West Nile encephalitis

West Nile encephalitis: Excerpt from Handbook of Diseases

West Nile encephalitis, part of a family of vectorborne diseases that also includes malaria, yellow fever, and Lyme disease, is an infectious disease that primarily causes encephalitis (inflammation) of the brain. It’s caused by the West Nile virus (WNV), a flavivirus commonly found in humans, birds, and other vertebrates in Africa, West Asia, and the Middle East. The first documented cases of WNV in the Western Hemisphere didn’t occur until late in August 1999, when numerous dead birds in the New York, New Jersey, and Connecticut region tested positive for WNV after genetic sequencing. Scientists traced the Western Hemisphere origin of the disease to the vicinity of New York’s Bronx Zoo and believe that mosquitoes feeding on diseased birds helped to spread the disease.

In temperate areas, West Nile encephalitis occurs mainly in the late summer or early fall. In southern climates with milder temperatures, West Nile encephalitis can occur year round.

All persons living in endemic areas carry a risk of contracting West Nile encephalitis, but persons older than age 50 or those with compromised immune systems have the greatest risk.

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

WNV is transmitted to humans by the bite of an infected mosquito (primarily the Culex species). Mosquitoes become infected by feeding on infected birds.

Ticks infected with WNV have been found in Africa and Asia, but their role in transmission and maintenance of the virus is uncertain; they aren’t considered vectors for WNV in the United States.

The Centers for Disease Control and Prevention has reported that there’s no evidence that a person can contract the virus from handling live or dead infected birds. However, barehanded contact when handling dead animals, including dead birds, should be avoided; if a dead animal must be handled, gloves or other protective measures should be used to dispose of the carcass. A dead bird is a sign that there may be infected mosquitoes in the area; findings should be reported to the nearest Emergency Management Office.

Signs and symptoms

Mild WNV infections are more common than severe infections and include symptoms such as fever, headache, and body aches, often accompanied by swollen lymph glands and a skin rash. Severe infections present with symptoms such as headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions, paralysis and, rarely, death.

The incubation period for West Nile encephalitis is 5 to 15 days after exposure.

CLINICAL TIP: Researchers estimate that only 1 in 300 people who are bitten by a mosquito infected with WNV actually develops the disease.

Diagnosis

The immunoglobulin M antibody capture–enzyme-linked immunosorbent assay is the test of choice for rapid definitive diagnosis. It has a high probability of accurate diagnosis of WNV infection when performed with acute serum or cerebrospinal fluid specimens obtained while the patient is hospitalized.

Encephalitis can also be caused by numerous viral and bacterial infections, so data must be carefully examined to determine a definitive diagnosis. St. Louis encephalitis, which is symptomatically similar to West Nile encephalitis, should be considered.

Treatment

No specific therapy is used to treat West Nile encephalitis, and no known cure exists. Treatment is generally aimed at controlling the specific symptoms. Supportive care, such as intravenous fluids, fever control, and respiratory support, is rendered when necessary.

Special considerations

❑ Obtain an extensive history of the patient’s whereabouts within the last 2 to 3 weeks. Ask him if he has spent time around a body of water, such as a lake or a pond, been in the presence of dead birds or other dead animals, or received a mosquito bite.

❑ Perform a comprehensive physical assessment. Report signs of fever, headache, lymphadenopathy, and maculopapular rash.

❑ Perform a complete neurologic exam. Report signs of confusion, lethargy, weakness, or slurred speech.

❑ Maintain adequate hydration with intravenous fluids.

❑ Strictly monitor intake and output.

❑ Utilize fever control methods.

❑ Provide respiratory support measures, if necessary.

❑ Use universal precautions when handling body fluids and blood.

❑ Report suspected cases of West Nile encephalitis to the state’s Department of Health.

To reduce your patient’s risk of becoming infected with West Nile encephalitis, tell him to:

❑ stay indoors at dawn, dusk, and in the early evening

❑ wear long-sleeved shirts and long pants whenever outdoors

❑ apply insect repellent containing N,N-diethyl-meta-toluamide (DEET) sparingly to exposed skin and clothing.

CLINICAL TIP: An effective insect repellent will contain 20% to 30% DEET. Tell patients to avoid products containing more than 30% DEET, because they may cause adverse effects, particularly in children. Also tell patients that whenever they use an insecticide or insect repellent, they should read and follow the manufacturer’s directions for use, as printed on the product.

age alert Because repellents may irritate the eyes and mouth, tell parents to avoid applying repellent to children’s hands. Insect repellents shouldn’t be applied to children younger than 3.

Book Source Details

• Book Title: Handbook of Diseases
• Author(s): Springhouse
• Year of Publication: 2003
• Copyright Details: Handbook of Diseases, Copyright © 2003 Lippincott Williams & Wilkins.

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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.

More About This Book: Title: Handbook of Diseases Authors: Springhouse Publisher: Lippincott Williams & Wilkins Copyright: 2003 ISBN: 1-58255-266-5

 » Next page: Kernig's sign (Alarming Signs and Symptoms: Lippincott Manual of Nursing Practice Series)

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