West Nile Virus Infection

Causative agent

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

Clinical features

Most infected people have no symptoms. About 20% may develop symptoms such as fever, headache, tiredness, body aches, nausea, vomiting, skin rash and swollen lymph glands. Less than 1% of infected patients will develop a more severe form of disease that affects the nervous system, including inflammation of the brain (encephalitis) and/ or inflammation of the membranes around the brain and spinal cord (meningitis). These patients may present with headache, high fever, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis. Elderly and immunocompromised people are at a higher risk of developing severe disease.

Mode of transmission

The infection is primarily transmitted by bites of infected mosquitoes, principally of the Culex species. The mosquitoes become infected when they feed on infected birds, and spread the virus to humans and other animals during feeding. It is not transmitted from person-to-person through close contact. Nevertheless, there have been reports that the virus spread through blood transfusion, organ transplantation, and from mother-to-baby during pregnancy or breastfeeding.

Incubation period

The incubation period usually ranges from 3 to 14 days.

Management

There is no specific treatment for WNV infection. In patients with severe disease, hospitalisation with supportive therapy may be indicated. The fatality rate among cases with neuro-invasive illness can be up to 17%.

Prevention

There is no human vaccine against WNV infection. The best preventive measure is to avoid mosquito bites and prevent mosquito proliferation.

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 repellents 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 the 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 (FEHD) at http://www.fehd.gov.hk/english/pestcontrol/handbook_prev_mos_breeding.html

Advice for travellers

• 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'.
• 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.
• 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.
• If you are feeling unwell, e.g. having a fever, during your visit abroad or after return, seek medical advice immediately and provide travel details to the doctor.