Transmission of Yellow Fever Virus

Yellow fever virus is an RNA virus that belongs to the genus Flavivirus. It is related to West Nile, St. Louis encephalitis, and Japanese encephalitis viruses. 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.

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.

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.

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.