West Nile virus

For Health Professionals

West Nile virus (WNV) 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 West Nile 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

WNV disease should be considered in any patient with:

• febrile or acute neurological illness
• recent exposure to mosquitoes, blood transfusion, or 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 WNV during pregnancy or while breastfeeding.

Differential Diagnosis

WNV should be considered in the differential diagnosis when the following diseases are suspected:

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

Signs and symptoms

Most human WNV infections are subclinical or asymptomatic.

Most symptomatic persons experience an acute systemic febrile illness which may include:

• headache
• weakness
• myalgia
• arthralgia
• gastrointestinal symptoms (nausea and vomiting) which have been frequently described and may lead to dehydration

Transient maculopapular rash may also be present and tends to be morbilliform and non-pruritic. It typically predominates over the torso and extremities, sparing the palms and soles. The rash may be transient, lasting less than 24 hours in some persons. Rash is more frequently observed among younger persons than among older persons.

Less than 1% of infected people develop neuroinvasive disease which typically manifests as aseptic meningitis, encephalitis, or acute poliomyelitis-like syndrome.

West Nile Neuroinvasive Disease

WNV meningitis is clinically indistinguishable from viral meningitis.

WNV encephalitis is a more severe clinical syndrome that usually manifests with fever and altered mental status, seizures, focal neurological deficits, or movement disorders such as tremor or parkinsonism. This manifestation is more commonly seen in older individuals, particularly those over the age of 50, as well as immunocompromised persons.

WNV acute flaccid paralysis is usually clinically and pathologically identical to poliovirus-associated poliomyelitis, with damage of anterior horn cells, and may progress to respiratory paralysis requiring mechanical ventilation.

WNV poliomyelitis often presents as isolated limb paresis or paralysis and can occur without fever or apparent viral prodrome.

Rarely – cardiac dysrhythmias, myocarditis, rhabdomyolysis, optic neuritis, uveitis, chorioretinitis, vitritis, orchitis, pancreatitis, hypertension, kidney disease and hepatitis have been described in patients with WNV disease.

West Nile virus disease and pregnancy

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

West Nile virus disease and children

Most children with symptomatic WNV infection present with fever. Of those who develop neuroinvasive disease, it most frequently manifests as meningitis. However, severe and fatal encephalitis, poliomyelitis, rhombencephalitis and hepatitis have all been described in children with WNV infection. 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 neuroinvasive disease, examination of cerebrospinal fluid (CSF) generally shows lymphocytic pleocytosis, but neutrophils may predominate early in the course of illness. Brain magnetic resonance imaging is frequently normal, but signal abnormalities in the basal ganglia, thalamus, and brainstem may be seen in patients with encephalitis, and in the anterior horn cells of the spinal cord in patients with poliomyelitis.

Laboratory diagnosis is generally accomplished by testing of serum or cerebrospinal fluid to detect WNV-specific IgM antibodies. WNV-specific IgM antibodies are usually detectable 3 to 8 days after onset of illness and 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 the diagnosis of WNV infection. The test may need to be repeated on a later sample.

Treatment

There is currently no definitive treatment for WNV disease. The clinical management is supportive. Patients with severe meningeal symptoms often require pain control for headaches and antiemetic therapy and rehydration for associated nausea and vomiting. Patients with encephalitis require close monitoring for the development of elevated intracranial pressure and seizures. Patients with encephalitis or poliomyelitis should be monitored for inability to protect their airway. Acute neuromuscular respiratory failure may develop rapidly and prolonged ventilator support may be required.

Prognosis

Most patients with non-neuroinvasive WNV disease or WNV meningitis recover completely, but fatigue, malaise, and weakness can linger for weeks or months.

Patients who recover from WNV encephalitis or poliomyelitis often have residual neurologic deficits. Among patients with neuroinvasive disease, the overall case-fatality ratio is approximately 10%. It is significantly higher for patients with WNV encephalitis and poliomyelitis than WNV meningitis.

Prevention

There is no WNV vaccine for humans. Prevention of WNV disease depends on:

• community-level mosquito control programs to reduce vector densities
• personal protective measures to decrease exposure to infected mosquitoes
• screening of blood and organ donors

Personal protective measures include use of mosquito repellents, wearing long-sleeved shirts and long pants, and limiting outdoor exposure from dusk to dawn. Using air conditioning, installing window and door screens, and reducing mosquito breeding sites around the house can further decrease the risk for WNV exposure.