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West Nile Virus Infection

West Nile Virus Infection

6 November 2017

Causative agent

West Nile Virus (WNV) belongs to the family Flaviviridae. WNV is commonly found in Africa, Europe, the Middle East, West Asia and recently North America. WNV is mainly transmitted between birds and mosquitoes. Humans, horses and other mammals can also be infected.

Clinical features

Most people who become infected have no symptoms at all, and about 20% may develop symptoms of fever, headache, body ache, nausea, vomiting, and sometimes swollen lymph glands or skin rash. Less than 1% of infected patients will develop severe disease such as inflammation of the brain (encephalitis) and/ or inflammation of the membrane around the brain and spinal cord (meningitis). These patients may have symptoms of high fever, headache, stiff neck, disorientation, muscle weakness, convulsion and coma. Elderly are at a higher risk of developing severe infection.

Incubation period

The incubation period is usually 3 – 14 days.

Mode of transmission

The infection is primarily transmitted by the bite of an infected mosquito, principally the Culex species mosquitoes. Mosquitoes become infected when they feed on infected birds. These infected mosquitoes can then spread the virus to humans and other animals during feeding. It is not transmitted from person-to-person through close contact. There have also been reports that the virus spread through blood transfusion, organ transplantation, breastfeeding and from mother-to-baby during pregnancy.


There is no specific treatment for WNV infection. In patients with severe disease, intensive supportive therapy may be indicated. The fatality rate among cases with neuro-invasive illness was about 4 – 14 %.


Currently, there is no human WNV vaccine. In the absence of an effective vaccine, preventive measures against mosquito breeding and avoidance of mosquito bites remain the main strategy to prevent from contracting the disease.

Travellers to areas with WNV outbreak should take measures to prevent mosquito bites. These include avoiding going outdoors from dusk till dawn when the mosquitoes are most active.

General measures on preventing mosquito-borne diseases

  1. Wear loose, light-coloured, long-sleeved tops and trousers, and use DEET-containing insect repellent on exposed parts of the body and clothing.
  2. Take additional preventive measures when engaging in outdoor activities:
  • Avoid using fragrant cosmetics or skin care products
  • Re-apply insect repellents according to instructions
  • Treat clothing and gears (such as tents, bed nets) with permethrin (an insecticide). Do NOT use permethrin directly on skin
  1. Travellers who return from affected areas and feel unwell, e.g. run a fever, should seek medical advice promptly, and provide travel details to doctor

Help prevent mosquito proliferation

  1. Prevent accumulation of stagnant water
  • Change the water in vases once a week
  • Clear the water in the saucers under potted plants every week
  • Cover water containers tightly
  • Ensure air-conditioner drip trays are free of stagnant water
  • Put all used cans and bottles into covered dustbins
  1. Control vectors and reservoir of the diseases
  • Store food and dispose of garbage properly

Pregnant women and children of 6 months or older can use DEET-containing insect repellent. For details about the use of insect repellents and the key points to be observed, please refer to 'Tips for using insect repellents'.

For more information about control and prevention of mosquito breeding, please visit the website of the Food and Environmental Hygiene Department (FEHD) at