Yellow Fever

Yellow fever is a serious viral disease. The virus belongs to the Flavivirus genus (the same group containing Japanese encephalitis, dengue fever, Murray valley encephalitis, and others). The disease is transmitted to humans via a mosquito bite.

Yellow Fever is found in tropical and subtropical areas of Africa and South America. The name, “yellow”, comes from jaundice due to the hepatitis and haemorrhagic complications observed in severe disease.

Most cases of yellow fever are self-limited but the mortality rate in severe disease can be as high as 50%. There is no specific antiviral therapy. Treatment can only be supportive.

The Yellow Fever virus is transmitted to humans through the bite of an infected Aedes or Haemagogus species mosquito. The mosquito is a daytime feeder. Nets have a limited role in protection. They might be very important for very young children in open accommodation.

An infected mosquito will acquire the virus by feeding on human or non-human primates, once in the ‘infectious period’ (shortly before the onset of fever and up to five days after onset) the virus can be continually transmitted between humans and monkeys, via the mosquito vector.

This is the main reason for it being subject to international health regulations. It is possible for an infected person to initiate an outbreak after international travel, but this is only possible if the receptive mosquito is present in the area.

Although many infected individuals can be asymptomatic or only experience very mild symptoms, there is a risk of severe disease in 15% of cases.

The incubation period (time from infection until illness) of the Yellow Fever virus is typically 3 – 6 days.

For 1 out of 7 infected individuals who display the initial symptoms, there will be a brief remission, often lasting less than 24 hours, followed by a more severe form of infection. A severe form of the yellow fever infection can be fatal, of those who develop the severe symptoms, 30-60% have a fatal outcome.

Yellow Fever typically begins with an abrupt onset of fever or chills, accompanied by a headache, jaundice (yellowing of the skin), muscle pain or back pain, general body aches, nausea or vomiting, and fatigue. As the condition progresses, more severe symptoms may develop, including a high fever, increased jaundice, bleeding, shock, and organ failure.

For most infected individuals, the initial symptoms improve within one week. However, for some, weakness and fatigue may last several months post-infection.

A diagnosis of Yellow Fever is based upon a combined observation of the infected individual’s clinical presentation and history of travel dates/location.

To confirm a diagnosis of Yellow Fever, laboratory testing of the individual’s blood is advised – virus specific antibodies can be detected early in the illness. Results should take 4– 14 days to be confirmed.

Once infected, the individual is likely to be protected from future Yellow Fever infections.

There is no cure for Yellow Fever. The Centers for Disease Control and Prevention (CDC) advises those infected with mild symptoms to rest, drink fluids and use pain relievers and medication to reduce fever and relieve aching. Medications, such as Aspirin or other non-steroidal anti-inflammatory medicines (e.g. ibuprofen, Advil, Motrin, Naproxen, Aleve, etc), should not be used, as they may increase the risk of bleeding.

Individuals infected with the severe form of Yellow Fever should be hospitalised for close observation and supportive care.

There are several factors which determine risk of exposure. These include:

• Immunisation status of the traveller
• Local immunisation levels
• Local viral transmission
• Season of travel
• Duration of travel
• Activities while travelling
• Occupational activities

The absence of case reports in a particular region does not equate to “no risk”.

Transmission is considered year-round in areas of risk in Africa. Peaks occur between the end of the wet season and the beginning of the dry (July to October).  In Central and South America, the highest risk is during the wet season (January to May) with a peak during February and March. It is more related to jungle areas in South America.

The Centers for Disease Control and Prevention (CDC) predict for a 2-week stay, the estimated risks for infection and death due to Yellow Fever, for an unvaccinated individual visiting an endemic area, are as follows:

Lower rates of morbidity and mortality in South America are attributed to higher vaccination rates in the local population and reduced human proximity to monkeys in forested areas.

See up to date Yellow Fever maps and areas of risk for Africa and South America.

For international travellers, the yellow fever vaccination may be required by law in some circumstances. It is important to have a travel medicine consultation if considering travel to/from endemic areas.

Although yellow fever vaccination is the mainstay for protection, mosquito avoidance measures are still very important. It is important to remember, the mosquito that transmits yellow fever, is also responsible for other diseases like dengue, chikungunya and zika.

Yellow Fever was a major public health concern from the early 1800’s to the 20th century. The disease was reported in most major centres in the USA and for one year (1878), an estimated 74,000 cases were reported and over 16,000 deaths in new arrivals.

In the 1880’s one in 5 workers on the Panama Canal died of yellow fever.

Following the development of the yellow fever vaccines in the 1930s, cases of yellow fever declined significantly. Occasional outbreaks of the disease are still reported in endemic areas that lack adequate vaccination programs.

An outbreak of yellow fever began in Brazil in December 2016. The WHO updated their recommendations for vaccination early 2018. Ten cases of the disease were reported from 2018. None had been vaccinated and 5 died.

A Cuban epidemiologist, Carlos Finlay, first described the epidemiology of yellow fever in 1886. This was the first publication citing transmission of the disease via a mosquito. He noted that peaks of the disease were highest when the mosquito population was high. His contribution was largely ignored until it was corroborated by the prominent American physician, Walter Reed, in 1900. Eradication programs were then successful in Cuba and Panama.