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.
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.
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
Help prevent mosquito proliferation
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 http://www.fehd.gov.hk/english/safefood/handbook_prev_mos_breeding.html