Open Outbreak

West Nile virus

Also known as: WNV, West Nile fever

West Nile virus is a mosquito-borne virus. It mainly cycles between birds and mosquitoes, especially Culex mosquitoes. People and horses can get infected, but they are usually considered dead-end hosts, meaning they do not usually pass the virus on to new mosquitoes. Most human infections cause no symptoms. WHO says about 80% are asymptomatic, about 20% cause a fever illness, and about 1 in 150 infections lead to severe disease affecting the brain, spinal cord, or the tissues around them. Severe illness can be fatal, especially in older adults and some people with weakened immune systems. The incubation period is usually 3 to 14 days. West Nile virus also can spread through blood transfusion, organ transplant, and other human-origin health products, though mosquito bites are the main route. There is no licensed human vaccine and no specific antiviral treatment in the sources reviewed here. Care is supportive, and prevention relies mainly on avoiding mosquito bites and reducing mosquito exposure.

Key facts

TransmissionMainly through bites from infected mosquitoes; WHO also notes transmission through blood transfusion, organ transplant, and other human-origin health products. Birds are the main amplifying hosts; humans and horses usually do not infect new mosquitoes.
IncubationUsually 3 to 14 days.
Case fatalityUnknown overall. Severe neuroinvasive disease can be fatal, but no single overall human case-fatality figure was stated in the sources reviewed here.
Reproduction numberunknown
VaccineNo licensed vaccine for humans in the WHO sources reviewed here. WHO notes vaccines are available for horses.
TreatmentNo specific antiviral treatment stated in the reviewed sources; treatment is supportive.
ReservoirBirds, especially migratory birds, are the main natural hosts.
Endemic regionsCommonly found in Africa, Europe, the Middle East, North America, and West Asia, according to WHO.

Transmission

West Nile virus is spread mainly by mosquito bites. WHO says the virus is generally carried by migratory birds and transmitted locally by mosquitoes to humans, horses, and other mammals. This bird-mosquito-bird cycle is the main way the virus persists in nature. Humans and horses are usually considered dead-end hosts, which means they can become ill but do not usually pass enough virus back to mosquitoes to keep transmission going. WHO also notes less common routes through human-origin health products such as blood transfusion and organ transplant. For most people, mosquito exposure is the key risk. The sources reviewed here do not provide a settled human reproduction number, and transmission is shaped by local mosquito species, bird movement, weather, and season. In plain terms: if infected mosquitoes are present, human risk rises, especially in warm months and places where birds and mosquitoes mix closely.

Symptoms and severity

Many infections cause no symptoms at all. WHO states that about 80% of infected people are asymptomatic. Of those who do become ill, about 20% develop West Nile fever, usually with fever, headache, tiredness, body aches, nausea, and vomiting. Rash and swollen lymph glands can occur. The incubation period is usually 3 to 14 days. A much smaller share become severely ill. WHO estimates about 1 in 150 infected people develop neuroinvasive disease, which can include encephalitis, meningitis, or a polio-like paralysis. Warning signs include severe headache, high fever, neck stiffness, confusion, tremors, convulsions, muscle weakness, or paralysis. Serious illness can happen at any age, but WHO says the risk is highest in people over 50 and in some immunocompromised people, including transplant patients. The sources reviewed here say severe disease can be fatal, but they do not give one clear overall case-fatality figure.

Treatment and vaccines

The sources reviewed here do not report a specific antiviral treatment that is established for routine human use. In practice, care is supportive. That means treating complications, helping with breathing if needed, managing fluids, and supporting recovery from neurological injury. Because severe disease can affect the brain, spinal cord, or nerves, some patients need hospital care and rehabilitation. Prevention matters because treatment options are limited. WHO states that no vaccine currently exists for humans. WHO also says vaccines are available for horses, which helps protect animal health but does not remove the need for mosquito control and personal protection. For the public, the most practical steps are to avoid mosquito bites, reduce standing water where mosquitoes breed, and follow blood-safety rules used by health systems in affected areas. In short, West Nile virus is one of those infections where prevention and supportive care are more developed than curative treatment.

Where it occurs

WHO’s 2015 notice says West Nile virus is commonly found in Africa, Europe, the Middle East, North America, and West Asia. The virus has also appeared in places where it had not previously been documented. WHO reported the first detected human case in Brazil in 2014 and the first detected human case in Barbados in 2024. In the Barbados notice, WHO says the virus has spread throughout the Caribbean, likely via infected migratory birds, and may circulate in birds and horses without being noticed at first. That matters because silent circulation can precede diagnosed human disease. On this site, the only listed record is monitoring in Occitanie, France. The user-provided archive note says there are 0 archived WHO notices since 1996 on this site for this pathogen, even though WHO’s own outbreak archive includes multiple notices in different countries. So the geographic picture is broad globally, but local detection depends heavily on surveillance of mosquitoes, birds, horses, blood supplies, and human neurological illness.

Recent history

Recent WHO outbreak reporting shows West Nile virus still appears sporadically in new or unexpected settings. WHO published a Disease Outbreak News item for Barbados on 2024-10-03 after the country reported its first confirmed human infection. WHO assessed the public-health risk there as low at that stage because only one human case had been recorded and response measures were in place. Earlier WHO notices included Portugal on 2015-09-17 and Brazil on 2014-12-15, and older archive entries from Canada, the United States, and Europe in 2002 and 2011. That pattern suggests repeated reappearance rather than one single continuous global emergency notice stream. For Open Outbreak specifically, the archive note provided with this task says there are 0 archived WHO notices on this site since 1996 for West Nile virus, and 1 site record: monitoring in Occitanie, France. So the site’s own archive is sparse, but WHO’s archive shows a longer international history of intermittent alerts and first detections.

What to watch

Several signals would make West Nile virus more concerning. One is spread into new regions or islands, especially places with suitable mosquitoes and migratory bird routes; WHO’s Barbados report shows that first detections can happen in places without earlier documented bird or horse infections. Another is any rise in severe neuroinvasive disease, especially among older adults or immunocompromised people. A third is evidence of transmission through blood transfusion, organ transplant, or other human-origin health products, because that would call for stronger screening and donor deferrals. A fourth is strain on mosquito-control systems during hot seasons, when vector populations can grow. Finally, vaccine supply matters for horses and any future human vaccine development, because WHO says there is no human vaccine at present. The reviewed sources do not show easy person-to-person spread, so that is not the main concern. The bigger watchpoints are geographic expansion, seasonal intensity, hidden circulation in birds and horses, and pressure on surveillance and blood safety.

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