West Nile encephalitis
West Nile encephalitis: Excerpt from Professional Guide to Diseases (Eighth Edition)
West Nile encephalitis is categorized as an infectious disease that primarily causes an inflammation or “encephalitis” of the brain. West Nile virus (WNV), a flavivirus commonly found in humans, birds, and other vertebrates in Africa, west Asia, and the Middle East, causes the disease, which is a part of a family of vector-borne diseases that also includes malaria, yellow fever, and Lyme disease.
The virus had not been previously documented in the Western Hemisphere until late August 1999. A virus found in numerous dead birds in New York, New Jersey, and Connecticut was definitively identified by genetic sequencing as the West Nile virus. Scientists in the United States discovered the rare strain initially in and around the Bronx Zoo and believe that infected birds may have carried the disease and that it was spread as mosquitoes fed on them.
In the temperate areas of the world, West Nile encephalitis cases occur mainly in the late summer or early fall. In the southern climates where temperatures are milder, West Nile encephalitis can occur all year round.
The risk of contracting West Nile encephalitis is greater for all residents of areas where active cases have been identified, but persons older than age 50 or those with compromised immune systems have the greatest risk. At present, there is no documented evidence that a pregnant woman's fetus is at risk due to an infection with WNV. The mortality rate of West Nile encephalitis is measured by case-fatality rates, which range from 3% to 15% (higher in the elderly population.)
Causes and incidence
WNV is transmitted to humans by the bite of a mosquito (primarily the Culex species) infected with the virus. It's considered the primary vector for WNV and the source of the August 1999 outbreak in New York, New Jersey and Connecticut. Mosquitoes become infected by feeding on birds contaminated with the West Nile virus and then transmitting it to humans and animals during a blood meal or “bite.” (See Transmission routes of West Nile virus, page 256.)
Ticks have been found infected with WNV in Africa and Asia only. The role of ticks in the transmission and maintenance of the virus remains uncertain, and to date they aren't considered vectors for WNV in the United States.
The Centers for Disease Control and Prevention has reported that there is no evidence that a person can contract the virus from handling live or dead infected birds. However, avoid barehanded contact when handling dead animals, including birds, and use gloves or double plastic bags to dispose of a carcass. Report the finding to the local health department.
Signs and symptoms
Mild infections of the virus are more common and include fever, headache, and body aches, usually accompanied by a skin rash and swollen lymph glands. Severe infections can be manifested by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions, paralysis and, rarely, death.
The incubation period for West Nile encephalitis is anywhere from 5 to 15 days after exposure. Most patients who are bitten by an infected mosquito won’t develop symptoms. It's estimated that only 1 in 300 people who are bitten by an infected mosquito will actually get sick.
Diagnosis
The immunoglobulin (Ig) M antibody capture enzyme-linked immunosorbent assay (MAC-ELISA) is the test of choice for rapid definitive diagnosis. The major advantage of MAC-ELISA laboratory analysis is the high probability of accurate diagnosis of WNV infection when performed with acute serum or cerebrospinal fluid specimens obtained while the patient is still hospitalized.
A new diagnostic test, the WNV MAC-ELISA, was recently approved by the Food and Drug Administration. This test detects levels of IgM antibodies in a patient's ser-um and is intended for use in patients with clinical symptoms consistent with viral encephalitis.
Other conditions to consider include St. Louis encephalitis, which is symptomatically similar.
Encephalitis can be caused by numerous viral and bacterial infections; all data must be examined to determine a definitive diagnosis.
Treatment
There is no specific therapy utilized to treat West Nile encephalitis and no known cure. Treatment is generally aimed at controlling the specific symptoms. Supportive care, such as I.V. fluids, fever control, and respiratory support, is rendered when necessary.
There is no vaccine present to prevent the transmission of West Nile encephalitis. Research trials are underway to determine if ribavirin, an antiviral drug, may be helpful.
Special considerations
❑Obtain an extensive history of the patient's whereabouts within the past 2 to 3 weeks (especially around bodies of water, such as lakes and ponds), the presence of dead birds, and recent mosquito bites acquired.
❑Perform a comprehensive physical assessment and report signs of fever, headache, lymphadenopathy, and a maculopapular rash.
❑Perform a complete neurological examination and report any signs of confusion, lethargy, weakness, or slurred speech.
❑Maintain adequate hydration with I.V. fluids.
❑Monitor strict intake and output.
❑Usefever control methods, such as cooling blankets and acetaminophen as ordered.
❑Provide respiratory support measures when applicable.
❑West Nile encephalitis isn’t transmitted from person to person, but use standard precautions when handling body fluids and blood.
❑Report any suspected cases of West Nile encephalitis to the applicable state health department.
❑Teach the patient ways to reduce his risk of becoming infected with West Nile encephalitis:
– Stay indoors at dawn, dusk, and in the early evening.
– Wear long-sleeved shirts and long pants outdoors.
– Apply insect repellent sparingly to exposed skin. Check the label. An effective repellent will contain 20% to 30% N,N-diethyl-meta-toluamide (DEET). DEET in high concentrations (greater than 30%) may cause adverse effects, particularly in children; avoid products containing more than 30% DEET.
– Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children. It's contraindicated to apply insect repellents to children younger than age 3 years.
– Spray clothing with repellents containing DEET, as mosquitoes may bite through thin clothing.
– Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's directions for use, as printed on the product.
Pictures
Book Source Details
- Book Title: Professional Guide to Diseases (Eighth Edition)
- Author(s): Springhouse
- Year of Publication: 2005
- Copyright Details: Professional Guide to Diseases (Eighth Edition), Copyright © 2005 Lippincott Williams & Wilkins.
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