What is yellow fever?
Yellow fever is a viral disease found in tropical regions of Africa and the Americas. The yellow fever virus, which belongs to the flavivirus group, is spread by the bite of Aedes and Haemogogus mosquitoes. Infection causes a wide spectrum of disease, from mild symptoms to severe illness and death. The name of the disease is derived from the jaundice that affects some patients.
How common is yellow fever and how is it spread?
Yellow fever is constantly present with low levels of infection (i.e. endemic) in some tropical areas of Africa and the Americas. Areas at risk are from 15° north to 10° south of the equator, including most of the north and central areas of Africa, nine South American countries, and several Caribbean islands. The viral presence periodically amplifies and causes large epidemics.
Yellow fever has two main transmission cycles:
- Sylvatic (or jungle) yellow fever – In tropical rainforests, yellow fever occurs in monkeys that are infected by wild mosquitoes. Infected wild mosquitoes bite humans entering the forest (mostly young men involved in logging etc) resulting in sporadic cases of yellow fever.
- Urban yellow fever – large epidemics occur when migrants introduce the virus into areas with high human population density. Domestic mosquitoes then transmit the virus from person to person.
What are the signs and symptoms of yellow fever?
Yellow fever has an incubation period of three to six days. Following this the ‘acute’ phase develops. While some infections are asymptomatic, the majority are associated with fever, muscle pain (particularly backache), headache, and vomiting. Often, the high fever is paradoxically accompanied by a slow pulse. After four days, most patients improve and their symptoms disappear.
However, 15% of patients then enter a ‘toxic’ phase during which fever reappears and multiple body systems are affected:
- Jaundice and severe abdominal pain with vomiting develop
- Bleeding can occur from the mouth, nose, and eyes
- Haemorrhage and erosion of the mucosal lining of the stomach leads to blood in the vomit and faeces
- Liver damage can occur
- Kidney function deteriorates ranging from elevated urinary protein (albuminuria) to complete kidney failure with no urine output
50% of patients in the ‘toxic’ phase will die within 10 to 14 days.
How is yellow fever diagnosed?
Serology assays (blood tests) can detect yellow fever immunoglobulins (antibodies) that are produced in response to the infection. In travellers the preferred method of testing is immunoglobulin M (IgM) testing by means of ELISA. This test is 95% sensitive when serum specimens are collected 7-10 days after the onset of illness.
Polymerase chain reaction can also be used to identify viral ribonucleic acid (RNA) during acute infection, but clinical experience is limited. Several other techniques are also used to identify the virus in blood specimens or liver tissue collected after death.
How is yellow fever treated?
There is no specific treatment for yellow fever. Current management is based around supportive and preventive care such as fluid replacement, renal dialysis, and medication to reduce gastric acid production. Intensive supportive care may improve outcomes for seriously ill patients, but is rarely available in developing countries.
Medications such as interferon and ribavirin are currently being researched as a treatment for yellow fever.
How is yellow fever prevented?
Vaccination is the best defence against yellow fever. A single dose of the vaccine provides protection for at least 10 years (and probably for life). Immunity occurs within one week in 95% of people vaccinated.
Over 300 million doses have been given worldwide and serious side effects are extremely rare. In recent years, a few patients have developed severe illness potentially related to yellow fever vaccination. However, the risk to life from yellow fever in those who may be exposed to the virus is far greater than the risk from the vaccine.
Vaccination is highly recommended for travellers to high-risk areas in South America and Africa. A vaccination certificate is required for entry to many countries, particularly for travellers arriving in Asia from Africa or South America. Children less than six months old and pregnant travellers should not be vaccinated. In case of outbreaks, the vaccine may be given to pregnant women if the risk of infection is high.
People can do the following to prevent themselves from being bitten by mosquitoes.