Background

West Nile Fever is caused by West Nile Virus (WNV). It is a mosquito-borne disease. WNV was first isolated in a woman in the West Nile district of Uganda in 1937. It was identified in birds (crows and columbiformes) in the Nile Delta region in 1953. WNV is commonly found in Africa, Europe, the Middle East, North America and South America.

WNV is maintained in nature in a cycle involving transmission between birds and mosquitoes. Birds are the reservoir of WNV. Human infection is most often the result of bites from mosquitoes which become infected when they feed on infected birds. Mosquitoes of the genus Culex are generally considered the principal vectors of WNV. The virus may also be transmitted through contact with other infected animals' blood or tissues. A very small proportion of human infections have occurred through organ transplant, blood transfusions and breast milk. There is one reported case of transplacental transmission. To date, no human-to-human transmission of WNV through casual contact has been documented, and no transmission of WNV to health care workers has been reported when standard infection control precautions have been put in place. Transmission of WNV to laboratory workers has been reported.

Infection with WNV is asymptomatic in around 70-80% of infected people. The incubation period is usually 2 to 6 days but ranges from 2 to 14 days and can be as long as 21 days in immunocompromised people. Most symptomatic people experience an acute systemic febrile illness that often includes headache, tiredness, arthralgia, myalgia, gastrointestinal tract symptoms such as vomiting or diarrhea, occasionally with a skin rash on the trunk of the body and swollen lymph nodes. Less than 1% of infected people develop neuroinvasive disease, which typically manifest as meningitis, encephalitis, or acute flaccid paralysis. Among patients with neuroinvasive disease, the overall case-fatality rate is approximately 10%. Treatment is supportive for patients with neuro-invasive WNV, often involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections. No vaccine is available for humans.

Epidemiology

Human infections attributable to WNV have been reported in many countries in the world for over 50 years. The largest outbreaks occurred in Greece, Israel, Romania, Russia and USA. Outbreak sites are on major birds’ migratory routes. In its original range, WNV was prevalent throughout Africa, parts of Europe, Middle East and Australia. In 1999, a WNV circulating in Israel and Tunisia was imported to New York producing a large and dramatic outbreak that spread throughout the continental United States of America (USA) in the following years. Since its introduction in 1999 into USA, the virus has spread and is now widely established from Canada to Venezuela. There are no WNV infected cases in Taiwan.

West Nile Fever Surveillance in Taiwan

1. Taiwan National Infectious Disease Statistics System–West Nile Fever
2. Fever screening at international airports and seaports.
3. Self–reporting through the toll–free 1922 hotline or local public health authority.

Figure: Fever screening at the Taiwan Taoyuan International Airport.

Prevention and Control

Since WNV outbreaks in animals precede human cases, the active animal health surveillance system is essential in providing early warning for human public health authorities. In the absence of a vaccine, the only way to reduce infection in people is by raising awareness of the risk factors and educating people about the measures to reduce the risk. To prevent getting West Nile fever through mosquito bites, use insect repellents, wear long-sleeved shirts and long pants, and avoid outdoor activity at peak biting times. People who have visited an area endemic for WNV should defer blood donation for 1 month following return; people who recently had West Nile virus should defer blood donation for 4 months.

FAQs

1. What is West Nile fever?
West Nile Fever is caused by West Nile Virus (WNV). It is a mosquito-borne disease. WNV is maintained in nature in a cycle involving transmission between birds and mosquitoes. Birds are the reservoir hosts of WNV. Human infection is most often the result of bites from mosquitoes which become infected when they feed on infected birds. Infection with WNV is either asymptomatic in around 70-80% of infected people, or can lead to West Nile fever or severe West Nile disease. The incubation period is usually 2 to 6 days but ranges from 2 to 14 days and can be as long as 21 days in immunocompromised people. Most symptomatic people experience an acute systemic febrile illness that often includes headache, tiredness, arthralgia, myalgia, gastrointestinal tract symptoms such as vomiting or diarrhea, occasionally with a skin rash on the trunk of the body and swollen lymph nodes. Less than 1% of infected people develop neuroinvasive disease, which typically manifest as meningitis, encephalitis, or acute flaccid paralysis. Among patients with neuroinvasive disease, the overall case-fatality rate is approximately 10%.

2. What is the treatment for West Nile fever?

Treatment is supportive for patients, often involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections. No vaccine is available for humans.

3. How to reduce the risk of acquiring West Nile fever?

The only way to reduce infection in people is by raising awareness of the risk factors and educating people about the measures to reduce the risk. To prevent getting West Nile fever through mosquito bites, use insect repellents, wear long-sleeved shirts and long pants, and avoid outdoor activity at peak biting times.

More Information

1. WHO|West Nile Virus
2. USA CDC|West Nile Virus
3. ECDC|West Nile Virus