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

West nile virus is transmitted to human beings by the culex mosquito after feeding on birds which are the reservoir of this virus. After an incubation period of 2-14 day, they can remain asymptomatic or present as west nile fever or as neuroinvasive disease which can be life threatening. The outcome of mild flu like symptoms are excellent while west nile meningitis/encephalitis can have residual neurologic deficits.

Natural history

West Nile Virus (WNV) is a member of Japanese encephalitis antigenic complex of the family Flaviviridae. West nile virus is spread by the bite of mosquito (most commonly Culex pipiens). Birds are the natural reservoir of the virus and the disease is transmitted when a mosquito that feed on the bird bites a human being. The virus can cause severe disease and death in horses.

Incubation period

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

Asymptomatic west nile infection

80% of the people infected by virus are not symptomatic. Most of them are not aware that they have the disease.

West nile fever

Around 20% of the people infected develop west nile fever and can present with any of following symptoms like fever, headache, tiredness, and body aches, nausea, vomiting, occasionally with a skin rash (on the trunk of the body) and swollen lymph glands. It usually presents as flu like disease.

Neuroinvasive disease

The symptoms of severe disease (also called neuroinvasive disease, such as West Nile encephalitis or meningitis or West Nile poliomyelitis) include headache, high fever, neck stiffness, stupor, disorientation,coma,tremors, convulsions, muscle weakness, and paralysis. It is estimated that approximately 1 in 150 persons infected with the West Nile virus will develop a more severe form of disease. Serious illness can occur in people of any age, however people over the age of 50 and some immunocompromised persons (for example, transplant patients) are at the highest risk for getting severely ill when infected with WNV.

West nile meningitis

It usually presents with fever and signs of meningeal irritation like headache, stiffneck, nuchal rigidity and photophobia.

West nile encephalitis

When the infectious process involves the brain parenchyma, west nile encephalitis develops and the patient may have altered level of conciousness, disorientation and focal neurological signs and symptoms due to the neuroinvasion of substantia nigra in brain stem , basal ganglia and cerebellum.

Acute flaccid paralysis

This is due to the selective lesion on anterior spinal cord by west nile virus. The main clinical feature is acute asymmetric flaccid paralysis with minimal or no sensory disturbances.

Possible Complications

Complications from mild West Nile virus infection are very rare.

Complications from severe West Nile virus infection are as follows

Neurologic complications

• Movement disorder
• Rhomboencephalitis
• Cerebellar dysfunction
• West nile poliomyelitis
• Brain damage

Musculoskeletal complications

• Permanent muscle weakness
• Dysphagia

Vascular complications

• Deep venous thrombosis
• Pressure ulcers

Ophthalmic complications

• Chorioretinitis (100% specificity for identifying West Nile virus infection in patients with possible West Nile virus encephalitis)

Other viseral organ complications

• Hepatitis
• Myocarditis
• Nephritis
• Pancreatitis
• Splenomegaly

• Death

Prognosis

• In general, the outcome of a mild West Nile virus infection is excellent. 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%, but it is significantly higher for patients with WNV encephalitis and poliomyelitis than WNV meningitis.
• For patients with severe cases of West Nile virus infection, the outlook is more uncertain. West Nile encephalitis or meningitis may lead to brain damage and death. Approximately 10% of patients with brain inflammation do not survive.