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West Nile Virus Encephalitis

Last updated Jan. 9, 2019

The West Nile Virus Encephalitis is a viral disease, which can cause an inflammation or swelling of the brain (termed encephalitis).


What are the other Names for this Condition? (Also known as/Synonyms)

  • Mosquito-Borne Encephalitis (West Nile Virus Encephalitis)
  • West Nile Encephalitis (WNE)
  • WNE (West Nile Encephalitis)

What is West Nile Virus Encephalitis? (Definition/Background Information)

  • The West Nile Virus Encephalitis is a viral disease, which can cause an inflammation or swelling of the brain (termed encephalitis). The West Nile virus (WNV) is a mosquito-transmitted virus
  • WNV is commonly found in Africa, West Asia, the Middle East, and Europe; it has also been reported from North America
  • Most people get infected with the West Nile virus through the bite of an infected mosquito, during the feeding process. Mosquitoes become infected when they feed on infected birds and can then go on to spread the virus to humans and other animals
  • Once an individual is infected with the virus, it is not transmissible from that individual to other human beings
  • Symptoms of West Nile Encephalitis (WNE) usually appear 5 to 15 days after the bite from an infected mosquito. The WNV crosses the coverings surrounding the brain (the blood-brain barrier) and infects the brain matter, causing damage and inflammation, resulting in encephalitis
  • Management of the condition is mostly by providing medical support, symptomatic treatment, and hospitalization, if necessary. The prognosis of West Nile Virus Encephalitis is generally excellent in most individuals

Who gets West Nile Virus Encephalitis? (Age and Sex Distribution)

  • West Nile Virus Encephalitis usually occurs during the summers, when mosquitoes, wild migratory birds, and humans are in close whereabouts or locations (outdoors). Thus, any individual living in an area where WNV is present, may be infected through mosquito bites
  • Worldwide, most cases tend to occur in young children and young adults; however, elderly adults are affected more severely
  • The West Nile virus is commonly found in world regions such as Africa, West Asia, the Middle East Asia, and Europe. It has also been reported from North America; the virus is prevalent across United States

What are the Risk Factors for West Nile Virus Encephalitis? (Predisposing Factors)

The risk factors for acquiring West Nile Virus Encephalitis include:

  • Living in an area where infected birds and mosquitoes reside
  • Most cases of WNE are reported during the summer months
  • Mosquito bites are particularly likely during feeding times (dawn and dusk) in the summer months
  • Time spent outdoors: A prolonged contact with mosquitoes or multiple mosquito bites enhance the risk of acquiring WNE

Even if one does get infected with the West Nile virus, the risk of developing a serious WNV-related illness in very small. Serious infections are more likely to develop in the following category of individuals:

  • Adults, older than 50 years of age
  • Individuals with weakened immune systems, such as after receiving an organ transplant, major surgeries, those suffering from cancer, diabetes, hypertension, and kidney disease

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases ones chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of West Nile Virus Encephalitis? (Etiology)

The West Nile Virus Encephalitis is transmitted to humans by the bite of mosquitoes that are infectious after feeding on a West Nile virus infected bird.

  • Birds tend to harbor the virus without actually being affected by them. The mosquitos are also responsible for transmitting the virus from one bird to another
  • Mosquitoes become infected by feeding on birds with the virus circulating in their blood. After an incubation period varying from 10-14 days, the infected mosquitoes have the ability to further transmit the virus to other animals and humans
  • Mosquito bites, which are particularly likely during peak feeding times, especially in the summer months, enable the transmission of WNV. A prolonged exposure to several mosquito bites can enhance the risk. The WNV may also be transmitted through organ transplants, in rare cases
  • When, WNV crosses the blood-brain barrier and infects the brain, it leads to brain inflammation or encephalitis

What are the Signs and Symptoms of West Nile Virus Encephalitis?

The incubation period of West Nile Virus Encephalitis (WNE) is 1-6 days and may be as long as 14 days in individuals with weakened immune systems.

  • Most individuals who become infected do not develop any symptoms; else, only have flu-like illnesses. A minority of people develop brain-related symptoms.
  • The signs and symptoms of WNE may include:
    • Fever, headache
    • Nausea and vomiting
    • Mental changes such as confusion, hallucination, or decreased consciousness
    • Sensitivity to light (photophobia)
    • Stiff neck (nuchal rigidity)
  • Lesser known symptoms also include:
    • Skin rash
    • Diarrhea
    • Sore throat
    • Backache, muscular and joint pain
  • The symptoms are most noticeable in very young and very old individuals

How is West Nile Virus Encephalitis Diagnosed?

In order to diagnose a West Nile Virus Encephalitis, the physician will initially perform a physical exam with evaluation of medical history (and recent travel history). During physical examination, the physician will look for:

  • Fever
  • Neck stiffness

Specific diagnostic tests for WNE include:

  • A lumbar puncture (or spinal tap) is the most important test for diagnosis of WNE. This test is performed by removing a sample of the cerebrospinal fluid (CSF); fluid that surrounds the brain and spinal cord
  • Other tests that may be performed include:
    • Blood culture
    • Serological tests on blood to detect antibodies that the immune system produces against the West Nile virus (WNV)
    • CSF examination for cell count, glucose, and protein
    • Gram stain, other special stains, and culture of CSF
    • CT scan or MRI scan of the head

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of West Nile Virus Encephalitis?

Complications from a mild West Nile Virus Encephalitis are very rare.

  • Most individuals with WNE recover completely, though fatigue and weakness can last for weeks or months
  • The possible complications from a severe episode of WNE may include:
    • Brain damage
    • Cranial nerves palsies
    • Convulsions (seizures)
    • Permanent muscle weakness and movement disorders

How is West Nile Virus Encephalitis Treated?

Currently, there is no specific medication to prevent a West Nile Virus Encephalitis.

  • Pain-relievers can be used to reduce fever and for some other symptoms
  • Antibiotics do not help cure viral infections, so they are not useful in the treatment of WNE
  • In more severe cases, the treatment of West Nile Virus encephalitis may include intensive supportive therapy, such as:
    • Hospitalization
    • Intravenous (IV) fluids
    • Airway management
    • Respiratory support (ventilator)

How can West Nile Virus Encephalitis be Prevented?

A few measures to prevent West Nile Virus Encephalitis include:

  • Avoiding exposure to mosquitoes, particularly from dawn to dusk, in areas where mosquitoes and West Nile Encephalitis (WNE) are known to be present
  • Limit outdoor activities around dawn and dusk and also in the early evenings, when possible
  • Wear long-sleeved shirts and long pants whenever you are outdoors
  • Make sure screen doors and windows are intact and remain closed during the evenings and early morning periods
  • Mosquito netting and mosquito-repellents may also be used
  • Avoid wearing bright colors and highly aromatic perfumes, deodorants, and hair products that would attract mosquitoes
  • Mosquitoes breed in standing water, hence, eliminate stagnant water around the house in discarded tires, blocked gutters, dirty birdbaths, poorly-maintained swimming pools, and any type of receptacle with decaying organic matter
  • When outdoors, apply insect repellant to exposed skin areas
  • Currently, there is no vaccine available to protect against the West Nile virus, although several are being tested.

What is the Prognosis of West Nile Virus Encephalitis? (Outcomes/Resolutions)

  • The prognosis for West Nile Virus Encephalitis is excellent in all individuals, except those who are elderly or weakened
  • Severe cases may infrequently result in death or neurologic sequelae; however, recovery is usually complete in most individuals

Additional and Relevant Useful Information for West Nile Virus Encephalitis:

  • West Nile Virus Encephalitis was first seen in the north eastern United States, along the eastern seaboard. It now prevails nationwide
  • A vaccine called chimerivax-VNW is being actively researched and has undergone clinical trials in 2011

What are some Useful Resources for Additional Information?


References and Information Sources used for the Article:


Helpful Peer-Reviewed Medical Articles:


Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: May 18, 2015
Last updated: Jan. 9, 2019