For health professionals: West Nile virus

Get detailed information on West Nile virus, its diagnosis, clinical assessment and prognosis.

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What health professionals need to know about West Nile virus

West Nile virus belongs to a family of viruses known as Flaviviridae. It was first identified in Africa in 1937. In Canada, West Nile virus disease is nationally notifiable and provincially and territorially reportable.

Health care providers are encouraged to monitor their patients for symptoms of the virus and request laboratory tests. All probable and confirmed cases of West Nile virus must be reported to local and provincial or territorial health authorities.

Diagnosis considerations

West Nile virus disease should be considered in any patient with:

  • febrile or acute neurological illness
  • recent exposure to:
    • mosquitoes
    • blood transfusion
    • organ transplantation

This is especially true during the summer months.

The diagnosis should also be considered in any infant born to a mother infected with the virus during pregnancy or while breastfeeding.

Differential diagnosis

West Nile virus should be considered in the differential diagnosis when the following diseases are suspected:

  • encephalitis and aseptic meningitis, such as:
    • herpes simplex virus
    • enteroviruses
  • other arboviruses, such as:
    • La Crosse
    • St. Louis encephalitis
    • Eastern equine encephalitis
    • Powassan virus

West Nile virus neurological syndrome

West Nile virus meningitis is clinically indistinguishable from viral meningitis.

West Nile virus encephalitis is a more severe clinical syndrome that usually manifests with:

  • fever and altered mental status
  • seizures
  • focal neurological deficits
  • movement disorders, such as tremor or parkinsonism

This manifestation is more commonly seen in:

  • older individuals (particularly those over the age of 50)
  • immunocompromised persons

West Nile virus acute flaccid paralysis is usually clinically and pathologically identical to poliovirus-associated poliomyelitis, with:

  • damage of anterior horn cells
  • possible progression to respiratory paralysis requiring mechanical ventilation

West Nile virus poliomyelitis often presents as isolated limb paresis or paralysis. It can occur without fever or apparent viral prodrome.

Rarely, the following symptoms have been described in patients with West Nile virus disease:

  • cardiac dysrhythmias
  • myocarditis
  • rhabdomyolysis
  • optic neuritis
  • uveitis
  • chorioretinitis
  • vitritis

Patients have also been described as having:

  • orchitis
  • pancreatitis
  • hypertension
  • kidney disease
  • hepatitis

West Nile virus disease during pregnancy and in children

Most women known to have been infected with West Nile virus during pregnancy have delivered infants without evidence of infection or clinical abnormalities.

Most children with symptomatic West Nile virus infection present with fever.
Of those who develop West Nile virus Neurological Syndrome (or neuroinvasive disease), it most frequently manifests as meningitis. However, children with West Nile virus have been described as having:

  • severe and fatal encephalitis
  • poliomyelitis
  • rhombencephalitis
  • hepatitis

Similar to adults, immunocompromised children may be more susceptible to more severe illness.

Clinical assessment

Routine clinical laboratory studies are generally nonspecific. In patients with West Nile virus Neurological Syndrome examination of cerebrospinal fluid generally shows lymphocytic pleocytosis. However, neutrophils may predominate early in the course of illness.

Brain magnetic resonance imaging is frequently normal, but signal abnormalities may be seen in patients with encephalitis. These abnormalities are found in the:

  • basal ganglia
  • thalamus
  • brainstem

In patients with poliomyelitis, the abnormalities are found in the anterior horn cells of the spinal cord.

Laboratory diagnosis is generally accomplished by testing of serum or cerebrospinal fluid to detect West Nile virus-specific IgM antibodies.

West Nile virus-specific IgM antibodies are usually detectable 3 to 8 days after onset of illness. They usually persist for 30 to 90 days, but longer persistence has been documented. Therefore, positive IgM antibodies occasionally may reflect a past infection.

If serum is collected within 8 days of illness onset, the absence of detectable virus-specific IgM does not rule out West Nile virus infection. The test may need to be repeated on a later sample.


Most patients with mild symptoms of West Nile virus Non-Neurological Syndrome (or non-neuroinvasive West Nile virus disease) recover completely.

Some symptoms, however, can linger for weeks or months, like:

  • fatigue
  • malaise
  • weakness

Patients who recover from West Nile viral encephalitis or poliomyelitis often have residual neurologic deficits.

Among patients with West Nile virus Neurological syndrome the overall case-fatality ratio is approximately 10%. It is significantly higher for patients with West Nile viral encephalitis and poliomyelitis than for West Nile viral meningitis.

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