What is yellow fever?
Yellow fever (YF) is a disease caused by infection with yellow fever virus. It is considered to be one of the most lethal viral diseases. It is transmitted to humans through the bite of infected mosquitoes.
Where does yellow fever occur?
YF occurs in tropical regions of Africa and in parts of South America. It is a very rare cause of illness in travellers to these areas. Endemic regions include countries (or areas within countries) where there is the potential for human infection because of the presence of the mosquito vector and of the YF virus in non-human primates. To date YF has not appeared in Asia or the Pacific region. The World Health Organization (WHO) estimates that there are approximately 200,000 cases of yellow fever, causing 30,000 deaths, worldwide each year, with 90% occurring in Africa.
The Centers for Disease Control and Prevention (CDC) in the United States maintain a list of countries affected by YF.
Information on recent outbreaks of YF can be accessed on the WHO website
What are the signs and symptoms of yellow fever?
YF varies in severity. The infection has an incubation period of 3 to 6 days. Initial symptoms include myalgia, pyrexia, headache, anorexia, nausea and vomiting. In many patients there will be improvement in symptoms and gradual recovery occurring three to four days after the onset of symptoms. However, within 24 hours of an apparent recovery, 15% to 25% of patients progress to a more serious illness. This takes the form of an acute haemorrhagic fever, in which there may be bleeding from the mouth, eyes, ears and stomach, pronounced jaundice (from which the disease gets its name) and kidney damage. The patient develops shock (very low blood pressure) and kidneys, liver and heart begin to fail. Twenty to 50% of patients who develop this form of the disease die within 7-10 days after the onset. Infection, however, confers lifelong immunity in those who recover. YF is a notifiable disease in Ireland.
What is the treatment for yellow fever?
There is no specific antiviral treatment. Supportive nursing care and symptomatic management are the standard.
How is yellow fever transmitted?
Jungle primates and humans are the vertebrate hosts for the YF virus. The female Aedes sp. or Haemogogus sp. (South America only) mosquito is the vector and a bite from an infected female of these species may transmit the virus.
There are three types of transmission cycle for yellow fever: sylvatic, intermediate and urban. All three cycles exist in Africa, but in South America, only sylvatic and urban yellow fever occur.
- Sylvatic (or jungle) yellow fever: In tropical rainforests, yellow fever occurs in monkeys that are infected by wild mosquitoes. The infected monkeys can then pass the virus onto other mosquitoes that feed on them. These infected wild mosquitoes bite humans entering the forest resulting in sporadic cases of yellow fever. The majority of cases are young men working in the forest (logging, etc). On occasion, the virus spreads beyond the affected individual.
- Intermediate yellow fever: In humid or semi-humid savannahs of Africa, small-scale epidemics occur. These behave differently from urban epidemics; many separate villages in an area suffer cases simultaneously, but fewer people die from infection. Semi-domestic mosquitoes infect both monkey and human hosts. This area is often called the "zone of emergence", where increased contact between man and infected mosquito leads to disease. This is the most common type of outbreak seen in recent decades in Africa. It can shift to a more severe urban-type epidemic if the infection is carried into a suitable environment (with the presence of domestic mosquitoes and unvaccinated humans).
- Urban yellow fever: Large epidemics can occur when migrants introduce the virus into areas with high human population density. Domestic mosquitoes (of one species, Aedes aegypti) carry the virus from person to person; no monkeys are involved in transmission. These outbreaks tend to spread outwards from one source to cover a wide area.
The Aedes mosquito is active during daylight hours and bites from dawn to dusk. Once infected with the virus, the mosquito remains infectious for life (2-3 months). Whilst the mosquito is killed by extremes of heat and cold, the virus can survive from season to season in mosquito eggs. This makes eradication of the disease difficult.
What is the risk to travellers from yellow fever?
The risk of contracting YF is determined by the following factors:
- Travel destination
- Intensity of YF transmission in area to be visited
- Season of travel
- Duration of travel
- Activities allowing exposure to mosquitoes
- Immunisation status
Although ongoing cases and outbreaks of YF are occurring in Africa and South America, the disease is preventable by vaccination and remains a very rare cause of illness in travellers. From 1970 through 2015, a total of 10 cases of yellow fever were reported in unvaccinated travellers from the United States and Europe who travelled to West Africa (5 cases) or South America (5 cases). Eight (80%) of these 10 travellers died. There has been only 1 documented case of yellow fever in a vaccinated traveller. This nonfatal case occurred in a traveller from Europe who visited several West African countries in 1988.
How is yellow fever prevented?
There are two methods to prevent YF:
YF immunisation has two primary aims:
- To protect individuals against YF. YF is often fatal in the unvaccinated; the vaccine is recommended for all travellers to areas that have a risk of transmission.
- To prevent international spread of YF. Under the International Health Regulations, proof of vaccination or prophylaxis may be required for travellers as a condition of entry to a State. Countries currently YF-free that consider that there is a risk of YF gaining a foothold can require travellers to produce proof of vaccination (in the form of a YF certificate). Some countries even require proof of vaccination from transiting travellers. Failure to produce a certificate may result in the traveller being placed in quarantine.
Information on YF vaccination can be found in Chapter 5 Immunisations and Health Information for Travel of the Immunisation Guidelines for Ireland, 2013.
Further information on yellow fever is available from:
The World Health Organization's International Travel and Health Manual
CDC's YF section in the Yellow Book
Last updated: 5 April 2017