:max_bytes(150000):strip_icc()/Health-GettyImages-1949513976-2ea2f125f14c44a2ae879698048c8560.jpg)
Jaromir / Getty Images
Yellow fever is a virus spread through specific types of mosquitoes found in Africa and South America. If bitten by an infected mosquito, the virus enters your bloodstream.
In several countries, yellow fever is so common it's considered endemic, meaning it is consistently present in a population.
While mosquitoes that transmit yellow fever don't live in the United States, it's possible to contract the virus if you're not properly vaccinated and travel to other countries where the virus is endemic.
If you're traveling or have recently traveled to a country where yellow fever spreads, it's important to know the symptoms and contact a healthcare provider if you feel sick. Most people only experience mild illness, but some people become severely ill and need medical care.
Yellow Fever Symptoms
Symptoms of yellow fever develop about 3-6 days after exposure to the virus. Most people have either mild symptoms or no symptoms at all, but some people will become severely ill from the virus.
Mild Symptoms
For most people with yellow fever, the illness will feel flu-like with symptoms such as:
- Fever
- Chills
- Headache
- Muscle pain
- Loss of appetite
- Fatigue or weakness
- Nausea and vomiting
These symptoms usually last 3-6 days before improving, though some people report feeling fatigued and weak for months after infection.
Severe Symptoms
About 15% of people with yellow fever will experience severe illness. They may initially have milder symptoms, but after a brief recovery period (24-48 hours), symptoms return and worsen as the virus affects the body's organs—particularly the liver and kidneys.
Symptoms of severe infection with yellow fever include:
- High fever
- Jaundice, or yellowing of the skin and whites of the eyes
- Nausea and vomiting
- Abdominal pain or bleeding
- Bleeding from the mouth, nose, and eyes
- Dark urine
People who become severely ill with yellow fever are at a much higher risk of organ failure and potentially death; the fatality rate for people with severe infection is between 30-60%.
What Causes Yellow Fever?
A virus spread through mosquitoes like the Aedes aegypti (yellow fever mosquito) causes yellow fever. Yellow fever is part of a group of viruses called flaviviruses, which also includes similar mosquito-borne diseases like dengue virus and West Nile virus.
The virus is initially transmitted when a mosquito bites an infected host (either a person or a non-human primate, like a monkey). The mosquito then contracts the virus, bites another host, and transmits the virus to that new host through the bloodstream.
People or non-human primates who contract yellow fever are viremic, meaning they can pass along the virus through mosquitos shortly before a fever begins and up to five days after symptoms start. If a non-infected mosquito bites a human who has yellow fever, the mosquito can acquire the virus and then pass it to the next person it bites.
It's technically possible to spread the virus through blood transfusions and organ transplants, but this kind of transmission is extremely uncommon. The virus cannot be spread from person to person through other bodily fluids (like coughing or sneezing) or from contact with contaminated surfaces.
Experts break yellow fever down into three distinct transmission cycles:
- Jungle (sylvatic) cycle: This cycle is common among people who visit or work in a jungle. Transmission occurs when mosquitos bite a non-human primate with yellow fever and then bite a human, transmitting the infection.
- Intermediate (savannah) cycle: This cycle is common in Africa when people live or work in areas bordering the jungle. A mosquito bites a non-human primate or human with yellow fever, then bites another human and transmits the infection.
- Urban cycle: This cycle occurs when a human contracts yellow fever while in the jungle or savannah and is bitten by a mosquito after returning to an urban area. This introduces the virus to urban species of mosquitoes, which then continue transmitting the virus to other humans living in urban areas.
Keep in mind that the virus doesn't change based on how it's transmitted, so however you contract yellow fever, the symptoms will be the same.
Risk Factors
Any person who isn't vaccinated and is exposed to mosquitoes with yellow fever is at risk for contracting the virus. Some people are more likely to contract yellow fever or have more exposure to it than others.
You may be at higher risk for yellow fever based on the following factors:
- Location: Yellow fever is fully or partially endemic to 29 countries in sub-Saharan Africa and 13 countries in tropical South America. If you aren't vaccinated and live, work, or visit these countries, you're at a higher risk for contracting the virus.
- Age: Infants, children, and younger unvaccinated people are more likely to get sick with yellow fever.
- Exposure to mosquitoes: The more mosquitoes you encounter in areas where yellow fever occurs, the more likely you are to encounter a mosquito infected with the virus.
How It's Diagnosed
Yellow fever can be hard to diagnose—especially early in the illness—because its symptoms can be confused with other illnesses, including other mosquito-borne viruses. Healthcare providers usually rely on blood tests.
Several blood tests can determine if a person has yellow fever, depending on how long they have been ill. Your provider may initially look for the presence of viral DNA associated with yellow fever, but the longer someone is sick, the less reliable this type of testing becomes.
They may use a blood test to detect antibodies to yellow fever instead. A provider may also run a complete blood count (CBC) to check your white blood cell levels, which typically decrease with any infection.
If a provider suspects yellow fever but can't confirm it, they may run tests to determine how well the liver is functioning. The levels of a substance called bilirubin, removed from your blood during healthy liver function, are often higher when a person has yellow fever.
Yellow Fever Treatment
Because yellow fever is a viral infection, there is no medication to treat the virus.
Most people recover on their own with the usual remedies prescribed for viral infections, such as rest, fluids, and over-the-counter (OTC) medications. OTC medications help relieve symptoms like fever, headache, and muscle pain. It's best to avoid aspirin and drugs like Advil (ibuprofen) because they increase the risk of bleeding.
People with severe yellow fever infections typically need to be hospitalized for symptom management and observation. Dehydration caused by fever and vomiting, as well as kidney and liver problems, are risk factors for potentially fatal illness. Receiving medical attention and care to manage these symptoms helps lower the risk of death in severe cases of yellow fever.
Medical care for yellow fever may include receiving intravenous (IV) fluids, oxygen supplementation, blood transfusions, and treatment for any bacterial infections resulting from illness.
How To Prevent Yellow Fever
While there is no cure or treatment for yellow fever, it's important to prevent infection if you live in or travel to areas where the virus is endemic.
The best way to prevent yellow fever is with a single-dose vaccination, which typically gives you lifelong immunity. The vaccine is highly effective, giving more than 99% of people full immunity to the virus after 30 days.
Plan Ahead
You should receive your yellow fever vaccine at least 10 days before traveling in order for it to be effective.
Although vaccination is not routinely recommended if you're traveling to places with a low exposure risk, anyone traveling for extended periods or who can't limit their exposure to mosquitoes may want to get vaccinated. Some countries also require proof of yellow fever vaccine before entering.
If vaccination isn't an option, the next best strategy to prevent infection is to avoid being bitten by mosquitoes:
- Wear protective clothing when spending time outdoors
- Treat clothing with insect-repellent products containing permethrin or DEET
- Sleep in air-conditioned spaces or utilize mosquito nets at night
- Avoid areas with standing water where mosquitoes breed
You may also want to look up when the rainy season occurs wherever you'll be traveling. Try to avoid the area (if possible) during or just after this wetter season since mosquitoes are more prevalent then. Keep in mind that mosquitoes still carry the virus throughout the year and that certain species are likely to spread infection year-round.
Complications
Mild infections with yellow fever typically don't cause complications, though it's important to drink plenty of fluids—especially if you have a fever or are vomiting—to avoid dehydration.
Severe yellow fever infections commonly cause complications, including excessive bleeding or blood clotting, kidney or liver failure, bacterial infections, organ failure, and even death.
It's important to seek urgent medical attention if you have any signs or symptoms of severe illness so you can be closely monitored by healthcare providers and receive the appropriate treatments.
A Quick Review
Yellow fever is a viral illness spread by mosquitoes infected with the yellow fever virus. It's common in some regions of Africa and South America. Yellow fever typically causes mild symptoms like fever, headache, vomiting, and muscle pain that go away within one week.
About 15% of people who contract the yellow fever virus will develop severe, potentially life-threatening symptoms requiring medical treatment. There is no cure for yellow fever, but there is a vaccine that prevents the illness. Experts recommend that everyone in areas where the virus is common should receive the vaccine.
If you think you may have yellow fever or were exposed to it while traveling, reach out to your healthcare provider for the best next steps.
Frequently Asked Questions
-
Yellow fever is not the same as malaria. Both illnesses are spread through the bites of certain types of mosquitoes, but a virus causes yellow fever and a parasite causes malaria.
-
Experts recommend yellow fever vaccination (or booster vaccination) for anyone who meets the following criteria:
- You're at least 9 months old and are spending time (living in, working in, or traveling to) in an area with a high risk of exposure to yellow fever
- You're traveling to an area experiencing an outbreak of yellow fever, and it's been ten or more years since you received a yellow fever vaccination
- You're traveling to a country where a yellow fever vaccine or booster is required
-
A single dose of yellow fever vaccine typically protects you from infection for life. In some circumstances, your healthcare provider may recommend a booster vaccine when needed. If it's been 10 years or longer since a person has had the yellow virus vaccine and is traveling to areas with ongoing outbreaks, they should consider getting the booster.