West Nile Virus: Symptoms, prevention, and treatment

West Nile Virus (WNV) is a potentially life-threatening viral infection.

It can pass to animals and humans if they are bitten by an infected mosquito

WNV is a virus of the Flaviviridae family, which includes the viruses responsible for Japanese encephalitis and dengue fever.

It mainly affects birds, but it can also be experienced by mammals and reptiles.

Between 70 and 80 percent of people have no symptoms. Up to one percent of those who become ill have serious and potentially fatal complications.

West Nile Virus (WNV) used to exist only in temperate and tropical areas, but in 1999, infections appeared in New York. It has since spread across most of the United States (U.S.), and is a notifiable disease.

Fast facts on West Nile Virus

Here are some key points about West Nile Virus (WNV). More detail is in the main article.

Mosquitoes transmit WNV from birds to humans.

WNV previously did not exist in the U.S., but in 1999, some imported cases triggered an outbreak.

Around 80 percent of people have no symptoms, but in up to 1 percent of cases, the virus can lead to life-threatening neurological complications.

The best way to prevent WNV is to avoid mosquito bites.

Signs and symptoms

West Nile Virus
West Nile Virus is transferred to humans by mosquitos. They pick up the virus from deceased birds.

WNV can affect humans in three different ways:

Asymptomatic infection: In about 80 percent of cases, there are no signs or symptoms.

West Nile Fever: Around 20 percent of people experience a mild febrile syndrome.

Neuroinvasive disease: About 1 percent of patients develop complications in the central nervous system (CNS) that affect the brain and spine.

West Nile fever

Symptoms appear 2 to 8 days after infection. This is known as the incubation period.

They may include:

backache and muscle aches

fever and excessive sweating

diarrhea, nausea, vomiting, and loss of appetite



skin rash

swollen lymph nodes, or glands

These symptoms resolve within 7 to 10 days. Fatigue may linger for several weeks, while glands may be swollen for up to 2 months.

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Neuroinvasive disease

Around 1 percent of infected individuals develop more serious neurological infections, and around 10 percent of these cases are fatal.

Possible complications are:

Encephalitis: Inflammation of the brain

Meningitis: Inflammation of tissues surrounding the brain and spinal cord

Myelitis, or West Nile poliomyelitis: Inflammation of the spinal cord

Acute flaccid paralysis: Sudden weakness in the arms, legs and breathing muscles.

Signs and symptoms may include:

confusion and disorientation


high fever

muscle jerking


Parkinson’s disease-like symptoms, including tremors

sudden weakness, poor coordination, and partial paralysis

severe headache

stiff neck



Most at risk are people aged over 60 years and those with existing conditions, such as kidney disease, diabetes, cancer, and conditions that weaken the immune system.

Some neurological effects can be permanent.

The American robin is one of the key culprits for the presence of WNV in the U.S.
The American robin is one of the key culprits for the presence of WNV in the United States.

Infected birds have high levels of the virus. In the U.S., the American robin and the American crow are common carriers.

If a mosquito bites an infected bird, and it then bites a person, the virus will enter the bloodstream of that individual.

The Culex Pipiens mosquito is known to pass on WNV in the U.S.

It remains unknown exactly how the virus works. WNV enters the bloodstream and reproduces, and sometimes it can cross the blood-brain barrier to cause inflammation in the brain.

Transmission is also possible through:

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