, unusual tiredness or weakness, and/or vomiting. The vaccine should not be given to people with weakened immune systems, pregnant women, or people who are allergic to eggs.