| 4 November 2009 |
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West Nile Virus Infection
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Causative agent
West Nile Virus (WNV) belongs to the family Flaviviridae. The virus can infect humans, birds, mosquitoes, horses and some other mammals. WNV is one of the most widely distributed flaviviruses with a geographic range including Africa, Europe, the Middle East, West Asia and recently North America.
Clinical features
Eighty percent of those infected have no symptoms at all, and 20% have a mild illness with 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. Severe infection may cause 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 to 14 days.
Mode of transmission
The infection is primarily transmitted by the bite of an infected mosquito, principally the Culex species mosquitoes. Mosquitoes are WNV vectors that become infected when they feed on infected birds. These infected mosquitoes can spread the virus to humans and other animals during feeding. In the United States, crows are the most commonly affected bird species. 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.
Management
There is no specific treatment for WNV infection. In patients with severe disease, intensive supportive therapy may be indicated. About 4-14% of cases have resulted in death in recent WNV outbreaks.
Prevention
Currently, there is no licensed human WNV vaccine. Research studies related to human WNV vaccine are still in progress. 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, wearing long-sleeved clothing and trousers, and applying effective insect repellents containing DEET to exposed parts of the body and clothes.
Prevent the breeding of mosquitoes and avoid mosquito bites:
1. Put all used cans and bottles into covered dustbins.
2. Change water for plants at least once a week, leaving no water in the saucers underneath flower pots.
3. Cover tightly all water containers, wells and water storage tanks.
4. Keep all drains like rain gutters, drain holes and surface channels free from blockage.
5. Top up all defective ground surfaces to prevent the accumulation of stagnant water.
6. Use mosquito screens or nets when the room is not air-conditioned.
7. Apply household pesticide to kill adult mosquitoes with a dosage according to the label instructions. Do not spray the pesticides directly against functioning electrical appliances or flame to avoid explosion.
8. Place mosquito coil or electric mosquito mat / liquid near possible entrance, such as window, to prevent mosquito bites.
9. When staying outdoors, wear long-sleeved clothes and long trousers
10. Apply insect repellent (containing DEET) on the clothes and exposed parts of the body when necessary.
For more information about control and prevention of mosquito breeding, please visit the website of Food and Environmental Hygiene Department at http://www.fehd.gov.hk/english/safefood/handbook_prev_mos_breeding.html
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