Epidemiology of Infectious Diseases: Yellow Fever
Causal agent
An arthropod borne virus of the Flaviviridae family with the genus flavivirus1.
Common clinical features
Two disease phases.
1 - Symptoms range from no symptoms to acute phase characterized by fever, muscle pain (with prominent backache), headache, shivers, anorexia, nausea and/or vomiting. Improvement in symptoms occurs within three to four days after the onset.
2 - A second phase (15-25% of cases) enter a 'toxic phase' within 24 hours (of apparent recovery). Fever reappears and several body systems are affected. Symptoms include, jaundice, abdominal pain and vomiting. Bleeding can occur from the mouth, nose, eyes and/or stomach. Kidney function deteriorates. 50% of all patients in the 'toxic phase' die within 10-14 days1,4.
Epidemiology
- Endemic in tropical areas of Africa and South America.
- WHO estimate that there are 200,000 cases and 30,000 deaths each year3.
- In Africa yellow fever is endemic in 33 countries in sub-Saharan Africa. West Africa is the most affected region. According to the WHO circulation of the virus in Africa has increased significantly in recent years. In addition the risk of large outbreaks in urban areas in Africa has increased significantly due to accelerated urbanization which has concentrated a non-immune population in areas of high population and vector density.
- In the Americas yellow fever is endemic in 10 countries and in several Caribbean islands. Bolivia, Brazil, Colombia, Ecuador and Peru are considered at the greatest risk4.
- According to the Health Protection Agency yellow fever is rare in travellers, but since 1996 there have been 6 fatal cases in European and US travellers.
Reservoir
Urban areas - humans and Aedes mosquitoes.
Forest areas - vertebrates (other than humans), mainly monkeys and possibly marsupials, and forest mosquitoes1.
Mode of transmission
Several different species of the Aedes and Haemogogus (South America only) mosquitoes transmit the virus.
There are three types of transmission cycle for yellow fever4.
Sylvatic (jungle)
In tropical rainforests of Africa and South America. The transmission cycle occurs between monkeys and wild mosquitoes. Humans may become infected when they live or work in these areas1,4.
Intermediate yellow fever
In humid or semi-humid savannahs of Africa, small-scale epidemics occur. Many separate villages in an area suffer cases simultaneously, but fewer people die from infection. Simi-domestic mosquitoes infect both monkey and human host. This area iscalled the 'zone of emergence', where increased contact between man and infected mosquito leads to disease. It is the most common type of outbreak seen in Africa in recent years4.
Urban yellow fever
Large epidemics occur when infected migrants introduce the virus into highly populated urban areas. Domestic mosquitoes (mainly Aedes aegypti) then transmit the virus from person to person (no monkeys are involved in transmission)1,4.
The Aedes mosquito is active during daylight hours and remains infectious for life (2-3 months)4.
Incubation period
3-6 days2
Period of Communicability
Blood of patients is infective for mosquitoes shortly before onset of fever and for the first 3-5 days of illness.
Infection confers lifelong immunity.
Prevention and control
Immunization- Single dose provides protection for 10 years (probably life).
Surveillance
Vector control
Avoidance of mosquitoes bites
References
- Health Protection Agency, Travel Health Surveillance Section, Yellow Fever factsheet, http://www.nathnac.org/pro/factsheets/yellow.htm
- Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association, 2004.
- World Health Organization, The Yellow fever situation in Africa and South America in 2004, Weekly Epidemiological Record. Vol. 80, 29, 2005, 250-256. Available at: http://www.who.int/wer/2005/wer8029.pdf
- World Health Organization, Information: Yellow Fever factsheet No100, 2001, available at: http://www.who.int/mediacentre/factsheets/fs100/en/index.html
© CM Kirwan 2006