Yellow fever is a viral haemorrhagic fever transmitted by infected mosquitoes. It can be recognized from historic texts stretching back 400 years. Infection causes a wide spectrum of disease, from mild symptoms to severe illness and death. The “yellow” in the name is explained by the jaundice that affects some patients, causing yellow eyes and yellow skin.
There are three types of transmission cycle: sylvatic, intermediate and urban. All three cycles exist in Africa, but in South America, only sylvatic and urban yellow fever occur.
- Sylvatic (or jungle) yellow fever occurs in tropical rainforests where monkeys, infected by sylvatic mosquitoes, pass the virus onto other mosquitoes that feed on them; these mosquitoes, in turn bite and infect humans entering the forest. This produces sporadic cases, the majority of which are often young men working in the forest e.g. logging.
- The intermediate cycle of yellow fever transmission occurs in humid or semi-humid savannahs of Africa, and can produce small-scale epidemics in rural villages. Semi-domestic mosquitoes infect both monkey and human hosts and increased contact between man and infected mosquito leads to disease. This is the most common type of outbreak seen in recent decades in Africa.
- Urban yellow fever results in large explosive epidemics when travellers from rural areas introduce the virus into areas with high human population density. Domestic mosquitoes, most notably Aedes aegypti, carry the virus from person to person. These outbreaks tend to spread outwards from one source to cover a wide area.
Yellow fever can be prevented by vaccination. In order to protect people living in areas at high risk of yellow fever transmission, WHO’s dual strategy for prevention of yellow fever epidemics relies on preventive mass immunization campaigns followed by infant routine immunization.
Yellow fever is caused by a virus carried by mosquitoes. You can catch this disease if you are bitten by a mosquito infected with this virus.
This disease is common in South America and in sub-Saharan Africa.
Anyone can get yellow fever, but the elderly have a higher risk of severe infection.
After being infected with yellow fever, it usually takes three to six days for the symptoms to appear. This is known as the incubation period.
The symptoms of yellow fever can occur in two distinct stages:
First stage. The symptoms of the first stage, also known as the ‘acute phase’, include:
- a high temperature (fever) of 38ºC (100.4ºF) or above
- chills (shivers)
- nausea and vomiting
- muscle pain, including backache
- loss of appetite
Most people improve after three to four days and their symptoms disappear. However, some people go on to develop more serious symptoms.
Second stage. After the initial symptoms of yellow fever, around 15% of people develop more severe symptoms. This is sometimes referred to as the ‘toxic phase’. The symptoms can include:
- a recurrent fever
- abdominal pain
- jaundice a yellow tinge to the skin and whites of the eyes caused by liver damage
- kidney failure
- bleeding from the mouth, nose, eyes or stomach, leading to blood in your vomit and stools.
Half of those who progress to the second, toxic phase of yellow fever die within 10-14 days. The other half recover with no major organ damage and are immune from the condition for the rest of their life.
Overall, this means about seven or eight people out of every 100 who develop yellow fever will die from it.
Exams and Tests
The health care provider will perform a physical examination and request selected blood tests. These blood tests may show liver and kidney failure and shock.
It is important to tell your doctor if you have traveled to areas where the disease is known to thrive. Blood tests can confirm the diagnosis.
As for other flavivirus infections, no cure is known for yellow fever. Hospitalization is advisable and intensive care may be necessary because of rapid deterioration in some cases. Different methods for acute treatment of the disease have been shown to not be very successful; passive immunisation after emergence of symptoms is probably without effect. Ribavirin and other antiviral drugs, as well as treatment with interferons, do not have a positive effect in patients. A symptomatic treatment includes rehydration and pain relief with drugs such as paracetamol (acetaminophen in the United States). Acetylsalicylic acid (aspirin) should not be given because of its anticoagulant effect, which can be devastating in the case of internal bleeding that can occur with yellow fever.
If your yellow fever symptoms are severe, you may be admitted to hospital so you can be monitored and receive supportive nursing care. Additional treatment may be necessary, including:
- a ventilator to help you breathe
- a blood transfusion of red blood cells to replace those lost through bleeding
- dialysis where a machine is used to filter your blood if your kidneys are no longer working
Vaccination is the most effective way of preventing yellow fever.
If you continue to live or travel in yellow fever-endemic areas, you should receive a booster dose of yellow fever vaccine after 10 years.
After receiving the vaccine, you should receive an International Certificate of Vaccination (yellow card) that has been validated by the vaccination center. This Certificate becomes valid 10 days after vaccination and lasts for 10 years. You will need this card as proof of vaccination to enter certain countries.
Some countries in Asia are theoretically in danger of yellow fever epidemics (mosquitoes with the capability to transmit yellow fever and susceptible monkeys are present), although the disease does not yet occur there. To prevent introduction of the virus, some countries demand previous vaccination of foreign visitors if they have passed through yellow fever areas. Vaccination has to be proven in a vaccination certificate which is valid 10 days after the vaccination and lasts for 10 years. A list of the countries that require yellow fever vaccination is published by the WHO. If the vaccination cannot be conducted for some reasons, dispensation may be possible. In this case, an exemption certificate issued by a WHO-approved vaccination center is required. Although 32 of 44 countries where yellow fever occurs endemically do have vaccination programmes, in many of these countries, less than 50% of their population is vaccinated.
By getting vaccinated
Yellow fever is preventable. The vaccine is safe and almost 100 percent effective. With few exceptions, vaccination is recommended for all travellers to countries or areas where there is a risk of yellow fever transmission.
By avoiding mosquitoes
The mosquitoes that transmit yellow fever are usually active during the day. All people who travel to or live in yellow fever endemic countries are advised to avoid mosquitoes. This can be done by taking the following measures:
- Wear a mosquito repellent containing DEET or Picaridin
- Wear light coloured, long-sleeved clothes when you’re outdoors
- Avoid wearing perfume or cologne (some of these can attract mosquitoes)
- Prevent mosquitoes entering your accommodation
- Use a mosquito net at night-time (if mosquitoes are likely to be present)
Mortality varies from 50% without medical treatment to about 5% with first-world medical care. Individuals recover or die from yellow fever in 7 to 12 days. Death usually occurs between the sixth and tenth day. Those who recover have life-long immunity. Weakness may prolong convalescence for 2 to 3 weeks. Initial prognosis is always guarded since a sudden change for the worse is common.