Yellow fever is a viral infection spread by mosquitoes. The word “yellow” is a reference to the yellowing of the skin (jaundice) as a result of the damage to the liver. Yellow fever does not often receive much public attention due to the effectiveness of immunisation, however, the incidence has increased in recent years for various reasons. It is more frequently seen in Africa and South America and about half of all the severe cases will result in death without treatment.
What is yellow fever?
Yellow fever is a viral hemorrhagic disease caused by a flavivirus that is spread to humans by the bite of an infected mosquito. The disease can vary in intensity from a mild, self-limiting illness to a severe life-threatening disease. It can cause damage to a number of organs and in particular to the the liver although the heart and kidney may also be affected in severe cases. Currently there is no treatment for yellow fever but the vaccine is highly effective in preventing the disease. It is important to note that yellow fever is not life-threatening in every instance and despite the lack of treatment, the body’s immune system is capable of overcoming the infection.
Signs and Symptoms of Yellow Fever
The incubation period for yellow is between 3 to 6 days and during this time after contracting the infection, a person may have no signs and symptoms (asymptomatic). This is then followed by the acute phase of infection where the following signs and symptoms may be evident :
- Muscle pain
- Nausea and vomiting
- Loss of appetite
The signs and symptoms of the acute phase may initially be intense but gradually improves and resolves after 3 to 4 days. The disease at this point may end but in some cases it can progress to the next, more severe phase. After about 24 hours of being symptom-free, the toxic phase may develop in a minority of cases. The signs and symptoms of the toxic phase includes :
- Jaundice – yellowing of the skin, ‘whites’ of the eyes, inside the mouth
- Abdominal pain
- Vomiting, sometimes vomiting of blood (haematemesis) due to bleeding in the stomach
- Bleeding from the mouth, nose or eyes
- Low urine output
- Irregular heartbeat
Liver and kidney failure may occur in the toxic phase and patients may slip into a coma. The toxic phase is potentially fatal.
Risk and Transmission
Travellers to endemic areas like sub-Saharan Africa and parts of South America need to be aware of the risk of yellow fever. While various factors may reduce the risk, vaccination is by far the most important preventative measure. Yellow fever is transmitted from one host to another by certain species of mosquitoes. It can be transmitted from human to human or even from animal (monkey) to human by these mosquitoes. Urban yellow fever is most often due to human-to-human transmission particularly by the Aedes species of mosquito.
Yellow Fever Vaccine
The vaccine should be taken about 10 to 14 days prior to travel to high risk areas. Within 7 days of taking the vaccine, about 95% of those vaccinated will have the necessary immunity. Immunisation offers protection for 30 to 40 years, or possibly throughout life.Vaccinating 65% to 80% of a population in mass immunisation campaigns is usually effective in preventing outbreaks. However, the vaccine is not suitable for every person and should be avoided in the following circumstances :
- Infants under the age of 6 months, and preferably the vaccination should be held back until 9 months of age
- Pregnant women
- People with allergies to egg protein
- People with HIV/AIDS and other diseases/treatments that may affect the the immune system like leukaemia, lymphoma, radiation therapy and cancer drugs.
Treatment for Yellow Fever
There is no treatment for yellow fever itself and antiviral drugs are often not of use in this regard. Instead supportive treatment is advised for patients with yellow fever preferably within the hospital environment. This may include oxygen administration and rehydration. In the toxic phase, transfusions may be necessary for blood loss, blood proteins for clotting disorders and dialysis for kidney dysfunction.