Yellow fever is a viral haemorrhagic disease.
It is transmitted by infected mosquitoes. The ‘yellow’ in the name refers to the jaundice that effects some patients. There is no treatment for the disease and vaccination is the most effective method of prevention.
Up to 50 percent of severely affected patients die within 14 days, according to the World Health Organisation.
The vast majority of yellow fever cases and deaths take place in sub-Saharan Africa, where the disease is a major public health problem.
Thirty-two African countries are now considered at risk of yellow fever, with a total population of 610 million people.
What causes yellow fever?
The yellow fever virus is transmitted through the bite of infected female Aedesmosquitoes. Yellow fever occurs only in humans, other primates and several species of mosquitoes.
In urban settings, the Aedes aegypti mosquito is involved in transmitting the virus, the same genus of mosquito responsible for transmitting Zika, dengue and chikungunya fevers.
When the mosquito takes the virus from an infected person, the virus develops in the mosquito’s body for about 12 days, after which the mosquito can transmit it to a healthy person.
Once a female mosquito is infected, it will continue to carry the virus throughout its life, which lasts one to two months. It can also transmit the virus to its offspring, although not systematically.
The disease is common in tropical and subtropical areas of South America and Africa. The A. aegypti is also found in tropical and subtropical areas of Asia, the Pacific and Australia, but no cases have been recorded in these regions.
Symptoms of yellow fever
Yellow fever begins after an incubation period of three to six days. Most cases only cause a mild infection with fever, with slight symptoms common to the majority of viruses: headache, chills, back pain, fatigue, loss of appetite, etc. In these cases, the infection lasts only three to four days.
In 15 percent of cases, however, people enter a second, toxic phase of the disease with recurring fever, this time accompanied by jaundice due to liver damage (hence the disease’s name). Bleeding in the mouth, the eyes, and the gastrointestinal tract will cause bloody vomit.
The toxic phase is fatal in about 20 percent of cases, making the overall fatality rate for the disease around three percent.
Surviving the infection provides lifelong immunity, and normally there is no permanent organ damage.
Diagnosing yellow fever
Yellow fever is difficult to diagnose, especially during the early stages. More severe cases can be confused with severe malaria, leptospirosis, viral hepatitis, other haemorrhagic fevers, infection with other flaviviruses (e.g. dengue fever), and poisoning.
Blood tests can sometimes detect the virus in the early stages of the disease. In later stages, testing to identify antibodies is needed.
Treating yellow fever
There is currently no specific anti-viral drug for yellow fever. We respond to the disease by admitting patients to hospital and help them to overcome the disease by attacking the symptoms.
During the second, more toxic, phase of the disease – when the virus disappears and the patient is no longer contagious – the most important thing is to prevent the liver from becoming necrotic.
The most effective method to combat yellow fever is prevention through vaccination. People should also protect themselves from being bitten using repellents, mosquito nets, long clothing, etc.
They should also avoid having anything in or around the house that can hold stagnant water, which encourages mosquito breeding. MSF also carries out vector control by using biocides (chemical or biological pesticides).