Yellow fever

Causative agent

Yellow fever is a mosquito-borne disease caused by the yellow fever virus. It is endemic in the tropical areas of Africa and Central and South America. In recent few years, yellow fever cases have been reported mostly in the African Region with major outbreaks in Cameroon, Central African Republic, Chad, Ghana and Nigeria.

Clinical features

The majority of persons infected with yellow fever virus have no or only mild symptoms, such as fever, muscle pain, backache, headache, loss of appetite, and nausea or vomiting. For most patients, these symptoms disappear after 3 to 4 days. A small proportion of the cases will progress to a more severe form of the disease, including high fever, jaundice, bleeding, shock, organ failure and sometimes death. Fatality rate among severe cases is about 50%.

Mode of transmission

Yellow fever virus is transmitted to humans through the bites of infective Aedes or Haemagogus mosquitoes, primarily Aedes aegypti. Mosquitoes feed on infected hosts (mainly monkeys), and then pass on the virus when they feed on other humans or animals.

Yellow fever virus can also be rarely transmitted from mother to newborn around the time of birth.

Incubation period

The incubation period ranges from 3 to 6 days.

Management

There is no specific antiviral drug treatment for yellow fever. Management is mainly for symptomatic relief. For severe forms of infection, hospitalization and treatment for dehydration, liver and kidney failure, and fever will improve the outcomes. Associated bacterial infections can be treated with antibiotics.

Patients carrying the virus should be isolated to prevent mosquito bites that may spread the disease to others.

Prevention

Vaccination

Vaccination is the most important means of preventing yellow fever. The yellow fever vaccine is generally safe, and a single dose provides life-long protection against the disease.

Vaccination for yellow fever is available in Hong Kong. The service is provided at the travel health centres of the Department of Health (DH). It is recommended for people aged 9 months or older who are travelling to or living in areas at risk of yellow fever. The vaccine provides effective immunity within 10 days for 80–100% of people vaccinated, and within 30 days for more than 99% of people vaccinated. To ensure a safe journey, it is important to seek information and advice from healthcare professionals well in advance; it is recommended to book a travel health consultation at least 8 weeks before departure. For more information on yellow fever vaccination, please visit the Travel Health Service website of the DH at https://www.travelhealth.gov.hk/english/travel_related_diseases/yellow_fever.html.

Some countries require proof of yellow fever vaccination as a condition of entry or transit for travellers arriving from certain countries. A list of countries with risk of yellow fever transmission and countries requiring yellow fever vaccination can be found on the World Health Organization website at https://www.who.int/publications/m/item/countries-with-risk-of-yellow-fever-transmission-and-countries-requiring-yellow-fever-vaccination-(november-2022).

Prevention of mosquito bites

1. Wear loose, light-coloured, long-sleeved tops and trousers

2. Use DEET-containing insect repellent on exposed parts of the body and clothing.

Pregnant women and children of 6 months or older can use DEET-containing insect repellent. In general, use DEET of up to 30% for pregnant women and up to 10% for children

3. Take additional preventive measures when engaging in outdoor activities:

• Avoid using fragrant cosmetics or skin care products
• Re-apply insect repellent according to instructions
• If both insect repellent and sunscreen are used, apply insect repellent after sunscreen

Prevention of mosquito proliferation

1. Prevent accumulation of stagnant water

• Change the water in vases once a week
• Avoid using saucers underneath flower pots
• Cover water containers tightly
• Ensure air-conditioner drip trays are free of stagnant water
• Put all used cans and bottles into covered dustbins

2. Control vectors and reservoir of diseases

• Store food and dispose of garbage properly

For more information about control and prevention of mosquito breeding, please visit the website of the Food and Environmental Hygiene Department at http://www.fehd.gov.hk/english/pestcontrol/handbook_prev_mos_breeding.html

Advice for travellers

1. Take preventive measures to avoid mosquito bites. For children who travel to countries or areas where mosquito-borne diseases are endemic or epidemic and where exposure is likely, children aged 2 months or above can use DEET-containing insect repellents with a concentration of DEET up to 30%. For details about the use of insect repellents and the key points to be observed, please refer to 'Tips for using insect repellents'.

2. If going to affected areas or countries, arrange medical consultation at least 6 weeks before the trip, have extra preventive measures to avoid mosquito bites.

3. If travelling in endemic rural areas, carry a portable bed net and apply permethrin (an insecticide) on it. Permethrin should NOT be applied to the skin. Seek medical attention promptly if feeling unwell.

4. Scientific studies have shown that infected persons can transmit the virus to mosquitoes through mosquito bites even if they remain asymptomatic or before their onset of symptoms, leading to further spread of the disease. Hence, travellers who return from affected areas should apply insect repellent for at least 12 days after arrival to prevent mosquito bites. If feeling unwell e.g. having a fever, the person should seek medical advice promptly, and provide travel details to the doctor.