What Is Yellow Fever?

Yellow fever is caused by a flavivirus, which is spread by mosquitos in Africa and South America, particularly in areas with dense forests or jungles. 

However, for about 10% to 15% of those infected, it can become serious, resulting in high fever, jaundice, and other concerns. Yellow fever can be fatal.

History, Impact, and Reach

Over the years, yellow fever has been better controlled than it was in the past, largely because there is a vaccine to protect against it. Still, the WHO estimates that approximately 84,000 to 170,000 people become infected with yellow fever each year. It is believed that many cases are not detected, so the full extent of the disease's impact is unclear. 

Yellow fever is present in geographic regions where the virus and mosquito can survive. This is largely dependent on the climate and the presence of a jungle environment.

In most of South America, the virus does not cause outbreaks in cities. It is found only in very specific regions, typically in remote jungle or forest areas, where the virus spreads in animals. 

There, it is centered in the Amazon, mainly in Brazil, reaching Peru, Ecuador, Bolivia, Colombia, Venezuela, and Argentina. Countries at risk also include Panama, Trinidad and Tobago, French Guiana, Guyana, Paraguay, and Suriname.

But about 90% of yellow fever is thought to occur in Africa, where most deaths from the infection occur. It is found in West and Central Africa, as well as in some parts of East Africa.

African countries for which there is a risk of yellow fever include: Angola; Benin; Burkina Faso; Burundi; Cameroon; Central African Republic; Chad; Republic of Congo; Cote d’Ivoire; Democratic Republic of the Congo; Equatorial Guinea; Ethiopia; Gabon; Gambia; Ghana; Guinea; Guinea-Bissau; Kenya; Liberia; Mali; Mauritania; Niger; Nigeria; Rwanda; Senegal; Sierra Leone; Sudan; South Sudan; Togo and Uganda.

In 2016, there was an outbreak in the capital of Angola, where over 100 people were confirmed to have died of the infection. The virus spread in the capital and in most provinces in the country. While it is not considered endemic in Asia, travelers and workers have returned home to China from Angola with the virus.

The virus used to extend much further geographically than it does now. It first reached the United States at the end of the 1600s. It is believed to have been brought by human trafficking between Africa and the Americas when mosquitoes and the virus were carried along with people who had lived in endemic areas. It reached as far north as Boston, New York, and Philadelphia, and remained in southern cities until the late 1800s. The virus was also spread by trade to European ports as far north as Cardiff and Dublin, though countries like Greece were at most risk.

Yellow Fever Symptoms

For most people, yellow fever causes a mild illness or goes unnoticed. There are usually about three to six days between being exposed to the virus through a mosquito bite and becoming sick. If you have mild disease and never become very ill, you are expected to make a full recovery. For some people, however, yellow fever causes fevers, chills, aches, bleeding, yellow eyes and skin, nausea, vomiting, confusion, shock, organ failure—even death.

For people who do experience symptoms of yellow fever, there are three stages of illness:

Early infection occurs three to six days after exposure. You may experience fever, muscle aches, nausea, vomiting, dizziness, and fatigue.

Remission occurs after a few hours, up to a day. The fever, if present, drops and symptoms improve. This may last for 24 to 48 hours. Most people recover at this point. About 15% of people infected with the virus go on to have a more severe disease.

Severe disease: Fever, nausea, and vomiting occur if you experience severe disease. New symptoms and signs emerge in serious infections:

• Jaundice: Many people who have severe disease experience yellowing of the skin (including the palms and soles), the whites of the eyes, and the skin under the tongue. This symptom gives yellow fever its name.
• Some people begin to bruise easily or to have bleeding from multiple places in the body. In particular, you may notice bleeding from your nose, other mucous membranes, or from an intravenous site, or you may see blood in your vomit.
• White blood cell counts may be low, meaning there are fewer immune cells during an infection.
• Blood tests may show that the liver is damaged, which can be detected by the presence of elevated liver enzymes in the blood. This may occur before jaundice develops. 
• If infection improves, liver enzymes are expected to rise until the second week of illness and then begin to drop toward normal. 
• Those who recover make antibodies that fight against the virus as it disappears. The virus lasts in the blood longer in those who get very sick. 
• A serious disease may cause symptoms of confusion, and ultimately, organ failure.

About 20% to 50% of those with severe disease may die. 

Causes

Yellow fever is caused by a flavivirus, a single-stranded RNA virus spread by the Aedes aegypti mosquito.

The yellow fever virus can also be spread by other mosquitoes, the Aedes africanus in Africa or Haemagogus and Sabethes mosquitoes in South America. 

The mosquitoes transmit the virus by feeding on the blood of an infected person or another primate, such as a monkey, and then biting another person or other primate. A mosquito can pick up the virus if it consumes infected blood right before the human or animal develops a fever and up to five days afterward.

Transmission Cycles

The virus has three different transmission cycles: jungle (sylvatic), inter­mediate (savannah), and urban. The effects of infection itself are the same whether it is transmitted in any of these cycles. 

When yellow fever spreads in the jungle, it largely spreads without humans. Instead, it spreads from non-human primate (like a monkey) to non-human primate by mosquitoes. If people visit the jungle area (say for mining, hunting, or tourism), they can also be bitten by a mosquito and become ill.

In the intermediate cycle (also called the savannah cycle), yellow fever spreads regularly between monkeys and humans through mosquitoes in areas on the edge of jungle areas. It can spread monkey to human, monkey to monkey, human to human, or human to monkey.

In the urban cycle, yellow fever spreads primarily between people through mosquitoes living in urban areas. It usually starts when someone who has been infected with the virus returns from a jungle area. It can lead to sudden and large outbreaks in crowded urban areas.

Diagnosis

Diagnosis of yellow fever is based on the clinical history of exposure to a mosquito bite in an endemic region, as well as a history of symptoms. There are a few tests that can support or confirm the diagnosis of yellow fever. 

• Antibody test: This test is the most well-established diagnostic test for yellow fever. It looks for the presence of immune proteins to yellow fever in your blood, which indicates that you are either fighting off or have effectively have fought the infection. It may take several days for your body to produce enough antibodies to be able to be detected, however. You may need to wait up to two weeks to get your results.
• Viral PCR test: PCR tests can detect the genetic material of the virus in your blood. A positive PCR test indicates that you have the virus in your body, so it is considered a stronger indicator of current infection than an antibody test. An important limitation: The viral RNA is detectable early after infection, but is not easily detectable after several days of infection. This means that you can get a negative PCR test result even if you have the infection. 
• Urine test: A relatively new test can detect the viral PCR in the urine, making the diagnosis more practical. However, this test, while promising, is not yet used widely.

Treatment

There is no specific antiviral treatment for yellow fever. However, the illness can become quite severe, and related complications can require medical care. 

This may include:

• Prevention of bleeding: Because of the risk of bleeding, medications that induce it, such as aspirin, ibuprofen, and naproxen, should generally be avoided.
• Hydration: Maintaining hydration throughout the illness with oral or IV fluids may be necessary, particularly if you experience vomiting or low blood pressure.
• Fever control: Generally, yellow fever is associated with low-grade fevers. But if your fevers become higher than expected, you may need medication to lower your temperature.
• Pain: If you experience muscle pain or generalized discomfort, it may be managed with oral or injected pain relievers.
• Blood pressure support: For those who are in shock, which occurs when blood pressure is extremely low, blood pressure can be raised with medications that constrict the blood vessels, often referred to as pressors.
• Management of organ failure: When yellow fever causes organ failure, those organs need to be supported while the infection improves. For example, a ventilator may be needed to assist with breathing; dialysis may be required to do the work of the kidneys.

Prevention

Because yellow fever has been around for a while, the transmission of the virus is well understood. Here are some effective ways to prevent the infection:

• Vaccination: From some people, vaccination is recommended. If you live in an endemic area, you might already be immune to the infection and should follow the official recommendations regarding vaccination for yourself and your children. If you are traveling to an area where yellow fever is endemic, you will likely need to have the vaccine. Usually, vaccines for travelers are not widely available, and you may need to make an appointment at a local travel clinic. It is also best to plan ahead, as you should get the vaccine at least 10 days before your trip.
• Mosquito precautions: If you are in an endemic area, you can protect yourself and your children against mosquitoes. While it is not always possible to completely avoid mosquito bites, you can wear layers, particularly when traveling in jungles and forests, and you can use insect spray. It is also recommended to sleep under protective nets, even if you will be in an enclosed room.
• Protecting others: In general, because the virus can spread from person to person via a mosquito, it is recommended that you remain under mosquito nets to avoid spreading the virus if you know that you have become infected.

A Word From Verywell

If you are traveling to an area where yellow fever is endemic, you should take the recommended precautions; doing so will greatly minimize your risk of infection.

Also be sure to familiarize yourself with the common symptoms so can seek medical attention if you do become infected. While most people with yellow fever have a good recovery, the chances of one are much higher if you get professional care before any complications occur.