Transmission of Yellow Fever Virus

Key points

  • Yellow fever virus is a mosquito-borne flavivirus, similar to dengue, West Nile, and St. Louis encephalitis viruses.
  • The virus is maintained in a cycle between mosquitoes and vertebrate hosts, primarily non-human primates.
  • Humans with yellow fever are infectious to mosquitoes shortly before the onset of fever and up to 5 days after onset.
  • One case of perinatal transmission of yellow fever virus has been documented.
Yellow fever virus image

Virus

Yellow fever virus is an RNA virus that belongs to the genus Flavivirus. It is in the same genus as several other medically important viruses, such as dengue virus, West Nile virus, and St. Louis encephalitis virus.

Photomicrograph depicting numerous yellow fever virions
Electron microscopic image of numerous yellow fever virions.

Transmission

Yellow fever virus is transmitted to people primarily through the bite of infected Aedes or Haemagogus species mosquitoes. Mosquitoes acquire the virus by feeding on infected primates (human or non-human) and then can transmit the virus to other primates (human or non-human). People infected with yellow fever virus are infectious to mosquitoes (referred to as being "viremic") shortly before the onset of fever and up to 5 days after onset.

Because of the high level of viremia, bloodborne transmission theoretically can occur via transfusion or needlesticks. One case of perinatal transmission of wild-type yellow fever virus from a woman who developed symptoms of YF 3 days prior to delivery has been documented; the infant subsequently tested positive for yellow fever viral RNA and died of fulminant yellow fever on the 12th day of life.

Yellow fever virus has three transmission cycles: jungle (sylvatic), inter­mediate (savannah), and urban.

  • The jungle (sylvatic) cycle involves transmission of the virus between non-human primates (e.g., monkeys) and mosquito species found in the forest canopy. The virus is transmitted by mosquitoes from monkeys to humans when humans are visiting or working in the jungle.
  • In Africa, an intermediate (savannah) cycle exists that involves transmission of virus from mosquitoes to humans living or working in jungle border areas. In this cycle, the virus can be transmitted from monkey to human or from human to human via mosquitoes.
  • 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 viremic human who was infected in the jungle or savannah.
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Image of yellow fever transmission cycle in an African setting. A mosquito and a non-human primate are shown on opposite sides of a circle with arrows connecting them to represent how yellow fever cycles between mosquitoes and animals in the jungle setting. A dashed line connects the circle to another circle in which the arrows connect a mosquito and a human demonstrating how yellow fever spreads in a city setting. Another dashed line connects the human to a third circle, in which the arrows connect mosquitoes, a human, and a non-human primate to show how yellow fever spreads in a savanna setting. The background of the first circle is a jungle landscape, the second is an urban city, and the third is a savanna.
Yellow fever virus transmission cycles in Africa.
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Image of yellow fever transmission cycle in a South American setting. A mosquito and a non-human primate are shown on opposite sides of a circle with arrows connecting them to represent how yellow fever cycles between mosquitoes and animals in the jungle setting. A dashed line connects the circle to another circle in which the arrows connect a mosquito and a human to show how yellow fever spreads in a city setting. The background of the first circle is a jungle landscape and the second is an urban city.
Yellow fever virus transmission cycles in South America.
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Content Source:
National Center for Emerging and Zoonotic Infectious Diseases (NCEZID)