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What is West Nile Encephalitis?

West Nile Encephalitis is an infection of the brain caused by the West Nile virus (WNV). West Nile virus is a member of the family Flaviviridae and belongs to the same family as the virus that causes St. Louis Encephalitis. West Nile virus was originally isolated from a febrile woman in the West Nile region of Uganda in 1937, and, since that time, has been isolated from humans, mosquitoes, and animals in Africa, southern Europe, Asia, and the Middle East. It was first found in the U.S. in 1999 in New York City. It is unknown how the virus came to the U.S. The virus was initially detected in Utah, in horses and sentinel chickens, in August 2003.

How do people get West Nile Encephalitis?

People are infected with WNV when an infected mosquito bites them while taking a blood meal. No person-to-person or animal-to-person transmission of WNV has been documented.

Who gets West Nile Encephalitis?

Anyone bitten by an infected mosquito can get the disease. However, the elderly are much more likely to develop serious symptoms. In an area where WNV is circulating, it has been estimated that about 1% of the mosquitoes would be infected with WNV. It has also been estimated that the risk to a person of getting infected with WNV from the bite of an infected mosquito is about 1%. While the risk of infection may be small, the disease can be quite serious. Of those cases that develop symptoms serious enough to require hospitalization, 3-15% are fatal.

How does the disease spread?

The normal life cycle for WNV involves wild birds as the primary reservoir and mosquitoes as the major vectors. The virus spreads outside of this life cycle when infected mosquitoes bite humans or other animals to take blood. No transmission of WNV to human from ticks or other insects has been documented, but the possibility of this type of transmission is being investigated.

What are the symptoms of West Nile Encephalitis?

Most cases of West Nile Encephalitis are asymptomatic or present with mild symptoms. Symptoms include fever, headache, myalgia, arthralgia, lymphadenopathy, and a maculopapular or roseolar rash affecting the trunk and extremities. Pancreatitis, hepatitis, and myocarditis have occasionally been reported. The most severe infections are marked with headache, high fever, nuchal rigidity, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death (usually in elderly persons).

What is the incubation period?

The incubation period is usually 6 days but ranges between 3 and 15 days.

What is the treatment for West Nile Encephalitis?

There is currently no specific treatment other than to treat the symptoms. In severe cases, hospitalization and intensive supportive therapy are needed. Currently, there is no vaccine available.

What are some clues to the diagnosis of West Nile Encephalitis?

Persons with West Nile Encephalitis in the outbreak in New York City experienced classic signs and symptoms of encephalitis accompanied by a severe, diffuse muscle weakness. In some individuals, the muscle weakness was severe enough to cause flaccid paralysis, axonal neuropathy was evident on electromyographic testing, and the condition was sometimes mistakenly diagnosed as Guillain-Barré syndrome.  The following case definition has been established to help distinguish West Nile encephalitis from other diseases. Persons who are hospitalized with any of the following clinical presentations may be considered to have a suspected diagnosis of West Nile encephalitis:

a) Viral encephalitis characterized by:

1) Fever greater than or equal to 38º C or 100º F, and

2) Central nervous system involvement, including altered mental status (altered level of consciousness, confusion, agitation, or lethargy) or other signs (cranial nerve palsies, paresis or paralysis, or convulsions), and

3) An abnormal CSF profile suggesting a viral etiology (a negative bacterial stain and culture with a pleocytosis [WBC between 5 and 1500 cells] and/or an elevated protein level [greater than or equal to 40 mg])

b) Any case of aseptic meningitis without another confirmed cause

c) Cases of Guillain-Barré syndrome, especially with atypical features, such as fever, altered mental status, and/or a pleocytosis.

What specimens are needed for laboratory testing in suspected West Nile Encephalitis patients?

Cerebrospinal fluid and sera need to be collected. Paired acute phase (collected as early as possible after the onset of illness) and convalescent phase (collected 8 days after clinical onset) specimens are needed for a complete laboratory analysis. Consultation with an infectious disease specialist may be desirable to address any questions about the diagnosis and treatment of patients with suspected West Nile Encephalitis. The Utah Public Health Laboratory has the ability to test specimens for WNV.

Is this disease reportable to the health department?

If you see a patient with known or suspected encephalitis or meningitis, or patients with any unusual disease, please notify your local or state health department as soon as possible. A number of reportable diseases of significant public health importance may present with signs and symptoms of encephalitis or meningitis.

What can be done to prevent West Nile Encephalitis?

To prevent West Nile Encephalitis and other diseases transmitted by mosquitoes, it is prudent to protect yourself from mosquitoes:

• Reduce time spent outdoors when mosquitoes are biting (i.e., between dusk and dawn)

• Wear long pants and long-sleeved shirts when outside

• Apply mosquito repellent to exposed skin areas (for adults use DEET at 30-35% concentration and for children less than 10% concentration)

• Make sure screen doors and window screens are in good condition

• Eliminate stagnant water around housing areas (i.e., water in old tires, cans, birdbaths, poorly kept swimming pools, etc.)

Mosquito control measures may be taken in the interest of protecting the public's health. These may include:

• Aerial spraying of pesticides

• Elimination of larval habitats

• Insecticides to kill juvenile (larvae) and adult mosquitoes

Where can I get more info?

• Your local health department, listed in your telephone directory

• The Utah Department of Health, Office of Epidemiology (801) 538-6191

UTAH DEPARTMENT OF HEALTH
Division of Epidemiology and Laboratory Services
August 2003