West Nile encephalitis

• West Nile virus (WNV) is the leading cause of mosquito-borne disease in the continental United States.
• It is most commonly spread to people by the bite of an infected mosquito.
• Cases of WNV occur during mosquito season, which starts in the summer and continues through fall.
• There are no vaccines to prevent or medications to treat WNV in people.

West Nile Virus

Transmission

• Image of the West Nile virus transmission cycle. The image is divided into two sides with a box in the middle. Inside the box there are images of mosquitoes on the top and birds on the bottom with arrows going from the mosquitoes to the birds and then from the birds to the mosquitoes. Mosquitoes transmit disease and birds are amplifier hosts. There is an arrow going from the mosquitoes to the left side of the image where a horse is in a field outside. The horse is a dead end host. There is also an arrow going from the mosquitoes to the right side of the image where there is a woman planting flowers outside of her house. The woman is also a dead end host.

• West Nile virus is most commonly spread to people by the bite of an infected mosquito.

Symptoms

• No symptoms in most people. Most people (8 out of 10) infected with West Nile virus do not develop any symptoms.
• Febrile illness (fever) in some people.

Mosquito bite

Mild symptoms

• About 1 in 5 people who are infected develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash.
• Most people with this type of West Nile virus disease recover completely, but fatigue and weakness can last for weeks or months.
• Serious symptoms in a few people. About 1 in 150 people who are infected develop a severe illness affecting the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord).
• Symptoms of severe illness include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.

Severe illness

• Severe illness can occur in people of any age; however, people over 60 years of age are at greater risk. People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk.
• Recovery from severe illness might take several weeks or months. Some effects to the central nervous system might be permanent.
• About 1 out of 10 people who develop severe illness affecting the central nervous system die.

Diagnosis

• See your healthcare provider if you develop the symptoms described above.
• Your healthcare provider can order tests to look for West Nile virus infection.

West Nile virus

WNV Antibody Testing

• Laboratory diagnosis is generally accomplished by testing of serum or cerebrospinal fluid (CSF) to detect WNV-specific IgM antibodies. Immunoassays for WNV-specific IgM are available commercially and through state public health laboratories.

• 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, and the test may need to be repeated on a later sample.

• The presence of WNV-specific IgM in blood or CSF provides good evidence of recent infection but may also result from cross-reactive antibodies after infection with other flaviviruses or from non-specific reactivity. According to product inserts for commercially available WNV IgM assays, all positive results obtained with these assays should be confirmed by neutralizing antibody testing of acute- and convalescent-phase serum specimens at a state public health laboratory or CDC.

• WNV IgG antibodies generally are detected shortly after IgM antibodies and persist for many years following a symptomatic or asymptomatic infection. Therefore, the presence of IgG antibodies alone is only evidence of previous infection and clinically compatible cases with the presence of IgG, but not IgM, should be evaluated for other etiologic agents.

• Plaque-reduction neutralization tests (PRNTs) performed in reference laboratories, including some state public health laboratories and CDC, can help determine the specific infecting flavivirus. PRNTs can also confirm acute infection by demonstrating a fourfold or greater change in WNV-specific neutralizing antibody titer between acute- and convalescent-phase serum samples collected 2 to 3 weeks apart.

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Global distribution of West Nile virus

Other testing for WNV disease

• Viral cultures and tests to detect viral RNA (e.g., reverse transcriptase-polymerase chain reaction [RT-PCR]) can be performed on serum, CSF, and tissue specimens that are collected early in the course of illness and, if results are positive, can confirm an infection. Immunohistochemistry (IHC) can detect WNV antigen in formalin-fixed tissue. Negative results of these tests do not rule out WNV infection.

Treatment

• There is no specific treatment for WNV disease; 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 ventilatory support may be required.

• Various drugs have been evaluated or empirically used for WNV disease, as described in a review of the literature for health care providers Cdc-pdf[PDF – 11 pages]. However, none have shown specific benefit to date. The National Institutes of Health maintains a registry of federally and privately supported clinical trials conducted in the United States and around the world.

Prevention

• No WNV vaccines are licensed for use in humans. In the absence of a vaccine, prevention of WNV disease depends on community-level mosquito control programs to reduce vector densities, personal protective measures to decrease exposure to infected mosquitoes, and 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 peridomestic mosquito breeding sites, can further decrease the risk for WNV exposure.

Screening in blood products

• Blood and some organ donations in the United States are screened for WNV infection; healthcare professionals should remain vigilant for the possible transmission of WNV through blood transfusion or organ transplantation.

West Nile encephalitis Resources
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