West Nile Virus Infection

3 December 2019

Causative agent

West Nile Virus (WNV) belongs to the family Flaviviridae. WNV is commonly found in Africa, Europe, the Middle East, West Asia and 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, tiredness, body aches, nausea, vomiting, skin rash and swollen lymph glands. 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 headache, high fever, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis. Elderly people are at a higher risk of developing severe disease.

Incubation period

The incubation period is usually 3 to 14 days.

Mode of transmission

The infection is primarily transmitted by the bite of an infected mosquito, principally of the Culex species. Mosquitoes may 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. Nevertheless, there have been reports that the virus spread through blood transfusion, organ transplantation, and from mother-to-baby during pregnancy or breastfeeding.

Management

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 to 14%.

Prevention

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 contracting the disease.

Travellers to areas with WNV outbreak should take measures to prevent mosquito bites, and avoid unprotected outdoor exposure from dusk till dawn when the mosquitoes are most active.

General measures on preventing mosquito-borne diseases

Wear loose, light-coloured, long-sleeved tops and trousers, and use DEET-containing insect repellent on exposed parts of the body and clothing.
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

Special notes when travelling abroad:

During the trip, if travelling in endemic rural areas, carry a portable bed net and apply permethrin on it. Travellers returning from affected areas if feel unwell, e.g. run a fever, should seek medical advice promptly and provide travel details to the doctor.

Help prevent mosquito proliferation

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

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 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'.

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