Yellow Fever
A viral disease with no specific treatment

Yellow Fever is a viral disease transmitted by the bite of an infected mosquito. “Yellow” in its name stands for jaundice which can be a symptom of the disease. It initially presents with fever, vomiting, fatigue; while jaundice and bleeding occur in the severe form. It is important to take preventive measures as there is no specific anti-viral drug for yellow fever.



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What causes Yellow Fever

Yellow fever is caused by a virus that belongs to the genus Flavivirus, like Dengue. This virus is transmitted by the bite of an infected Aedes or Haemagogus mosquito. The mosquito acquires the virus by feeding on an already infected person. A person is considered infective to a mosquito shortly before the onset of fever and up to 5 days after onset.

The transmission of Yellow Fever is in the form of three cycles:

  • The jungle cycle operates in forests between monkeys and mosquitoes. A person can get infected when visiting or working in a jungle.
  • The intermediate cycle involves both monkeys and humans. It exists in semi-domestic or areas bordering jungles. It is common in Africa.
  • The urban cycle involves a person who was infected by the jungle or intermediate cycle. It may lead to outbreaks in urban areas where transmission occurs between Aedes mosquitoes and humans.
Photo: Aedes aegypti mosquito

Where is Yellow Fever found

Yellow Fever is found in the tropical and subtropical areas of Africa, South America, and Central America. Proof of vaccination may be required for travelers visiting the endemic areas to prevent the spread of this disease in areas that are currently free of it.

The evolutionary origins of yellow fever most likely lie in Africa, with transmission of the disease from nonhuman primates to humans. The virus is thought to have originated in East or Central Africa and spread from there to West Africa. As it was endemic in Africa, local populations had developed some immunity to it.

Image: geographical distribution of Yellow Fever.

What are the symptoms of Yellow Fever

There is an incubation period (time from infection to development of illness) of about 3 to 6 days. A person may be asymptomatic or complain of non-specific features like fever, headache, nausea, vomiting, loss of appetite, general body aches, and fatigue. These symptoms generally resolve in a week.

Few people develop a severe form of the disease in which fever returns along with jaundice (yellowish discolouration of the skin and whites of the eyes), bleeding, abdominal pain, and vomiting. This may progress to organ failure, primarily affecting the liver and kidneys. A fraction of people who develop the severe form of the disease die in about two weeks.

The overall fatality rate is estimated to be between 3 and 7.5%. Surviving the infection provides lifelong immunity

How can Yellow Fever be prevented

Controlling mosquito populations is the main way to prevent and reduce Yellow Fever. This includes source reduction, pesticide spraying for larval control and fumigation for adult control, and/or the use of mosquito traps.

Wear long-sleeved clothing and long trousers when outdoors.

The Yellow Fever vaccine is an important part of the prevention of this disease. A single dose provides life-long protection and a booster dose is not required. An International Certificate of Vaccination is required to travel to high-risk areas. For activation of the vaccine, it is imperative to get the vaccine ten days before traveling.

The vaccine is not recommended for infants less than 9 months of age and people above 65 years of age, pregnant women and nursing mothers, people with a severe allergy to egg protein, and people with compromised immune systems.

It is a safe vaccine. A mild reaction of low-grade fever and muscle aches can occur. Rarely, a life-threatening reaction is possible. No vaccine is 100% effective. A person may still develop the disease despite vaccination but the severity of the disease is reduced.


How is Yellow Fever diagnosed

The diagnosis of Yellow Fever is typically made based on symptoms in endemic areas. However, early disease can be difficult to differentiate from other viral infections.

Serologically, an enzyme-linked immunosorbent assay during the acute phase of the disease using specific IgM against yellow fever or an increase in specific IgG titer (compared to an earlier sample) can confirm yellow fever. A four-fold increase in IgG titer is considered sufficient indication for yellow fever. As these tests can cross-react with other flaviviruses, such as dengue virus, these indirect methods cannot conclusively prove yellow fever infection.

Yellow Fever can be detected by Polymerase chain reaction (PCR) tests, such as the ones made available by the WoIDM. The PCR tests are more accurate than antigen detection.

How is Yellow Fever treated

There is no specific anti-viral drug for Yellow Fever. Supportive care is given in the form of rest, hydration, and medication to relieve pain and fever. Some medicines that relieve fever increase the risk of bleeding and should not be taken without a doctor’s consultation. Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) are to be avoided  because of an increased risk of gastrointestinal bleeding due to their anticoagulant effects. Patients with the severe form will need to be hospitalized as there is a risk of rapid deterioration.

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