West Nile Neuroinvasive Disease

West Nile virus is a mosquito borne flavivirus transmitted by Culex species from infected birds. Most infections (about 80%) are asymptomatic, about 20% cause fever, and fewer than 1% (1 in 150) cause neuroinvasive disease (meningitis, encephalitis or acute flaccid paralysis).

Risk factors for neuroinvasive disease are age over 60 years, immunosuppression, diabetes, hypertension and kidney disease. Case fatality in neuroinvasive disease is about 10%.

Pathophysiology. The virus crosses the blood brain barrier and infects neurons of the deep grey matter, brainstem and anterior horn cells.

Fever, headache, myalgia and rash are followed by meningitis (neck stiffness), encephalitis (confusion, tremor, parkinsonism, seizures, coma) or poliomyelitis like asymmetric flaccid paralysis with preserved sensation, with or without respiratory failure.

Red flags.

• Acute asymmetric limb weakness with fever.
• Falling consciousness, bulbar or respiratory weakness.
• Immunosuppression or age over 60 years.

Serum and CSF West Nile virus IgM by antibody capture assay (IgM in CSF is diagnostic). PCR on CSF or serum is positive early and often negative in immunocompetent hosts. CSF shows lymphocytic pleocytosis (can be neutrophilic early) and raised protein. MRI shows T2 hyperintensity in the thalami, basal ganglia and brainstem, and anterior horn cells of the cord. Nerve conduction studies show motor axonal loss. Exclude herpes simplex virus encephalitis.

Differential diagnosis.

• Herpes simplex virus encephalitis, enterovirus, other arboviruses.
• Guillain-Barre syndrome, bacterial meningitis and autoimmune encephalitis.

No specific antiviral treatment is established.

Supportive care. Fluids and electrolytes, analgesia and antipyretics, seizure control, intracranial pressure management, and early intubation for respiratory or bulbar failure. Monitor forced vital capacity every 4 to 6 hours in flaccid paralysis.

Empirical treatment. Give aciclovir 10 mg/kg IV every 8 hours until herpes simplex virus is excluded, with adjustment to renal function. Add antibiotics if bacterial meningitis is possible.

Seizures. Levetiracetam 500 mg twice daily, increased to 1500 mg twice daily (adjust to eGFR).

Rehabilitation. Early physiotherapy, as motor recovery may continue for up to 1 to 2 years. Poliomyelitis like paralysis recovers poorly.

Prevention. Mosquito control, repellents, protective clothing and screening of blood and organ donations in affected areas.

Monitoring. Respiratory function, swallowing, neurological recovery and cognition.

Referral and escalation. Admit for monitoring, with intensive care for respiratory or consciousness compromise. Report notifiable cases to public health authorities.