Yellow fever is a viral disease spread by infected mosquitoes. It occurs in parts of Africa and Central and South America. The risk of yellow fever infection for international travellers is low, particularly for those who travel only to urban areas. Although Canadians stand a small risk of getting yellow fever when travelling abroad, they cannot spread the disease on their return to Canada. The mosquito which spreads the virus is not present in Canada.

Symptoms, which begin from three to six days post-infection include:

• sudden onset of fever
• chills
• headache
• backache
• all-over muscle pain
• nausea
• vomiting

The disease progresses to cause problems with the person's blood count; bleeding from the nose, mouth, and intestine; and a yellowing of the skin and eyeballs (jaundice).

There are 3 cycles of yellow fever:

• jungle or sylvatic cycle, with the virus infecting non-human primates bitten by Aedes orHaemagogus mosquitoes, and occasionally infecting humans in tropical regions of Africa and Latin America 
• urban cycle, with the virus cycling between humans and Aedes aegypti mosquitoes, and
• intermediate cycle involving Aedes mosquitoes in humid and semi-humid portions of Africa (Control of Communicable Diseases Manual, Heymann, 2008)

The case fatality rate is 20 to 50 per cent.

There are 3 different ways to determine if someone has yellow fever:

• growing the virus from a blood specimen in suckling mice, mosquitoes or cell cultures
• using the ELISA test to identify antigens in the blood or tissues
• using the PCR test to find evidence (RNA) of the virus in the blood

Other tests may be necessary to distinguish infection from a recent yellow fever vaccination (Control of Communicable Diseases Manual, Heymann, 2008).

Yellow fever vaccination is available to international travellers through designated yellow fever vaccination centres. Yellow fever vaccination is a legal requirement for entry to some countries, either for all travellers or for those coming from other countries where the yellow fever virus is found. Consult a travel clinic to find out which countries have this requirement. 

The travel clinic which vaccinates you will give you a special booklet. It contains an "International Certificate of Vaccination or Revaccination Against Yellow Fever" which records the date of your vaccination, and a validation stamp. You are responsible for keeping track of this booklet. It is an official record of your vaccinations. 

Travellers nine months of age and older receive one injection of vaccine in the arm. Infants 4 to 9 months of age, pregnant women, and persons with HIV infection should only be considered for vaccination if travelling to high-risk areas when travel cannot be postponed and a high level of prevention against mosquito exposure is not feasible. Infants less than 4 months of age should not be given vaccine. 

Vaccination is also recommended for laboratory personnel who work with yellow fever virus. Immunity develops 10 days after vaccination and lasts for more than 10 years. Revaccination every 10 years is required for travel and is recommended for those laboratory personnel who work with the virus. Any person not receiving yellow fever vaccination because of a medical contraindication should travel with a doctor's letter, written on letterhead stationery and stamped with the Health Canada stamp used to validate the International Certificate of Vaccination. It should clearly state the contraindication to yellow fever vaccination. 

Occasionally, side effects may appear 5 to 10 days after immunization. These side effects (which happen to about 5% of people) may include: 

• mild headache
• slight fever, or
• aching muscles lasting a day or two.

These can be relieved by using acetaminophen. With any vaccine or drug there is a possibility of a shock-like allergic reaction (anaphylaxis). This can be hives, wheezy breathing, or swelling of some part of the body. If this happens, particularly swelling around the throat, immediately get to your family doctor or hospital emergency. It is suggested that persons stay in the clinic for at least 15 minutes after receiving any type of immunization. Report serious reactions to your local health unit, travel clinic or doctor.

• have diseases or take drugs which reduce their immunity;
• have a moderate or severe illness;
• are less than four months old;
• have HIV infection, are pregnant, or are 4 to 9 months of age, unless the risk of exposure is very high; or
• have had a known shock-like (anaphylactic) reaction to eating eggs, to a previous dose of yellow fever vaccine or to any component of the vaccine (sorbitol and gelatin).

While every effort is made to seek parental or guardian consent prior to immunizations, children under the ago of 19 who are able to fully understand the risks and benefits of specific immunizations may consent to, or refuse such immunizations regardless of parental/guardian wishes. Appropriate steps are taken to avoid peer influence in these decisions. Parents/guardians and their minor children are advised to discuss consent issues. For further information, please contact your local health unit.

The best way to prevent this disease is to avoid being bitten by mosquitoes. Mosquitoes that carry yellow fever are most active during the day, particularly at dawn and at dusk. Some of the ways you can avoid mosquito bites are:

• If you are taking part in outdoor activities between dusk and dawn, use insect repellent on all exposed skin (one containing a 30% concentration of DEET, if available, for longest protection). Follow application instructions carefully.
• Wear light-colored clothing including long sleeves, trousers and socks.
• Sleep in places with screened windows and doors, or places which have air conditioning (so you can close the windows and doors without being too hot and uncomfortable).
• Sleep under an intact, small-mesh mosquito net which has been sprayed with a permethrin (insecticide) preparation. Tuck mosquito net under mattress before nightfall. The mosquito net is particularly important if it is not possible to stop mosquitoes from entering your room.
• Use insecticidal spray indoors in the evening.
• Use non-perfumed cosmetics and toiletries. Mosquitoes are attracted to scented products.