West nile virus natural history

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Ammu Susheela, M.D. [2]

Overview

WNV is usually transmitted to humans by the culex mosquito after feeding on infected birds with high-level viremia. Following an incubation period of 2-14 day, untreated patients can remain asymptomatic or present with West Nile fever or with life-threatening neuroinvasive disease. Common complications of WNV infections include neurological impairment. The prognosis of mild disease is excellent; whereas West Nile meningitis and encephalitis may have residual neurologic deficits.

Natural History

WNV is a member of Japanese encephalitis antigenic complex of the family Flaviviridae. It is transmitted by a mosquito bite, most commonly Culex pipiens. Birds are the natural reservoir of the virus and the disease is generally transmitted to humans when a mosquito that previously fed on a bird with high-level viremia bites a human.

Incubation Period

The incubation period for WNV disease is typically 2 to 6 days. It ranges from 2 to 14 days and can be several weeks in immunocompromised patients. An estimated 70-80% of human WNV infections are subclinical or asymptomatic. Less than 1% of infected individuals develop neuroinvasive disease, which typically manifests as meningitis, encephalitis, or acute flaccid paralysis.

Asymptomatic West Nile Infection

When left untreated, approximately 80% of the people infected by virus remain asymptomatic.

West Nile Fever

Around 20% of the patients infected with WNV develop West Nile fever and usually present with fever and other constitutional symptoms. If left untreated, the infection generally self-resolves with no complications or sequelae.

Neuroinvasive Disease

Approximately 1 in 150 persons infected with WNV will develop a severe form of disease if left untreated. Serious illness can occur in patients of any age. However, advanced age, systemic diseases such as malignancy and cardiovascular disease, and immunosuppressed patients are considered high risk for developing neuroinvasive disease.

Possible complications

• Complications from mild West Nile virus infection are very rare.
• Complications from severe West Nile virus infection are as follows

Neurologic complications

• Meningitis
• Encephalitis
• Parkinsonism
• Rhomboencephalitis
• Cerebellar dysfunction
• West nile poliomyelitis
• Dysphagia
• Permanent motor weakness
• Cranial nerve palsy

HEENT complications

• Chorioretinitis
• Papilledema
• Hearing loss

Respiratory complications

• Ataxic breathing
• Apnea

Vascular complications

• Deep venous thrombosis
• Pressure ulcers

Other visceral organ complications

• Hepatitis
• Myocarditis
• Nephritis
• Pancreatitis
• Splenomegaly

Prognosis

• In general, the prognosis of a mild WNV infection is excellent. Most patients with non-neuroinvasive WNV disease or WNV meningitis recover completely. However, fatigue, malaise, and weakness can linger for weeks or months.
• Patients who recover from WNV encephalitis often have residual neurologic deficits. Among patients with neuroinvasive disease, the overall case-fatality ratio is approximately 10%. The rate is significantly higher among patients with WNV encephalitis compared to patients with WNV meningitis.