Yellow Fever: 10 Frequently Asked Questions About This Viral Infection

Yellow fever once stalked U.S. and European cities. It still stalks many parts of the world quietly. In 2016, it roared back, this time in the capital of Angola.

Over 200 people have died in Angola; the virus has spread in the capital and in most provinces in the country. The virus has also been recorded in Asia. Yellow fever had never been recorded in Asia before—even though the virus has reached across Africa and South America.

Travelers and workers from China have returned home from Angola with the virus.

There is a vaccine, however. It's worked well for years. The problem is: the world is now running low on stockpiles of the vaccine needed to prevent the virus.

Is Yellow Fever New?

Not at all. Every year many people are infected with yellow fever. It's just not recently caused as large an outbreak as seen in Angola.

Cases are reported around the year, everywhere from Peru, Brazil, the DRC, to Cote d'Ivoire have had cases.

The virus used to extend much further. The virus first reached what is now the U.S. at the end of the 1600s. It was brought largely by the trafficking of people—the Slave Trade—between Africa and the Americas. The mosquitoes and the virus were carried along with people taken from their homes. 

It used to reach as far north as Boston, New York, and Philadelphia, but especially Southern cities, where there were cases until the late 1800s.

 The virus also spread by trade to European ports, as north as Cardiff and Dublin, though countries like Greece were at most risk.

How Many People Get Sick With Yellow Fever?

The WHO estimates there are about 84 thousand to 170 thousand people, perhaps 200 thousand a year, who get sick with yellow fever.

As many as 30,000 die a year worldwide; this figure is estimated between 29,000 and 60,000 deaths a year. This number is sometimes thought to be even higher, up to 60,000 or 78,000 a year.

Most deaths occur in Africa. In South America, the virus persists, but it does not cause outbreaks in cities; it has been restricted to remote jungle or forest areas, where the virus spreads in animals. 

Can Yellow Fever Only Spread Around Africa and South America?

Nope, yellow fever used to spread a lot further. Interestingly, though, yellow fever transmission has never been reported to be transmitted in Asia. The mosquitoes found in many parts of Asia place the region at risk. As very few have immunity to yellow fever in Asia, this is a risk for Asia. 

The recent outbreak in Angola has led to multiple travelers returning to China with yellow fever. These were the first reported cases of yellow fever in China—but no local transmission has been seen. Mosquitoes that can spread Dengue (and Zika) can spread yellow fever.

Given the large outbreaks of Dengue in recent years, yellow fever can pose a risk.

How Is Yellow Fever Spread?

Yellow fever is a viral infection spread by mosquitoes. In particular, it can be spread through the same mosquitoesAedes aegypti—that spread Zika and Dengue. Aedes aegypti is, in fact, called the yellow fever mosquito. It can also spread by Haemagogus mosquitoes. The virus can also be spread by other mosquitoes (Aedes africans 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 other primates (like monkeys) and then biting another person or other primate. Mosquitoes that feed on an infected person right before they develop a fever and up to 5 days later can spread the virus to others. This is important as sick people don't always swat away mosquitoes. Those who are infected should remain inside under mosquito nets to avoid spreading the virus.

Yellow fever spreading in a city is very different than in a forest or jungle. The virus has 3 different transmission cycles: jungle (sylvatic), inter­mediate (savannah), and urban. It's the urban type that's the most worrisome.

  • When yellow fever spreads in the jungle it largely spreads without humans. It spreads from a 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 bit 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 found in urban areas. It usually starts when someone returns sick - or about to be sick - from a jungle area. It can lead to sudden and large outbreaks in crowded urban areas.

The infection is usually mild or not even noticed. In some, however, it can be deadly, with fevers, chills, aches, bleeding, yellow eyes and skin, nausea, vomiting, confusion, shock, and organ failure.

How Is Yellow Fever Diagnosed?

There's a blood tests that can look for antibodies to the virus. There are also ​PCR tests that look at the blood for the virus.

What Is it Like to Have Yellow Fever?

For most people, it's actually no big deal. Nothing much happens for most who catch the virus that causes yellow fever. It causes just a mild illness or none at all.

For those who do get sick, there are 3 stages of illness:

3-6 days after exposure, infection occurs, including fever, muscle aches, nausea, vomiting, dizziness, fatigue.

2-3 days later, remission: fever drops, symptoms improve. This may last for 24-48 hours. Most recover at this point. About 15 percent of cases go on to have more severe disease.

Finally, severe disease: Fever, nausea, and vomiting return. Of those patients who go on to severe disease, many become jaundiced (hence the name yellow fever). Jaundice can be seen when the whites of the eyes go yellow, skin under the tongue turns yellow, or skin appears yellow (including the palms and soles). Some will have bleeding—from the nose or other mucus membranes, in vomit, or from an IV site. Some may bruise easily. 

Patients can go into shock, be confused, and have failure of their organs. Blood tests will show that their livers are damaged (elevated liver enzymes), which can be seen before jaundice (when lab tests show elevated bilirubin values). In those who get better, the live enzymes rise until the 2nd week and then begin to drop towards normal. White counts will also be low, meaning there will be less white blood cells (immune cells) during infection.

Those who recover make antibodies against the virus, as the virus disappears. The virus lasts in the blood longer in those who get very sick. 

Can People Die from Yellow Fever?

Usually the time between being exposed by a mosquito bite and getting sick is about 3-6 days.

Those who go on to have severe disease can die. About 1 in 5 to 1 in 2 of those with severe disease may die. It seems about 1 in 5 in West Africa who become sick with jaundice die. Probably about 60,000 die a year from yellow fever.

Those who have mild disease (and never become very ill) do well. They are expected to make a full recovery.

How Long Before You Get Sick?

Yellow fever is found in both Africa and South America. 

Most cases—about 90 percent—are thought to occur in Africa. It is found in West and Central Africa, as well as in some parts of East Africa. Most recently, there has been an outbreak in Angola. Many cases are thought to not be diagnosed, so the full extent of the disease's impact is unclear.

In the South American region, it is centered in the Amazon, largely in Brazil, reaching Peru, Ecuador, Bolivia, Colombia, Venezuela, and neighboring countries. Countries at risk also include Panama and Trinidad and Tobago. Other countries at risk include: Argentina, French Guiana, Guyana, Paraguay, Suriname, Venezuela. In most of South America the virus is not found throughout a country, but only in very specific regions.

Specifically, 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, Uganda.

There are areas of Asia that have mosquitoes that could spread the virus but never have. Travelers have recently returned to China with the virus, and as a result the disease has been reported for the first time in Asia, but without transmission.

How Do You Treat Yellow Fever?

There's no specific treatment. The best thing is to prevent getting sick by being vaccinated and protecting against mosquitoes.

Patients who become sick from yellow fever should be hospitalized. Treatment in the hospital will be supportive. Nurses and doctors keep patients hydrated (with fluids, either oral or through IV's). Pain relievers can be provided. For those who are in shock, especially with organ failure, there can be ways to keep people breathing (ventilation), raise blood pressures that are too low (pressors), and replace what their kidneys cannot do (dialysis), while their bodies slowly get better.

Because of the risk of bleeding, aspirin and NSAIDS (like ibuprofen, naproxen) should generally be avoided.

If you think you or someone else is sick with yellow fever, please seek treatment. Neither you nor anyone else should try to tough it out on your own.

There are, however, many who never realize they have the virus because they do not have symptoms. It's only if you have a fever or any other symptom that you would need help.

What Sort of Virus Causes Yellow Fever?

Yellow fever is caused by a Flavivirus. It is a single-stranded RNA virus. Other members of the flavivirus genus include: West Nile, ZikaDengue, Japanese Encephalitis, St Louis, and tick-borne encephalitis viruses. Hepatitis C is found in the Flavivirus family, like yellow fever and the other members of the Flavivirus genus, but is not part of the same genus (instead in the Hepacivirus)

The name Flavivirus—for the family and genus—comes from yellow fever. Flavus means yellow in Latin. Yellow fever is so named because patients often become yellow - due to liver problems during the illness.

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