Yellow fever

Written by Dr Charlie Easmon, specialist adviser in travel medicine

What is yellow fever?

Yellow fever is a serious viral infection, transmitted by mosquitoes in tropical regions. It has both an urban cycle and a jungle cycle that relies on monkeys as carriers ('sylvatic cycle').

In mild cases the symptoms are similar to influenza, but serious cases develop a high temperature and may have a series of after effects, such as internal bleeding, kidney failure and meningitis.

A classic feature of yellow fever is hepatitis, which is the reason for the yellow colouring of the skin (jaundice) and the name of the disease.

Yellow fever can cause sudden epidemics, with a mortality rate of almost 50 per cent. Although a safe, efficient vaccine has been available for the last 60 years, epidemics still occur, constituting a health risk in tropical regions.

The disease is covered by the International Quarantine Regulations, which are taken very seriously by authorities everywhere. Therefore, the vaccine has to be administered by a specially authorised doctor.

In the UK you can only obtain the yellow fever vaccination from a designated Yellow Fever Clinic.

What causes yellow fever?

Yellow fever virus belongs to the Flaviviridae family, other members of which cause dengue fever and Japanese encephalitis.

The virus is introduced into the bloodstream via the saliva of the mosquito as it bites.

The virus can then be transported around the body and reproduce itself in a variety of the body's cells, usually the liver, kidneys and blood vessels. In serious cases, these cells may become damaged themselves.

In addition, the cells of the immune system are affected and release large quantities of signalling substances. These substances are the cause of the normal disease symptoms, such as muscular pain and fever, which are also observed in influenza.

How is yellow fever passed on?

The virus is transmitted among humans by a couple of species of mosquito, including Aedes egyptii, which can also transmit dengue fever. It is an unexplained fact that despite the presence of the Aedes mosquito in Asia, yellow fever is limited to Africa and South America.

In its original jungle cycle, the mosquito sucks the blood of an infected monkey. The mosquito develops a permanent infection, in which the virus accumulates in its salivary glands. Then the mosquito bites another monkey, which then also becomes infected with the virus.

A person travelling through the jungle may also become infected by an infected mosquito. When this person returns to urban areas, a new cycle begins.

Urban cycles start when an infected traveller returns from the jungle. A mosquito bites the traveller, who then becomes infected and passes the virus on to other people, and either an epidemic breaks out, or an endemic situation is perpetuated.

Where does yellow fever occur and how many people are affected?

The virus is permanently prevalent, with a more or less constant number of sufferers (ie it is endemic) in several tropical regions of Africa and on the continent of America.

In addition, there is an increasing number of epidemics, in which a large number of people suddenly develop yellow fever.

Every year about 200,000 cases of yellow fever are recorded, and 30,000 of these die, but the figures are underestimated because of poor record-keeping.

In total, yellow fever occurs in 33 countries and 468 million people are at risk of catching the disease.

As yet there is no yellow fever in Asia, but it is feared that the high level of international travel could introduce the virus by means of infected people. Mosquitoes do live here, and they could potentially transmit the disease and create a new reservoir.

Consequently, the countries of Asia have very strict quarantine regulations, which apply if you arrive without a valid vaccination certificate travelling from areas in Africa and Latin America, where yellow fever occurs.

What are the symptoms of the disease?

The incubation period from infection to developing yellow fever is 3 to 16 days.

Mortality ranges from 5 per cent to 40 per cent. Some people do not develop a serious form of the disease. They may have no symptoms at all or just a mild influenza-like illness.

In the actual full-blown disease there is:

high fever

generalised symptoms like violent headache, muscular pain, upset stomach and loss of fluid.

The fever dies down after three to four days and the general condition improves. However, in about 15 per cent of cases the fever returns within 24 hours and a dangerous situation develops.

The danger signals

Various systems in the body are affected during this phase. Anaemia (lack of red blood cells) develops as well as liver inflammation, hepatitis and jaundice.

The kidneys are also affected and bleeding from the mouth, nose and stomach may occur, which leads to blood in vomit and faeces.

The majority of patients who experience bleeding die in a short space of time.

What can you do yourself?

There is a vaccine, which is very effective against yellow fever. It protects you from 10 days after the vaccination, which is administered in a single injection. Current advice is to have it repeated every 10 years.

It is recommended for all areas where the disease occurs.

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