What is West Nile encephalitis?
"Encephalitis" means an
inflammation of the brain and can be caused by viruses and bacteria,
including viruses transmitted by mosquitoes. West Nile encephalitis is an
infection of the brain caused by West Nile virus, a flavivirus commonly
found in Africa, West Asia, and the Middle East. It is closely related to
St. Louis encephalitis virus found in the United States.
Where did West Nile virus come from?
West Nile virus has been commonly found in
humans and birds and other vertebrates in Africa, Eastern Europe, West Asia,
and the Middle East, but until 1999 had not previously been documented in
the Western Hemisphere. It is not known from where the U.S. virus
originated, but it is most closely related genetically to strains found in
the Middle East.
How long has West Nile virus been in the
U.S.?
It is not known how long it has been in
the U.S., but CDC scientists believe the virus has probably been in the
eastern U.S. since the early summer of 1999, possibly longer.
How many cases of West Nile encephalitis
in humans have occurred in the U.S.?
In 1999, 62 cases of severe disease,
including 7 deaths, occurred in the New York area. In 2000, 21 cases were
reported, including 2 deaths in the New York City area. In 2001, there were
66 human cases of severe disease and 9 deaths. No reliable estimates are
available for the number of cases of West Nile encephalitis that occur
worldwide.
Do the findings indicate that West Nile
virus is established in the Western Hemisphere?
The continued expansion of the West Nile
virus in the United States indicates that it is permanently established in
the Western Hemisphere.
Is the disease seasonal in its occurrence?
In the temperate zone of the world (i.e.
between latitudes 23.5° and 66.5°
north and south), West Nile encephalitis cases occur primarily in the late
summer or early fall. In the southern climates where temperatures are
milder, West Nile virus can be transmitted year round.
How do people get West Nile encephalitis?
People become infected by the bite of
mosquitoes infected with West Nile virus.
What is the
basic transmission cycle?
Mosquitoes become infected when they feed
on infected birds, which may circulate the virus in their blood for a few
days. Infected mosquitoes can then transmit West Nile virus to humans and
animals while biting to take blood. The virus is located in the mosquito’s
salivary glands. During blood feeding, the virus may be injected into the
animal or human, where it may multiply, possibly causing illness.
If I live in an area where birds or
mosquitoes with West Nile virus have been reported and a mosquito bites me,
am I likely to get sick?
No. Even in areas where the virus is
circulating, very few mosquitoes are infected with the virus. Even if the
mosquito is infected, less than 1% of people who get bitten and become
infected will get severely ill. The chances you will become severely ill
from any one mosquito bite are extremely small.
Can you get
West Nile encephalitis from another person?
No. West Nile encephalitis is NOT
transmitted from person-to-person. For example, you can not get West Nile
virus from touching or kissing a person who has the disease, or from a
health care worker who has treated someone with the disease.
Besides
mosquitoes, can you get West Nile virus directly from other insects or
ticks?
Infected mosquitoes are the primary source
for West Nile virus. Although ticks infected with West Nile virus have been
found in Asia and Africa, their role in the transmission and maintenance of
the virus is uncertain. However, there is no information to suggest that
ticks play any role in the cases identified in the United States.
How many types of animals have been found
to be infected with West Nile virus?
Although the vast majority of infections
have been identified in birds, WN virus has been shown to infect horses,
cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.
Can you get
West Nile virus directly from birds?
There is no evidence that a person can get
the virus from handling live or dead infected birds. However, persons should
avoid bare-handed contact when handling any dead animals and use gloves or
double plastic bags to place the carcass in a garbage can.
Among those with severe illness due to
West Nile virus, case-fatality rates range from 3% to 15% and are highest
among the elderly. Less than 1% of persons infected with West Nile virus
will develop severe illness.
If a person contracts West Nile virus,
does that person develop a natural immunity to future infection by the
virus?
It is assumed that immunity will be
lifelong; however, it may wane in later years.
What can be done to prevent outbreaks of
West Nile virus?
Prevention and control of West Nile virus
and other arboviral diseases is most effectively accomplished through
integrated vector management programs. These programs should include
surveillance for West Nile virus activity in mosquito vectors, birds,
horses, other animals, and humans, and implementation of appropriate
mosquito control measures to reduce mosquito populations when necessary.
Additionally, when virus activity is detected in an area, residents should
be alerted and advised to increase measures to reduce contact with
mosquitoes. Details about effective prevention and control of West Nile
virus can be found in CDC’s Guidelines for Surveillance, Prevention,
and Control.
Is there a vaccine against West Nile
encephalitis?
No, but several companies are working
towards developing a vaccine.
What can I do to reduce my risk of
becoming infected with West Nile virus?
Stay indoors at dawn, dusk, and in the
early evening.
Wear long-sleeved shirts and long pants
whenever you are outdoors.
Spray clothing with repellents
containing permethrin or DEET since mosquitoes may bite through thin
clothing.
Apply insect repellent sparingly to
exposed skin. An effective repellent will contain 35% DEET (N,N-diethyl-meta-toluamide).
DEET in high concentrations (greater than 35%) provides no additional
protection.
Repellents may irritate the eyes and
mouth, so avoid applying repellent to the hands of children. (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.)
Install or repair window and door
screens so that mosquitoes cannot get indoors.
Note: Vitamin B and
"ultrasonic" devices are NOT effective in preventing mosquito
bites.
Who is at risk for getting West Nile
encephalitis?
All residents of areas where virus
activity has been identified are at risk of getting West Nile encephalitis;
persons over 50 years of age have the highest risk of severe disease.
What are the symptoms of West Nile
encephalitis?
Most infections are mild, and symptoms
include fever, headache, and body aches, occasionally with skin rash and
swollen lymph glands. More severe infection may be marked by headache, high
fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions,
muscle weakness, paralysis, and, rarely, death.
What is the incubation period in humans
(i.e., time from infection to onset of disease symptoms) for West Nile
encephalitis?
Usually 3 to 15 days.
I think I have symptoms of West Nile
virus. What should I do?
Contact you health care provider if you
have concerns about your health. If you or your family members develop
symptoms such as high fever, confusion, muscle weakness, and severe
headaches, you should see your doctor immediately.
How do health care providers test for West
Nile virus?
Your physician will first take a medical
history to assess your risk for West Nile virus. People who live in or
traveled to areas where West Nile virus activity has been identified are at
risk of getting West Nile encephalitis; persons older than 50 years of age
have the highest risk of severe disease. If you are determined to be at high
risk and have symptoms of West Nile encephalitis, your provider will draw a
blood sample and send it to a commercial or public health laboratory for
confirmation.
How is West Nile encephalitis treated?
There is no specific therapy. In more
severe cases, intensive supportive therapy is indicated, often involving
hospitalization, intravenous fluids, airway management, respiratory support
(ventilator), prevention of secondary infections (pneumonia, urinary tract,
etc.), and good nursing care.
Can West Nile virus cause illness in dogs
or cats?
There is a published report of West Nile
virus isolated from a dog in southern Africa (Botswana) in 1982. West Nile
virus has been isolated from several dead cats in 1999 and 2000. A
serosurvey of dogs and cats in the epidemic area showed a low infection
rate.
Can infected dogs or cats be carriers
(i.e., reservoirs) for West Nile virus and transmit the virus to humans?
West Nile virus is transmitted by
infectious mosquitoes. There is no documented evidence of person-to-person,
animal-to-animal, or animal-to-person transmission of West Nile virus.
Veterinarians should take normal infection control precautions when caring
for an animal suspected to have this or any viral infection.
Should a dog or cat infected with West
Nile virus be destroyed? What is the treatment for an animal infected with
West Nile virus?
No. There is no reason to destroy an
animal just because it has been infected with West Nile virus. Full recovery
from the infection is likely. Treatment would be supportive and consistent
with standard veterinary practices for animals infected with a viral agent.
