What Is Yellow Fever? What Causes Yellow Fever?
Yellow fever is an acute systemic illness - a hemorrhagic fever - caused by the Flavivirus. Acute means it comes on (onset) rapidly, while systemic means it affects the whole body. In severe cases yellow fever causes a high fever, bleeding into the skin and the death of cells in the liver and kidney. Liver damage results in severe jaundice - yellowing of the skin; hence the name "yellow fever".
The mosquito Aedes aegypti, and some other species transmit the virus to humans. Aedes aegypti is found in tropical and subtropical parts of South America, parts of the Caribbean, and Africa (not Asia). Apart from mosquitoes, the only other known hosts of the virus are primates and humans.
Experts believe yellow fever originated in Africa and was introduced to South America via the slave trade in the 16th century. Several major yellow fever epidemics have taken place in Europe, the Americas and Africa since the 17th century. It was deemed one of the most dangerous infectious diseases in the 19th century.
According to the World Health Organization (WHO), there are approximately 200,000 cases of yellow fever worldwide each year, and 30,000 deaths. About 90% of all cases occur in Africa.
In the middle of the 20th century a safe and effective vaccine was created. However, since the 1980s the number of yellow fever cases has increased, making it a reemerging disease.
There are two classifications of yellow fever:
Jungle yellow fever - it is spread by jungle mosquitoes and mainly affects monkeys. Humans who go into the jungle can become infected by mosquitoes that became infected by biting infected monkeys, and can be the source of urban outbreaks of the disease.
Urban yellow fever - this is spread when an Aedes aegypti mosquito bites an infected human or monkey and then goes on to bite other people. Urban yellow fever is most likely to occur in settlements close to the jungle, where infected monkeys and mosquitoes live. Urban yellow fever is extremely rare in the Americas. Put simply, a human may go into the jungle, be bitten by an infected mosquito, come back to their settlement, and infect mosquitoes who bite them there; those mosquitoes will then infect other people.
According to Medilexicon's medical dictionary: Yellow fever is "a tropical mosquito-borne viral hepatitis, due to yellow fever virus, a member of the family Flaviviridae, with an urban form transmitted by Aedes aegypti, and a rural, jungle, or sylvatic form from tree-dwelling mammals by various mosquitoes of the Haemagogus species complex; characterized clinically by fever, slow pulse, albuminuria, jaundice, congestion of the face, and hemorrhages, especially hematemesis; used to occur in epidemics mainly in port cities, especially in late summer, with 20-40% case fatality rates; immunity to reinfection accompanies recovery."
Jungle yellow fever is "a form occurring in South America, transmitted by Aedes leucocelaenus and various treetop mosquitoes of the Haemagogus complex; transmitted normally to primates, occasionally by chance to humans to set off a human outbreak of classical yellow fever transmitted by Aedes aegypti."
What are the signs and symptoms of yellow fever?Yellow fever has an incubation period of between 3 and 6 days - i.e. it takes from 3 to 6 days for signs and symptoms to appear after a person is infected. The disease cannot spread from human-to-human; it is spread by infection-carrying mosquitoes to humans. Signs and symptoms are categorized into two stages:
The acute stage (initial stage)
Initial symptoms of yellow fever may include:
- Aching muscles, particularly the back and knees
- An elevated body temperature (fever)
- Loss of appetite
- Shivers (chills)
Toxic second stage
Although signs and symptoms may go away a few days after the toxic stage, approximately 15% of patients subsequently develop more severe symptoms, and enter the toxic stage. These more severe and sometimes life-threatening signs and symptoms may include:
- Recurring fever
- Abdominal pain
- Vomiting, sometimes with blood
- Tiredness, sluggishness, lethargy
- Jaundice (skin and whites of the eyes take on a yellow tinge)
- Kidney failure
- Liver failure
- Delirium, seizures, and sometimes coma
- Arrhythmias (irregular heartbeats)
- Bleeding from the nose, mouth and eyes
Experts say that out of all people who are infected with yellow fever, about 3% to 7.5% eventually die from it. Those who recover do not generally have any organ damage and are immune for life.
What are the causes of yellow fever?Yellow fever is caused by a flavivirus; it is transmitted by the bite of mosquitoes, usually the Aedes aegypti mosquito, which had become infected by biting an infected human or animal (a monkey). An infected mosquito is a source of infection for the rest of its life.
Experts say that the flavivirus is endemic among monkeys that live on the tree tops of the jungle (the jungle canopy) in many parts of Africa and the Americas.
If the infected mosquito passes the flavivirus on to a person who is in the jungle, that person may become a source of infection when they return to their community (mosquitoes bite them and become infected and then go on to infect other people).
What are the risk factors for yellow fever?A risk factor is something which increases the likelihood of developing a condition or disease. For example, obesity significantly raises the risk of developing diabetes type 2. Therefore, obesity is a risk factor for diabetes type 2.
Anybody who travels to an area which is known to have the yellow fever virus is at risk of becoming infected - these areas include parts of Africa (especially sub-Saharan Africa), tropical South America, as well as some parts of the Caribbean.
Do not rely just on reports indicating there have been no cases recently. This could be due to unreliable collection of data, or simply that the local population has been vaccinated. A yellow fever vaccine taken 10 to 14 days before traveling provides effective protection from the disease.
How is yellow fever diagnosed?Diagnosis will be confirmed after the doctor has detected the signs and symptoms and carried out a blood test. Relying on signs and symptoms alone is not enough, because the following illnesses may also have similar signs and symptoms:
- Dengue fever
- Viral hepatitis
- Some other viral hemorrhagic fevers
What are the treatment options for yellow fever?There is currently no effective antiviral medication for the treatment of yellow fever. The medical team's focus is mainly on supportive care in a hospital. This includes providing fluids, oxygen, making sure the patient's blood pressure is adequate, replacing lost blood, kidney dialysis if there is kidney failure, and treating any secondary infections.
Some patients may be given plasma transfusion to replace proteins that help with clotting.
The patient should be kept away from mosquitoes - if they bite the patient the mosquitoes will become infected and then pass the disease on to other people.
Prevention of yellow feverVaccine
There is a very effective and safe vaccine which prevents yellow fever. If you are traveling to an area where yellow fever is known to exist, see your doctor and ask about having the vaccine at least 10 to 14 days before your departure. Some countries may insist on a valid immunization certificate before they let you in. A single vaccine dose provides at least 10 years' protection. Vaccine side effects may include:
- Low grade fevers
- Muscle pain
- Soreness at the injection site
- Infants and elderly people may develop more serious reactions, such as encephalitis (very rare)
The following groups of people should not have the vaccination:
- Children aged less than 9 months in the USA, 6 months in the UK (unless the risk of yellow fever is unavoidable)
- Pregnant women (unless the risk of yellow fever is unavoidable)
- Breastfeeding mothers
- People with weakened immune systems, (unless the risk of yellow fever is unavoidable) such as those with HIV, or patients receiving chemotherapy and/or radiotherapy.
- Patients who are allergic to eggs
- If the patient is over 60 years of age he/she should discuss whether to have the vaccine with a doctor
To reduce your exposure to mosquitoes:
When possible, avoid outdoor activities during dawn, dusk and early evening - when mosquitoes are most prevalent.
Cover your skin as much as possible; wear long-sleeved shirts and long pants (UK: trousers) if you plan to go into areas where there are mosquitoes.
Stay indoors in places that have air-conditioning and good screening (window nets, etc).
Apply mosquito repellent which contains permethrin to clothing, shoes, camping equipment and bed netting. Do not use permethrin directly on your skin.
Skin repellents containing DEET or picaridin provide protection from mosquitoes for longer periods, compared to other products. The higher the concentration the longer it will last. Do not use DEET on young children's hands or infants aged less than 2 months - in such cases cover their stroller (push-chair) or playpen with mosquito netting when outdoors.
Oil of lemon eucalyptus - the CDC (Centers for Disease Control and Prevention) says that oil of lemon eucalyptus offers the same protection as DEET when used in the same concentrations. However, do not use on children less than 3 years of age.