Yellow fever is a potentially fatal viral disease transmitted to humans by infected mosquitoes. It generally occurs only in Africa, South America and the Caribbean. Each year, there are about 200,000 cases of yellow fever, which cause 30,000 deaths, according to the World Health Organization (WHO).

People who live in certain regions of the world are at risk for yellow fever. In rare cases, tourists and visitors to other countries may contract yellow fever. However, affected nations require vaccination for yellow fever before a person is allowed into the country.

Symptoms associated with yellow fever can vary dramatically from case to case. They tend to appear between three and six days after a person is bitten by a mosquito. Some patients only experience mild discomfort, while others develop severe and even life-threatening complications.

Yellow fever may be difficult to diagnose in its early stages because symptoms are similar to those of several other illnesses (e.g., malaria, influenza). No specific treatment exists for yellow fever except for supportive care, such as replacing fluids.

Yellow fever vaccine provides the best protection against this infection for people who are at least 9 months old. This live virus vaccine causes the body to produce antibodies to the virus, creating immunity from the infection for a decade or more.

People can also lower their risk of contracting yellow fever by taking steps to help prevent the mosquito bites that transmit the illness. Finally, government efforts to control mosquito populations have been effective in reducing the incidence of yellow fever.

Yellow fever is a viral disease transmitted from humans and monkeys to other humans through mosquito bites. It occurs only in Africa, South America and the Caribbean, although it previously occurred in North America, Central America and Europe. Each year, there are about 200,000 cases of yellow fever, which cause 30,000 deaths, according to the World Health Organization (WHO).

Typically, mosquitoes of several different species (e.g., Aedes, Haemogogus) pass yellow fever on to both human and monkey hosts through bites. Mosquitoes also can pass the virus on to their offspring via infected eggs. These offspring then go on to infect more humans and monkeys.

Three types of mosquitoes transmit yellow fever: wild (breed in jungles), domestic (breed around houses) and semi-domestic (breed in a mixture of habitats). The transmission cycle for yellow fever can be divided into three types:

• Sylvatic yellow fever. Also known as “jungle fever,” it is associated with tropical rainforests, where monkeys are infected by wild mosquitoes. Other mosquitoes that bite infected monkeys may become infected themselves. Forestry workers sometimes are infected by these mosquitoes while working in the forest.
  
  
• Intermediate yellow fever. Occurs in the humid or semi-humid savannas of Africa and often manifests in small-scale epidemics. Separate villages in the same region may suffer epidemics at the same time, but fatalities tend to be lower than in urban yellow fever. Semi-domestic mosquitoes are involved in infecting both humans and monkeys.
  
  
• Urban yellow fever. Occurs when the virus is introduced by domestic mosquitoes of the Aedes aegypti species to areas of high population density. Outbreaks begin with a single source and spread out to cover a large area. Monkeys are not involved in the transmission.

All three of the transmission cycles occur in Africa, while sylvatic and urban yellow fever occur in South America.

A type of virus known as a flavivirus causes yellow fever. These are spherical, enveloped, single-strand ribonucleic acid (RNA) viruses that reproduce within infected cells.

People who live in certain regions of the world are at risk for yellow fever. Outbreaks occur throughout Africa, especially during the rainy season in the savanna zones of west and central Africa. They also occur in urban locations and villages and (to a lesser extent) in jungle areas. Some outbreaks are also known to occur in the Caribbean.

In South America, yellow fever infections occur sporadically and are usually associated with workers in the forestry and agricultural industries who are exposed to the virus. South American countries with the highest incidence of yellow fever include Bolivia, Brazil, Colombia, Ecuador and Peru.

People who live in these regions are at the highest risk of yellow fever. In rare cases, tourists and visitors to other countries may contract yellow fever. However, many nations require vaccination for yellow fever before a person is allowed into the country. Elderly people are at risk for more severe infections than younger people.

Improved mosquito control efforts have helped to end the threat of yellow fever in areas such as the United States and Europe.

Symptoms associated with yellow fever can vary dramatically from one case to another. They tend to appear between three and six days after a person is bitten by a mosquito. Some patients only experience mild discomfort, while others develop severe and even life-threatening complications.

In its early stages, yellow fever typically causes the following:

• Fever
• Headache
• Loss of appetite
• Muscle pain (especially backache)
• Nausea
• Slow pulse
• Vomiting

In most cases, these symptoms clear within a few days and the patient recovers.

About 15 percent of patients experience a return of the illness within 24 hours after yellow fever appears to subside, according to the World Health Organization (WHO). This toxic phase can affect several body systems and may include symptoms of bleeding from the mouth, nose and eyes, or from the stomach, which shows up as blood in stools or vomit. Patients may experience a return of the fever and rapidly develop abdominal pain with vomiting and jaundice, and kidney and liver function often deteriorates. Other symptoms associated with this stage of infection include coma, delirium and seizures. Half of the patients who enter this toxic phase die within two weeks, according to the WHO. The rest recover and usually do not incur significant organ damage.

In diagnosing yellow fever, a physician will perform a complete physical examination and compile a thorough medical history.

Yellow fever may be difficult to diagnose in its early stages, because symptoms are similar to those of illnesses such as malaria, typhoid, rickettsial diseases, other hemorrhagic viral fevers, arboviral infections, leptospirosis, viral hepatitis and poisoning. For this reason, blood tests are performed and analyzed in a laboratory to look for the presence of antibodies to yellow fever.

No specific treatment exists for yellow fever. However, oral rehydration salts are often used to treat dehydration and acetaminophen is used to treat fever. Antibiotics are often used to treat related bacterial infections that may develop.

Yellow fever vaccine provides the best protection against this infection for most people. This is a live virus vaccine that causes the body to produce antibodies to the virus, creating immunity from the infection for a decade or more. People who continue to spend time in areas prone to yellow fever are urged to receive a booster shot every 10 years.

The vaccine itself is available at designated vaccination centers around the United States. Local health departments can provide information about center locations. Some people may experience mild side effects after receiving the vaccine, including mild headache or muscle pain. This tends to occur between five and 10 days after vaccine administration and quickly passes. More than 300 million doses of this vaccine have been administered, and side effects have been rare, according to the World Health Organization (WHO).

There are four types of people who typically are advised not to receive yellow fever vaccine. They are:

• Infants under 6 months of age. These children are at risk for developing viral encephalitis after being vaccinated for yellow fever. In general, children should be at least 9 months old before receiving the vaccine.
  
  
• Pregnant women. There is a risk that the developing fetus could be infected from the vaccine, resulting in the potential for birth defects. The vaccine does not appear to be a risk for breastfeeding infants.
  
  
• People with hypersensitivity to eggs. Yellow fever vaccine is prepared from an egg base. In some cases, people with this hypersensitivity may receive a small dose of the vaccine to see how they react before getting a larger dose.
  
  
• People with immune system deficiencies. This includes patients with HIV and AIDS and other diseases (e.g., leukemia, lymphoma) and those who are undergoing radiation treatments or taking certain medications.

People in these groups may be able to obtain a waiver letter that allows them to enter countries that usually require yellow fever vaccination. To find out the proper procedure, people are urged to contact the U.S. embassy or consulate in the country they plan to visit.

In other cases, a physician may recommend vaccination on the grounds that the risk associated with not being vaccinated is higher than the potential risks associated with vaccination.

People can also lower their risk of contracting yellow fever by taking steps to help prevent the mosquito bites that transmit the illness. This includes using an effective mosquito repellent, such as one containing N,N-diethyl-meta-toluamide (DEET) on exposed skin. Other protective measures include use of mosquito netting (while sleeping) and wearing protective clothing.

Finally, government efforts to control mosquito populations have been effective in reducing the incidence of yellow fever in the past. However, these efforts have declined over the past three decades allowing mosquito populations to increase and the incidence of yellow fever to rise, according to the WHO.

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to yellow fever:

1. What countries require me to be vaccinated for yellow fever?
   
   
2. Where can I get this vaccination?
   
   
3. Should I be vaccinated if I’m in a group at higher risk for complications?
   
   
4. If I am pregnant, do you advise that I cancel my trip until after the pregnancy so I can receive vaccination?
   
   
5. What side effects should I watch for following vaccination?
   
   
6. How long will it take for my body to establish immunity?
   
   
7. Should I call a doctor if I experience side effects?
   
   
8. What signs should I look for that might indicate I have been infected?
   
   
9. What are my treatment options?
   
   
10. What can I do before, during and after my trip to reduce the risk of yellow fever?