Yellow fever is a viral hemorrhagic fever caused by the yellow fever virus. The yellow fever virus is a single-stranded enveloped virus that belongs to the flavivirus group. The disease can result in mild symptoms or severe illness and death (mortality rate 5-70%). Yellow fever derives its name from the yellowing of the skin and whites of the eyes (jaundice) that occur in some people infected with the virus. Jaundice is caused by the presence of bile pigment (bilirubin) in the bloodstream and results from damage to liver cells (hepatocytes) during severe infection.
The yellow fever virus infects mainly monkeys and humans: monkeys are the animal reservoir. Infection is transmitted from human to human, monkey to monkey, monkey to human, and human to monkey by daytime-biting mosquitos. Several species of Aedes and Haemoagogus mosquitos can serve as vectors, transmitting the virus during a blood meal.
Three types of transmission cycles exist for yellow fever: sylvatic (jungle), intermediate and urban. Although all three transmission cycles occur in Africa, only sylvatic and urban transmission cycles occur in South America.
Sylvatic yellow fever (monkey to human)
- Occurs in monkeys infected by wild mosquitos in tropical rainforests
- Infected monkeys pass the virus to mosquitos during feeding
- Infected wild mosquitos bite humans entering the rainforest (accidental infection)
Intermediate yellow fever (monkey to human; human to monkey)
- Small-scale epidemics that occur in humid or semi-humid grasslands of Africa
- Separate villages experience simultaneous infections transmitted by semi-domestic mosquitos that infect both monkey and human hosts
- Most common type of outbreak in Africa
Urban yellow fever (human to human)
- Large epidemics occurring when the virus is introduced into high human population areas by migrants
- Domestic mosquitos of one species (Aedes aegypti) transmit the virus from person to person
- Monkeys are not involved in transmission
- Outbreaks spread from one source to cover a wide area
Although viral replication begins in cells at the site of the mosquito bite, symptoms of infection are not usually noted for a period of three to six days when the acute phase of infection presents. Acute yellow fever infection is characterized by high fever, muscle pain, backache, headache, shivers, loss of appetite, nausea and/or vomiting. Most people infected improve after three to four days.
However, within 24 hours of the disappearance of symptoms, up to 15% of those infected enter a toxic phase during which fever resumes, and the yellow fever virus quickly spreads to the kidneys, lymph nodes, spleen, bone marrow and liver. Liver invasion of one of the last stages to occur: as the liver is increasingly damaged, patients develop jaundice as bilirubin is released from damaged liver cells, experience abdominal pain and vomiting, and develop coagulopathies (inability of the blood to clot) characterized by bleeding from the mouth, nose, eyes and stomach, and presence of blood in vomit and stool. Up to 50% of people who enter the toxic phase die within two weeks of infection.
Yellow fever may be difficult to diagnose, especially during the early stages, and may be confused with malaria, typhoid, other hemorrhagic fevers (dengue, Rift Valley, Venezuelan, Bolivian, Argentine, Lassa, Crimean-Congo, Marburg and Ebola), rickettsial infection, leptospirosis, viral hepatitis, other causes of liver failure and toxic hepatitis (e.g. carbon-tetrachloride poisoning).
The treatment for yellow fever is supportive: control of fever, fluids to treat dehydration, and intensive support related to organ damage.
The World Health Organization estimates 200,000 cases of yellow fever per year with approximately 30,000 deaths.
Further Reading:
Mosquito-Borne Disease Prevention
Sources:
Yellow fever (World Health Organization)
Yellow fever (Centers for Disease Control and Prevention)
