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Published On: Saturday February 11, 2017
Yellow fever virus is found in tropical and subtropical areas in South America and Africa. The virus is transmitted to people by the bite of an infected mosquito. Illness ranges in severity from a self-limited febrile illness to severe liver disease with bleeding.
Yellow fever disease is diagnosed based on symptoms, physical findings, laboratory testing, and travel history, including the possibility of exposure to infected mosquitoes. There is no specific treatment for yellow fever; care is based on symptoms. Steps to prevent yellow fever virus infection include using insect repellent, wearing protective clothing, and getting vaccinated.
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.
- The majority of persons infected with yellow fever virus have no illness or only mild illness.
- In persons who develop symptoms, the incubation period (time from infection until illness) is typically 3–6 days.
- The initial symptoms include sudden onset of fever, chills, severe headache, back pain, general body aches, nausea, and vomiting, fatigue, and weakness. Most persons improve after the initial presentation.
- After a brief remission of hours to a day, roughly 15% of cases progress to develop a more severe form of the disease. The severe form is characterized by high fever, jaundice, bleeding, and eventually shock and failure of multiple organs.
- No specific treatments have been found to benefit patients with yellow fever. Whenever possible, yellow fever patients should be hospitalized for supportive care and close observation.
- Treatment is symptomatic. Rest, fluids, and use of pain relievers and medication to reduce fever may relieve symptoms of aching and fever.
- Care should be taken to avoid certain medications, such as aspirin or other nonsteroidal anti-inflammatory drugs (e.g. ibuprofen, naproxen), which may increase the risk of bleeding.
- Yellow fever patients should be protected from further mosquito exposure (staying indoors and/or under a mosquito net) for up to 5 days after the onset of fever. This way, yellow fever virus in their bloodstream will be unavailable to uninfected mosquitoes, thus breaking the transmission cycle and reducing risk to the persons around them.
- The majority of infected persons will be asymptomatic or have mild disease with complete recovery.
- In persons who become symptomatic but recover, weakness and fatigue may last several months.
- Among those who develop severe disease, 20–50% may die.
- Those who recover from yellow fever generally have lasting immunity against subsequent infection.
Prevention of Yellow Fever
Avoid Mosquito Bites
- Use insect repellent.When you go outdoors, use an EPA-registered insect repellent such as those containing DEET, picaridin, IR3535, or oil of lemon eucalyptus on exposed skin. Even a short time outdoors can be long enough to get a mosquito bite. For details on when and how to apply repellent.
- Wear proper clothing to reduce mosquito bites.When weather permits, wear long-sleeves, long pants and socks when outdoors. Mosquitoes may bite through thin clothing, so spraying clothes with repellent containing permethrin or another EPA-registered repellent will give extra protection. Clothing pre-treated with permethrin is commercially available. Mosquito repellents containing permethrin are not approved for application directly to skin.
- Be aware of peak mosquito hours.The peak biting times for many mosquito species is dusk to dawn. However, Aedes aegypti, one of the mosquitoes that transmits yellow fever virus, feeds during the daytime. Take extra care to use repellent and protective clothing during daytime as well as during the evening and early morning. Staying in accommodations with screened or air-conditioned rooms, particularly during peak biting times, will also reduce risk of mosquito bites.
Get Vaccinated if Recommended
- Yellow fever vaccine is recommended for persons aged ≥ 9 months who are traveling to or living in areas at risk for yellow fever virus transmission inSouth America and
- Yellow fever vaccine may be required for entry into certain countries. Yellow fever vaccination requirements and recommendations for specific countries are available on theCDC Travelers’ Health page.
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