West Nile Virus

How West Nile virus spreads

West Nile virus normally circulates between birds and mosquitoes. Culex mosquitoes become infected when they feed on infected birds, then pass the virus to people and horses when they bite. People and horses are "dead-end" hosts: they do not develop enough virus in the blood to infect mosquitoes.

The virus was first found in Uganda's West Nile district in 1937 and first appeared in the US in New York in 1999. It has since spread to every state in the continental US. Cases are reported each year, often in the hundreds to a few thousand, with larger outbreaks in some years.

Rarely, it spreads through:

• blood transfusions and organ transplants (donated blood in the US is screened)
• from a pregnant person to the baby, or through breast milk
• lab exposures

It does not spread through casual contact, touching, kissing or caring for an infected person, or from handling live animals.

West Nile virus symptoms

No symptoms (most people)

About 8 in 10 infected people never feel ill. They develop immunity and usually never know they were infected.

West Nile fever (about 1 in 5)

Symptoms typically start 2 to 14 days after the bite:

• fever
• headache
• body aches and joint pains
• tiredness and weakness
• vomiting or diarrhea
• a rash on the chest, back or arms, more common in younger people
• swollen lymph glands

Most people recover within a week or so, but tiredness and weakness can last weeks or months.

Neuroinvasive disease (about 1 in 150)

The virus enters the nervous system and causes:

• Meningitis: inflammation of the lining of the brain and spinal cord, with fever, headache, stiff neck and sensitivity to light.
• Encephalitis: inflammation of the brain, with confusion, drowsiness, tremor, muscle jerks, Parkinson-like stiffness, seizures or coma.
• Acute flaccid paralysis: sudden weakness of one or more limbs, often without numbness, similar to polio. It can affect breathing muscles.

About 1 in 10 people with neuroinvasive disease die. Many survivors have lasting weakness, fatigue, memory problems or depression.

Who is at highest risk

Anyone can be infected, but serious illness is much more likely in:

• people over 60, with risk rising with age
• organ transplant recipients
• people with cancer, diabetes, high blood pressure, kidney disease or alcohol use disorder
• anyone with a weakened immune system

Time spent outdoors at dusk and dawn during summer and early fall increases exposure.

How West Nile virus is diagnosed

• Blood test for IgM antibodies: the main test. IgM usually appears within 3 to 8 days of symptoms.
• Spinal fluid (lumbar puncture): in people with signs of meningitis or encephalitis, the fluid shows inflammation and can be tested for West Nile IgM.
• Confirmatory testing: state public health labs or the CDC may confirm results, because antibodies to related viruses, such as dengue or Zika, or recent yellow fever or Japanese encephalitis vaccination, can cross-react.
• PCR: sometimes used early in illness or in people with weak immune systems who may not make antibodies.
• MRI and EEG: help assess brain involvement.

Mild illness is often diagnosed clinically without testing. Cases are reported to public health authorities, which track outbreaks and target mosquito control.

Treatment

There is no antiviral medicine for West Nile virus. Treatment supports the body while the immune system clears the infection:

• Mild illness: rest, fluids, and over-the-counter pain and fever relief such as acetaminophen (paracetamol) or ibuprofen. Follow the label dose, and do not give aspirin to children or teenagers.
• Severe illness: hospital care with IV fluids, pain control, seizure treatment, nursing care to prevent complications, and sometimes a ventilator for breathing failure.
• Rehabilitation: physical, occupational and speech therapy help recovery after paralysis or encephalitis.

Treatments such as immune globulin and interferon have been tried, but none has proven benefit.

Prevention

There is no human vaccine (a vaccine exists for horses). Prevention relies on avoiding mosquito bites:

• Use EPA-registered repellent containing DEET, picaridin, IR3535, oil of lemon eucalyptus (OLE) or PMD, or 2-undecanone. Apply sunscreen first, then repellent. Do not use OLE or PMD on children under 3.
• Cover up with long sleeves and pants, especially at dusk and dawn, when Culex mosquitoes bite most. Permethrin-treated clothing adds protection.
• Mosquito-proof your home: use screens on windows and doors and air conditioning when possible.
• Remove standing water weekly from buckets, gutters, flowerpot saucers, birdbaths, tires and toys, where mosquitoes breed.
• Report dead birds if your local health department requests it, since they can signal virus activity.
• Protect babies with mosquito netting over strollers and carriers.

Frequently asked questions

What are the first symptoms of West Nile virus?

Most people have no symptoms. When symptoms occur, they usually start 2 to 14 days after a mosquito bite with fever, headache, body aches, tiredness, sometimes vomiting, diarrhea or a rash. Severe headache, stiff neck, confusion or weakness need emergency care.

How do you get West Nile virus?

Almost always from the bite of an infected Culex mosquito, which picks up the virus from birds. Rarely, it spreads through blood transfusions, organ transplants, or from mother to baby. It does not spread from person to person through casual contact.

Is West Nile virus curable?

There is no specific antiviral cure, but most people recover on their own. Mild cases need rest, fluids and pain relief. Severe cases need hospital supportive care. Recovery from brain or nerve involvement can take months, and some effects may be permanent.

How long does West Nile virus last?

West Nile fever usually lasts a few days to a week, though fatigue and weakness may persist for weeks or months. Neuroinvasive disease requires longer recovery, and some people have lasting weakness, memory or mood problems.

Can you get West Nile virus twice?

It is thought that infection provides lifelong immunity to West Nile virus, so getting it a second time is considered very unlikely. However, immunity does not protect against other mosquito-borne viruses.