VOL: 102, ISSUE: 17, PAGE NO: 52
Joyce Skeet, MSc, RGN, is practice nurse, Holbrook Surgery, Horsham, West SussexYellow fever is caused by the yellow fever virus that is transmitted by the bite of a mosquito in tropical and sub-...
Yellow fever is caused by the yellow fever virus that is transmitted by the bite of a mosquito in tropical and sub-Saharan Africa and South America. The main vector is the Aedes aegypti mosquito in Africa and the forest mosquitoes of the Haemagogus and Sabethes genera in South America.
About 90% of an estimated 200,000 annual cases of yellow fever occur in Africa (Tomori, 2004).
Since 1996 there have been six fatal cases of yellow fever reported in European and US travellers, but it should be noted that none of these people had been immunised (Health Protection Agency, 2006).
At the beginning of the 20th century, control of yellow fever in West Africa was by compulsory immunisation of the population, but the World Health Organization (WHO) is concerned that many of those countries have gradually lost that protection because of non-immunised people migrating to areas of high risk owing to armed conflict in their homelands, which prevents immunisation programmes being carried out.
Yellow fever has never been reported as occurring in Asia, but should it accidentally be imported there, the potential for outbreaks exists because the appropriate mosquito vector is present in that part of the world (WHO, 2005a).
Types of yellow fever
In Africa, urban yellow fever is usually spread by the Aedes aegypti mosquito. The mosquito breeds in stagnant water or pools near urban dwellings and the virus is passed from human to human by the bite of an infected female mosquito during daylight hours. Jungle yellow fever occurs when monkeys infected by the virus are bitten by forest mosquitoes that feed on them which, in turn, bite humans who enter the forest to work - forestry workers, for example - or un-immunised travellers. Once the mosquito is infected it remains so for its lifetime.
Over the past 10 years there has been an increase in the number of countries reporting yellow fever to the WHO: 18 countries in Africa have reported cases since 2000. Yellow fever poses a risk to all travellers, tourists and immigrants visiting countries where it is endemic. Some countries require proof of vaccination.
Sign and symptoms
Following a bite from a mosquito infected by the yellow fever virus, there is an incubation period of between three and six days. The disease then evolves through three stages:
- Stage 1: patients suddenly present with fever (39 degs-40 degsC), severe headache, muscular pains, abdominal pain, nausea and vomiting. They are very ill, with a slow pulse (Faget's sign), which is unusual considering the pyrexia and dark urine. (A pyrexia is normally accompanied by a rapid pulse.) At this stage of the disease patients are highly infectious to mosquitoes. They should therefore be isolated in a well-screened room that has been sprayed with an insecticide;
- Stage 2: this is a short remission stage, which lasts for up to a day, when the pyrexia is lower and the headache disappears. The patient consequently feels a little better. Some people may recover slowly after this stage;
- Stage 3: about 15-25% of patients infected by the virus will go on to this third stage; the temperature, headache, pains and sickness return, and they become jaundiced. It is the jaundice that gives the disease its name.
Bleeding of the mucous membranes, epistaxis and petechiae (tiny purple or red spots on the skin resulting from minor haemorrhages in the dermal or submucosal layers) are common during this third stage. Severe bleeding of the gastrointestinal tract may present as haematemesis or melaena, and laboratory blood tests will show thrombocytopenia, with platelet and clotting disorders. Patients also become dehydrated from vomiting, and urine output is reduced. Between 20 and 50% of patients lapse into a coma and die between seven and 10 days from the onset of the disease.
Patients who survive yellow fever may suffer from fatigue for a prolonged period and may have abnormal liver function test results for months after recovery. Some may contract pneumonia as a result of a bacterial infection, and if they have renal impairment they may require dialysis. Patients who survive a primary yellow fever infection have protection for life against getting the disease again.
Diagnosis and treatment
Diagnosing yellow fever can be difficult, as it presents in a similar way to many other conditions that involve the liver or kidneys: leptospirosis, hepatitis A and severe malaria, for example. Because there are no specific laboratory tests to diagnose yellow fever, a microbiologist who can isolate the virus and interpret the serology results is required. Liver biopsy is contraindicated as it may cause haemorrhage (Tomori, 2004).
Unfortunately, treatment of yellow fever is supportive, and many of those who become infected with it will not have access to basic hospital care or drugs. The death rate can be as high as 50% in those who are not immunised (Spira, 2005). It is therefore important to minimise the risk of infection. Methods of doing so are listed in Box 1.
The are two main reasons for vaccination:
- To protect travellers who may be exposed to the yellow fever virus because of living in or travelling through areas where it is endemic;
- To protect countries from the risk of importing the yellow fever virus.
Yellow fever vaccine is based on the 17D strain of the yellow fever virus. It is a live, attenuated virus strain produced in chick embryos. Ten days after vaccination, 95% of those vaccinated will have measurable antibody levels, and 99% will have antibodies 30 days following vaccination. Revaccination is required under the International Health Regulations every 10 years, although studies have shown that antibody levels are still detectable 30 - 35 years following vaccination (Tomori, 2004).
Only yellow fever vaccine approved by WHO can be used for vaccination.
In order for a yellow fever vaccination to be officially recognised, it must be administered in an approved vaccination centre and registered on an international certificate. The certificate must be correctly completed in English or French, signed by an authorised person, dated, and the official stamp of the centre used. It must also include the name of the manufacturer of the vaccine, the batch number and the expiry date. This certificate is valid from the tenth day after vaccination for 10 years.
If an individual cannot be immunised for medical reasons, for example, if she/he is immunocompromised (Weir and Haider 2004), or has an allergy to egg protein (WHO, 2005b) a physician may complete an exemption certificate. The British National Formulary lists contraindications and precautions. See Box 2 for further useful information on yellow fever.
Reactions following vaccination
Vaccination reactions include pain, swelling, redness and induration at the injection site, pyrexia, headache and influenza-like symptoms. Since 1996 there have been reports worldwide of yellow fever vaccine causing viscerotropic disease (a syndrome of fever and multi-organ failure), with 14 deaths. This has led to more research into the vaccine's safety profile and it has been shown that four of the 23 people who reported the syndrome had a history of thymus dysfunction: either they had a thymoma or had had a thymectomy. These rare conditions increase with age and with autoimmune disorders. Healthcare professionals therefore need to include questions in their risk assessment about thymus disorders, and consider the risks and benefits of vaccination for older travellers.
Yellow fever is a serious disease, and can be prevented by vaccination. Those considering travelling to areas where it is endemic, need to ensure that they take the necessary precautions to prevent its transmission.