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What causes yellow fever?
Yellow fever is caused by the yellow fever virus, of the genus Flavivirus.
Transmission
The yellow fever virus is transmitted by species of Aedes and Haemogogus mosquitoes, which – when infected by the virus – can cause infection when they bite their host, either a human or a monkey. The mosquito can also transmit the virus via infected eggs to its offspring.
There are three possible types of transmission cycle for yellow fever: sylvatic, intermediate and urban.
Sylvatic (or jungle) yellow fever mainly affects monkeys and is rare in humans. It spreads from infected mosquitoes to monkeys in the tropical rain forest. Infected monkeys can then pass the virus onto other mosquitoes that feed on them. The infected "wild" mosquitoes occasionally infect humans entering the forest.
Intermediate yellow fever occurs in small-scale epidemics in Africa. Many separate villages in an area can suffer cases simultaneously. "Semi-domestic" mosquitoes infect both monkeys and humans. This can evolve into a more severe urban-type epidemic if the infection is carried into areas that have large numbers of unvaccinated humans and "domestic" mosquitoes.
Urban yellow fever is spread by Aedes aegypti mosquitoes that have been infected by feeding on other infected people. Large epidemics can occur when migrants introduce the virus into areas with high human population density.
Who gets yellow fever and who is at risk?
Yellow fever can affect anyone living or travelling in areas of high incidence, where it continues to cause epidemics: several tropical and subtropical areas of Central America, the northern half of South America, and Central and West Africa.
There has been a marked resurgence of yellow fever in these parts since the 1980s. These areas have large, unvaccinated populations, and environmental changes such as deforestation and urbanisation have caused increased contact with infected mosquitoes. Mosquito numbers and habitats are also increasing.
Until the start of the twentieth century, yellow fever outbreaks also occurred in Europe and North America. Even though these areas are now free of yellow fever, small numbers of imported cases do occur and there is still some risk of an epidemic.
Yellow fever has never been reported in Asia, but the existence of the appropriate primates and mosquitoes supports the potential for the disease to occur in the area.
Although yellow fever is a very rare cause of illness in travellers to high-incidence areas, it is a potentially fatal illness and is preventable by taking certain precautions as well as by vaccination (see later).
Symptoms and signs
Symptoms typically occur within three to six days after exposure. There are two disease phases. The first phase may include:
- Fever
- Muscle pain, especially backache
- Headache
- Shivers
- Loss of appetite
- Nausea and/or vomiting
- Unusually red tongue
- Flushed face
- Reddened eyes
After three to four days, most cases improve and symptoms disappear. Fifteen to twenty percent of cases enter a second phase within 24 hours, which may include:
- Fever
- Slow, weak pulse
- Jaundice, which may lead to liver failure
- Abdominal pain
- Vomiting
- Bleeding from the mouth, nose, eyes and/or stomach
- Impaired kidney function, ranging from abnormal protein levels in the urine to complete kidney failure with no urine production
- Delirium and seizures followed by shock and coma
Half the number of people who enter the second phase die within two weeks. The remainder recover without significant organ damage.
How is yellow fever diagnosed?
Yellow fever is difficult to recognise, especially during the early stages, and can easily be confused with several other conditions, such as malaria.
A doctor may suspect yellow fever after taking a medical history and performing a medical examination, but laboratory analysis is required for a definitive diagnosis. Blood tests (serology assays) can detect yellow fever antibodies, which the immune system produces in response to the infection.
Several other techniques may be used to identify the virus itself in blood specimens, and these tests are only carried out in highly specialised laboratories due to the dangerous nature of the virus itself. Liver biopsy is not normally carried out for diagnostic purposes as there is a risk of serious bleeding from the liver if a biopsy is taken from a patient with yellow fever.
How is yellow fever treated?
There is no specific treatment for yellow fever, other than supportive care. The doctor will recommend rest and drinking plenty of fluids. In more serious cases, intravenous fluids may be given to treat dehydration, low blood pressure and low blood sugar. Antacids and H2 histamine blockers are often prescribed to prevent bleeding in the stomach. Fever is controlled with medications such as acetaminophen.
Dialysis may be required if kidney failure occurs. Blood transfusions may be needed to control severe bleeding disorders.
Can yellow fever be prevented?
Vaccination
Vaccination is the most important way to prevent yellow fever. The vaccine is safe and highly effective: immunity (protection) occurs within a week in 95% of people vaccinated, and lasts for at least 10 years. If you are at continued risk of infection, a booster dose is needed every 10 years. Serious side-effects are very rare.
Incorporating the vaccine into routine national vaccination programmes is more effective – and cheaper – than emergency vaccination campaigns to control an epidemic. Unfortunately, at least 80% of the population must have immunity in order to prevent an epidemic, and few African countries have this vaccination level.
Vaccination is recommended for travellers to high-risk areas. International regulations require proof of yellow fever vaccination for travel to and from certain countries. People who get vaccinated should receive an International Certificate of Vaccination.
The following groups should not be vaccinated unless the risk of contracting the disease exceeds the small risk associated with the vaccine:
- Infants under six months, because of the risk of viral encephalitis.
- Pregnant women, because of a theoretical risk that the vaccine may infect the foetus.
- People allergic to eggs, as the vaccine is grown in eggs.
- People with compromised immune systems, such as those with HIV/Aids.
If you fall into one of these groups, ask your doctor for advice before being vaccinated.
Surveillance
In populations where vaccination coverage is low, ongoing surveillance is vital for prompt detection of outbreaks and rapid response with emergency vaccination campaigns. Improvement in yellow fever surveillance is needed in many countries, as many cases go unreported.
Mosquito control
Eliminating potential mosquito breeding sites is an important means of controlling mosquito-transmitted diseases, and helping prevent transmission until vaccination has taken effect. However, priority is given to vaccination programmes for prevention and control of yellow fever.
General precautions should be taken to avoid mosquito bites, such as using insect repellent, protective clothing and mosquito netting.
What is the outcome of yellow fever?
Many yellow fever infections are mild, and some may show no symptoms. Most people regain health after a long recovery period. However, the disease can cause severe, life-threatening illness, and the mortality rate remains high, even with good supportive care.
Complications may include:
- Kidney failure
- Liver failure
- Disseminated intravascular coagulation (a blood disorder that causes excessive bleeding)
- Secondary infections
When to call the doctor
Call your doctor immediately if you live in or have recently visited an area where yellow fever is endemic, and you develop any of the symptoms of yellow fever.
Ask your doctor for advice about preventative measures if you are planning a trip to an area with a high incidence of yellow fever.
