West Nile Fever (Human)

Primary reference(s)

WHO, 2017. West Nile virus. World Health Organization (WHO). Accessed 26 October 2020.

Additional scientific description

West Nile virus disease can cause neurological disease and death in people. West Nile virus (WNV) is commonly found in Africa, Europe, the Middle East, North America and West Asia. WNV is maintained in nature in a cycle involving transmission between birds and mosquitoes. Humans, horses and other mammals can be infected (WHO, 2017).

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. Before 1997, WNV was not considered pathogenic for birds, but at that time in Israel a more virulent strain caused the death of different bird species presenting signs of encephalitis and paralysis. Human infections attributable to WNV have been reported in many countries for over 50 years. In 1999, a WNV circulating in Israel and Tunisia was imported into New York producing a large and dramatic outbreak that spread throughout the continental USA in the following years. The WNV outbreak in the USA (1999–2010) highlighted that importation and establishment of vector-borne pathogens outside their current habitat represent a serious danger to the world. The largest outbreaks occurred in Greece, Israel, Romania, Russia and the USA. Outbreak sites are on major bird migratory routes. In its original range, WNV was prevalent throughout Africa, parts of Europe, Middle East, West Asia, and Australia. Since its introduction in 1999 into the USA, the virus has spread and is now widely established from Canada to Venezuela (WHO, 2017).

Human infection is most often the result of bites from infected mosquitoes. Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood for a few days. The virus eventually gets into the mosquito’s salivary glands. During later blood meals (when mosquitoes bite), the virus may be injected into humans and animals, where it can multiply and possibly cause illness (WHO, 2017).

The virus may also be transmitted through contact with other infected animals, their blood, or other 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 (mother-to-child) WNV 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. However, transmission of WNV to laboratory workers has been reported (WHO, 2017).

Infection with WNV is either asymptomatic (no symptoms) in around 80% of infected people or can lead to West Nile fever or severe West Nile disease. About 20% of people who become infected with the WNV will develop West Nile fever. The symptoms of severe disease (also called neuroinvasive disease, such as West Nile encephalitis or meningitis or West Nile poliomyelitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that approximately 1 in 150 persons infected with the WNV will develop a more severe form of disease. Serious illness can occur in people of any age, however people over the age of 50 years and some immunocompromised persons (for example, transplant patients) are at highest risk for getting severely ill when infected with WNV. The incubation period is usually 3 to 14 days (WHO, 2017).

Diagnosis is by laboratory tests, including molecular tests (PCR) which can detect the virus genome during the acute phase of infection, and by serological tests to detect the individual’s immune response to recent or past infection (WHO, 2017).

West Nile Virus is maintained in nature in a mosquito-bird-mosquito transmission cycle. Mosquitoes of the genus Culex are generally considered the principal vectors of WNV, in particular C. pipiens. WNV is maintained in mosquito populations through vertical transmission (adults to eggs). Birds are the reservoir hosts of WNV. In Europe, Africa, Middle East and Asia, mortality in birds associated with WNV infection is rare. In striking contrast, the virus is highly pathogenic for birds in the Americas. Members of the crow family (Corvidae) are particularly susceptible, but the virus has also been detected in dead and dying birds of more than 250 species. Birds can be infected through a variety of routes other than mosquito bites, and different species may have different potential for maintaining the transmission cycle. Horses, just like humans, are ‘dead-end’ hosts, meaning that while they become infected, they do not spread the infection. Symptomatic infections in horses are also rare and generally mild, but can cause neurological disease, including fatal encephalomyelitis (WHO, 2017).

Metrics and numeric limits

The Centers for Disease Control and Prevention and the European Commission have published guidance on case classification and surveillance standards (CDC, 2015; European Union, 2018).

Examples of drivers, outcomes and risk management

Preventing transmission in horses is important. Since WNV outbreaks in animals precede human cases, the establishment of an active animal health surveillance system to detect new cases in birds and horses is essential to provide early warning for veterinary and human public health authorities. In the Americas, it is important to help the community by reporting dead birds to local authorities (WHO, 2017).

Vaccines have been developed for horses. Treatment is supportive and consistent with standard veterinary practices for animals infected with a viral agent (WHO, 2017).

In the absence of a vaccine (although there are vaccine candidates for a human vaccine under development), the only way to reduce infection in people is by raising awareness of the risk factors and educating people about the measures they can take to reduce exposure to the virus. Public health educational messages should focus on the following (WHO, 2017):

• Reducing the risk of mosquito transmission. Efforts to prevent transmission should first focus on personal and community protection against mosquito bites through the use of mosquito nets, personal insect repellent, by wearing light coloured clothing (long-sleeved shirts and trousers) and by avoiding outdoor activity at peak biting times.
• Reducing the risk of animal-to-human transmission. Gloves and other protective clothing should be worn while handling sick animals or their tissues, and during slaughtering and culling procedures.
• Reducing the risk of transmission through blood transfusion and organ transplant.

Effective prevention of human WNV infections depends on the development of comprehensive, integrated mosquito surveillance and control programmes in areas where the virus occurs. Studies should identify local mosquito species that play a role in WNV transmission, including those that might serve as a ‘bridge’ from birds to human beings. Emphasis should be on integrated control measures including source reduction (with community participation), water management, chemicals, and biological control methods (WHO, 2017).

Health-care workers caring for patients with suspected or confirmed WNV infection, or handling specimens from them, should implement standard infection control precautions (WHO, 2017).

The World Health Organization (WHO) Regional Office for Europe and the WHO Regional Office for the Americas/Pan American Health Organization are intensively supporting WNV surveillance and outbreak response activities, respectively, in Europe and in North America, Latin America and the Caribbean, together with country offices and international partners (WHO, 2017).