West Nile Fever

Unique identifier / Notation

BI0240

Synonyms

West Nile Disease WNV

West Nile virus disease is a potentially fatal neurological disease caused by a virus transmitted through the bites of infected mosquitoes. The virus is a member of the flavivirus genus and belongs to the Japanese encephalitis antigenic complex of the family Flaviviridae (WHO, 2017).

Primary reference(s)

WHO, 2017. West Nile virus. World Health Organization (WHO). Accessed 30 May 2025.

Annotations

Additional scientific description

Human

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 the 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, the 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).

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, the 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).

Animal

WNV has a broad host range. It replicates in birds, reptiles, amphibians, mammals, mosquitoes and ticks (WOAH, no date). Birds are the natural hosts of WNV and can be infected through various routes. The virus has been detected in more than 250 bird species. Mortality of birds due to WNV in the Americas is high, especially in crows, while rare in Europe, Africa, the Middle East and Asia (WHO, 2017). Mammals e.g. horses, can become infected from the bite of an infected mosquito but are considered dead-end hosts (ECDC, no date). In horses, clinical signs of the neurologic disease caused by WNV may include loss of appetite, depression, stumbling, muscle twitching, partial paralysis, impaired vision, head pressing, teeth grinding, aimless wandering, convulsions, circling, and an inability to swallow. Weakness, usually in the hind limbs, is sometimes followed by paralysis. Coma and death may occur. Fever has been seen in some but not all cases. Vaccines for horses are available. In areas where the disease is common, vaccination of horses is considered to be an effective control measure (WOAH, no date).

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).

Key relevant UN convention / multilateral treaty

International Health Regulations (2005), 3rd ed. (WHO, 2016).

Drivers

Human

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).

People and animals can also become infected by coming into contact with other infected animals, their blood, or other tissues (WHO, 2024).

WNV can also be transmitted through organ transplants, blood transfusions, during pregnancy delivery, and through breast milk, though this is rare. To date, no human-to-human transmission of WNV through everyday contact has been documented (WHO, 2024).

A very small proportion of human infections have occurred through organ transplants, 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).

Animal

Infection in mosquito.

Impacts

Human

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 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 the 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).

Animal

WN viral encephalitis occurs in only a small percentage of infected horses; the majority of infected horses do not display clinical signs. The mortality rate is approximately one in three clinically affected unvaccinated horses (WOAH, 2024a).

Multi-hazard context

Multiple factors can contribute to alterations in vector-host contact ratios, including mosquito feeding preferences and changes in habitat structures. For example, urbanisation, which is projected to increase the urban population to approximately two-thirds by 2050, has been shown to cause notable changes in the distribution and abundance of mosquito species at the local level, with implications for disease transmission dynamics of vector-borne diseases including WNV.

Risk Management

Human

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).

Animal

Vector control, vaccination (horses).

Monitoring

The table below offers an overview of monitoring for West Nile fever. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.

Which institution(s) produce(s) Disaster Risk Data/Information?

Ministry of Agriculture, Ministry of Livestock, Ministry of Health, FAO Reference Centres, WOAH Reference Centres, WHO

How is the Hazard Observed/Monitored/Forecast?

Through its global early warning system, FAO has been supporting Members with risk monitoring, assessment and forecasting for animal health threats to enhance preparedness and response to animal health threats:

The Global Animal Disease Information System, EMPRES-i+, plays a crucial role in supporting countries with early warning and early detection of animal disease outbreaks. By providing a comprehensive platform for real-time disease tracking, EMPRES-i+, enables monitoring and analysis of animal health threats effectively, collaborating closely with countries and regions on disease information sharing. This supports the dissemination of risk assessments and alerts for evidence-based decision-making.
FAO develops and maintains decision support tools that provide risk maps of existing and predicted risks of animal disease emergence, as well as other threats to the agri-food chain.
Additionally, FAO collaborates closely with international organizations like WHO and WOAH to verify disease information through integrated mechanisms such as the Epidemic Intelligence from Open Sources (EIOS), the Global Early Warning System (GLEWS+), and the World Animal Health Information System (WAHIS) for WOAH-listed diseases. These collaborations allow for the dissemination of early warning alerts and risk assessments to Members, to enable timely responses to potential outbreaks.

FAO empres-i+ https://empres-i.apps.fao.org/diseases

WOAH WAHIS https://wahis.woah.org/#/event-management

WHO supports countries to conduct all-hazards strategic risk assessment in the contexts of health emergencies and disasters, which results in the development of a country risk profile. Empowered with the country's risk profile, inclusive of a seasonal risk calendar, countries can anticipate potential emergencies before they occur to trigger early alerts and inform early actions (WHO, 2021).

WHO's Early Warning, Alert and Response System (EWARS) has been designed to improve disease outbreak detection in emergency settings, such as in countries in conflict or following a natural disaster. It is a simple and cost-effective way to rapidly set up a disease surveillance system. EWARS is deployed during an emergency as an adjunct to the national disease surveillance system. WHO works with Ministries of Health and health sector partners to train local health workers to use the system. After the emergency, EWARS should re-integrate back into the national system (WHO, 2023).