Yellow fever

Yellow fever affects countries in Africa and Central and South America. Increased contact between people and infected mosquitoes, the vector of yellow fever, can lead to an increase in transmission. Large epidemics happen when people who are infected introduce the virus in densely populated urban areas with a high density of Aedes aegypti mosquitoes and where most people have little or no immunity. There is no anti-viral drug but the vaccine is sufficient to provide immunity for life. The Eliminate Yellow Fever Epidemics (EYE) Strategy launched in 2017 aims at building a global coalition to protect at-risk populations, prevent international spread and contain outbreaks rapidly.

The following are standard case definitions to allow national health authorities to interpret data in an international context. However, during an outbreak case definitions may be adapted to the local context and the Red Cross Red Crescent should use those agreed/established by national health authorities. NB: Consider that during community-based surveillance, volunteers should use broad (simplified) case definitions (referred to as community case definitions) to recognize most or all possible cases, provide relevant risk communication and appropriate actions and encourage them to seek care. Other actors such as healthcare workers or investigators studying the cause of a disease, on the other hand, can use more specific case definitions that may require laboratory confirmation.

Suspected case: Any person with acute onset of fever, with jaundice appearing within 14 days of onset of the first symptoms.

Probable case: A suspected case and one of the following: presence of yellow fever IgM antibodies in the absence of yellow fever immunization within 30 days before onset of illness; OR

positive post-mortem liver histopathology; OR

epidemiological link to a confirmed case or an outbreak.

Confirmed case: a) A probable case AND absence of yellow fever immunization within 30 days before onset of illness AND one of the following: detection of yellow fever-specific neutralizing antibodies.; OR

b) absence of yellow fever immunization within 14 days before onset of illness AND one of the following: detection of yellow fever virus genome in blood or other organs by polymerase chain reaction (PCR) OR detection of yellow fever antigen in blood, liver or other organs by immunoassay OR isolation of yellow fever virus.

WHO case definition source of information here.

One confirmed case is sufficient to identify a potential outbreak, but this needs to be interpreted in context (e.g. vaccination history, specific serologic test). For example one confirmed case in an unvaccinated urban population requires immediate intervention.

• Environments where mosquitoes breed such as tropical rainforests, humid, and semi-humid environments, as well as around bodies of stagnant water in and close to human habitations in urban settings.
• Increased contact between humans and infected mosquitoes, particularly in urban areas where people have not been vaccinated for yellow fever, can create epidemics.
• Outbreaks of the disease are of particular concern when they occur in overcrowded settings with inadequate water supply and waste management services that allow the mosquitoes to breed easily.
• Seasonality is a risk factor for yellow fever, though this will vary geographically. In west Africa for instance, risk is higher at the end of the rainy season and beginning of the dry season. However, in South America the risk of infection is highest during the rainy season.

Attack rates will vary from one outbreak to another. In case of an outbreak, consult the latest information provided by health authorities.

• Infancy and older age are associated with increased severity and lethality of infection with yellow fever virus.
• Immunosuppressed persons such as those receiving chemotherapy, transplant recipients or HIV carriers.
• People with chronic diseases such as renal disease, cancer, chronic lung or liver disease, and diabetes.

Yellow fever virus.

Vector: Mosquitoes (Aedes and Haemagogus) that carry the yellow fever virus.

Monkeys and humans.

Vector borne: Yellow fever virus is transmitted to people primarily through the bite of infected Aedes or Haemagogus species mosquitoes. It is primarily a day biting mosquito. The transmission of yellow fever has three different cycles: jungle (sylvatic), intermediate (savannah) and urban. It is important to know the difference to ensure the right prevention measures are being used:

• The jungle (sylvatic) cycle happens when mosquitoes found in the forest canopy infect monkeys. The virus can be transmitted by mosquitoes from monkeys to humans when humans are visiting or working in the jungle. This does not cause large epidemics but should be prevented and controlled quickly.
• In Africa, an intermediate (savannah) cycle occurs when the virus is transmitted from mosquitoes to humans living or working at the border of the jungle. Human-to-human transmission can occur.
• The urban cycle involves transmission of the virus between humans and urban mosquitoes, primarily Aedes aegypti. The virus is usually brought to the urban setting by a human who was infected in the jungle or savannah.  This is the type of transmission that can create large epidemics.

Six to seven days (range three to ten days).

People infected with yellow fever virus can infect a mosquito shortly before they suffer from a fever and up to five days after they first get sick. Infected mosquitoes go on to infect other people.

• Most people infected with the yellow fever virus have no illness or only have a mild illness which improves after the initial presentation.
• The initial symptoms include sudden onset of fever, chills, severe headache, back pain, general body aches, nausea, vomiting, fatigue and weakness.
• Approximately 15 per cent of cases progress to develop a more severe form of the disease after a periodic decrease of symptoms for one to two days. This is characterized by high fever, jaundice, bleeding and eventual shock and failure of multiple organs.

Other haemorrhagic fevers (dengue fever, Rift Valley fever, Crimean-Congo fever, Lassa fever, Marburg haemorrhagic fever, Ebola virus disease, South American haemorrhagic fevers), viral and non-infectious hepatic diseases, malaria, leptospirosis, tick-borne relapsing fever, typhus and typhoid fever.

• Laboratory diagnosis of yellow fever by testing serum to detect virus-specific IgM antibodies and neutralizing antibodies.
• Sometimes the virus can be found in blood samples taken early in the course of the disease.

Please refer to the appropriate local or international guidelines for clinical management. All clinical management including the administration of any treatment or vaccine should be conducted by health professionals.

Important therapy principles include:

• No specific treatments have been found to benefit patients with yellow fever. Whenever possible, these patients should be hospitalized for supportive care and close observation.
• Yellow fever patients should be protected from further mosquito exposure (staying indoors and/or under a mosquito net) for up to five days after the onset of fever. This way, the yellow fever virus in their bloodstream will be unavailable to uninfected mosquitoes, thus breaking the transmission cycle and reducing risk to the people around them.
• Yellow fever is vaccine-preventable.

• Those who recover from yellow fever generally have lasting immunity against subsequent infection.
• The one-time vaccine provides effective immunity within 30 days for 99 per cent of persons vaccinated. Vaccination provides life-long immunity.