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]

Natural history

West Nile Virus (WNV) is a member of the flavivirus genus and belongs to the 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 bite the bird bites a human being. The virus can cause severe disease and death in horses.

Transmission

Human infection is most often the result of bites from infected mosquitoes. Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood for a few days. The virus eventually gets into the mosquito's salivary glands. During later blood meals (when mosquitoes bite), the virus may be injected into humans and animals, where it can multiply and possibly cause illness. The virus may also be transmitted through contact with other infected animals, their blood, or other tissues. A very small proportion of human infections have occurred through organ transplant, blood transfusions and breast milk. There is one reported case of transplacental (mother-to-child) WNV transmission. To date, no human-to-human transmission of WNV through casual contact has been documented, and no transmission of WNV to health care workers has been reported when standard infection control precautions have been put in place. Transmission of WNV to laboratory workers has been reported.

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 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.

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.

Possible Complications

Complications from mild West Nile virus infection are very rare.

Complications from severe West Nile virus infection include:

• Permanent muscle weakness (sometimes similar to polio)
• Dysphagia
• Deep venous thrombosis
• Pressure ulcers
• Chorioretinitis (100% specificity for identifying West Nile virus infection in patients with possible West Nile virus encephalitis)
• Hepatitis
• Myocarditis
• Nephritis
• Pancreatitis
• Splenomegaly
• Brain damage
• Death

Prognosis

In general, the outcome of a mild West Nile virus infection is excellent.

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.