West Nile Virus
Questions and Answers
Q. What is West Nile
encephalitis?
A. "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.
Q. Where did West Nile
virus come from?
A. 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.
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Q. Historically, where has West Nile
encephalitis occurred worldwide?
A. See the map describing distribution of flaviviruses,
including West Nile virus:
Q. How long has West Nile
virus been in the U.S.?
A. 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.
Q. How many cases of West
Nile encephalitis in humans have occurred in the U.S.?
A. 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.
Q. I understand West Nile
virus was found in "overwintering" mosquitoes in the New
York City area in early 2000. What does this mean?
A. One of the species of mosquitos found to carry West
Nile virus is the Culex species which survive
through the winter, or "overwinter," in the adult stage.
That the virus survived along with the mosquitoes was
documented by the widespread transmission the summer of
2000.
Q. Do the findings
indicate that West Nile virus is established in the Western
Hemisphere?
A. The continued expansion of West Nile virus in the
United States indicates that it is permanently established
in the Western Hemisphere.
Q. Is the disease
seasonal in its occurrence?
A. 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.
Transmission of West Nile Virus
Q. How do people get West
Nile encephalitis?
A. People become infected by the bite of a mosquitoes
infected with West Nile virus.
Q. What is the basic
transmission cycle?
A. 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.
Q. 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?
A. No. Even in areas where mosquitoes do carry the
virus, very few mosquitoes—much less than
1%—are infected. 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.
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Q. Can you get West Nile encephalitis from
another person?
A. No. West Nile encephalitis is NOT transmitted from
person-to-person. For example, you cannot 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.
Q. Is a woman's pregnancy
at risk if she gets West Nile encephalitis?
A. There is no documented evidence that a pregnancy is
at risk due to infection with West Nile virus.
Q. Besides mosquitoes,
can you get West Nile virus directly from other insects or
ticks?
A. 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
played any role in the cases identified in the United
States.
Q. How many types
of animals have been found to be infected with West Nile
virus?
A. 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.
Q. Can you get West Nile
virus directly from birds?
A. 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.
Q. Can I get infected
with West Nile virus by caring for an infected horse?
A. West Nile virus is transmitted by infectious
mosquitoes. There is no documented evidence of
person-to-person or animal-to-person transmission of West
Nile virus. Normal veterinary infection control precautions
should be followed when caring for a horse suspected to
have this or any viral infection.
Q. How does West Nile
virus actually cause severe illness and death in
humans?
A. Following transmission by an infected mosquito, West
Nile virus multiplies in the person's blood system and
crosses the blood-brain barrier to reach the brain. The
virus interferes with normal central nervous system
functioning and causes inflammation of brain
tissue.
Q. What proportion of
people with severe illness due to West Nile virus die?
A. 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 those infected
with West Nile virus will develop severe
illness.
Q. If a person contracts West Nile virus,
does that person develop a natural immunity to future
infection by the virus?
A. It is assumed that immunity will be lifelong;
however, it may wane in later years.
Q. What can be done to
prevent outbreaks of West Nile virus?
A. 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
(286 KB, 111 pages).
Q. Is there a vaccine
against West Nile encephalitis?
A. No, but several companies are working towards
developing a vaccine.
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Q. Where can I get more
information on mosquito repellents?
A. Visit the American College of Physicians website:
"Mosquitoes and mosquito repellents: A
clinician's guide" (Mark S. Fradin, MD. Annals of
Internal Medicine. June 1, 1998;128:931-940). You
can also find information on insect repellents containing
DEET at the Environmental Protection Agency (EPA)
website.
Q. Where can I get information about
the use of pesticide sprays that are being used for
mosquito control?
A. The federal agency responsible for pesticide
evaluation is the Environmental Protection Agency (EPA).
See the EPA website for detailed
answers to the questions about pesticides used for mosquito
control.
Q. Who is at risk for
getting West Nile encephalitis?
A. All residents of areas where virus activity has been
identified are at risk of getting West Nile encephalitis;
persons older than 50 years have the highest risk of severe
disease.
Q. What are the symptoms
of West Nile encephalitis?
A. 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.
Q. What is the incubation
period in humans (i.e., time from infection to onset of
disease symptoms) for West Nile encephalitis?
A. Usually 3 to 15 days.
Testing and Treating West Nile Encephalitis in Humans
Q. I think I have
symptoms of West Nile virus. What should I do?
A. Contact your 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.
Q. How do health care
providers test for West Nile virus?
A. 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.
Q. How is West Nile
encephalitis treated?
A. 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.
Q. Do birds infected with
West Nile virus die or become ill?
A. In the 1999 New York area epidemic, there was a
large die-off of American crows. West Nile virus has been
identified in more than 70 species of birds found dead in
the United States. Most of these birds were
identified through reporting of dead birds by the
public.
Q. How can I report a sighting of dead
bird(s) in my area?
A. State and local health departments may start
collecting reports of dead birds at different times in the
year. Some will wait until the weather becomes warming
before initiating their surveillance program. For
information on your specific area, please contact your
state or local
health department.
West Nile Virus and Dogs and Cats
Q. Can West Nile virus
cause illness in dogs or cats?
A. 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.
Q. Can infected dogs or
cats be carriers (i.e., reservoirs) for West Nile virus and
transmit the virus to humans?
A. 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.
Q. How do dogs or cats
become infected with West Nile virus?
A. The same way humans become infected—by the
bite of infectious mosquitoes. The virus is located in the
mosquito's salivary glands. During blood feeding, the virus
is injected into the animal. The virus then multiplies and
may cause illness. Mosquitoes become infected when they
feed on infected birds, which may circulate the virus in
their blood for a few days. It is possible that dogs and
cats could become infected by eating dead infected animals
such as birds, but this is unproven.
Q. Can a dog or cat
infected with West Nile virus infect other dogs or
cats?
A. No. There is no documented evidence that West Nile
virus is transmitted from animal to animal.
Q. How long can a dog or
cat be infected with West Nile virus?
A. The answer is not known at this time.
Q. Should a dog or cat
infected with West Nile virus be destroyed? What is the
treatment for an animal infected with West Nile virus?
A. 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.
Q. Has West Nile virus
caused severe illness or death in horses?
A. Yes, while data suggest that most horses infected
with West Nile virus recover, results of investigations
indicate that West Nile virus has caused deaths in horses
in the United States.
Q. How do the horses
become infected with West Nile virus?
A. The same way humans become infected—by the
bite of infectious mosquitoes. The virus is located in the
mosquito's salivary glands. When mosquitoes bite or
"feed" on the horse, the virus is injected into its blood
system. The virus then multiplies and may cause illness.
The mosquitoes become infected when they feed on infected
birds or other animals.
Q. How does the virus
cause severe illness or death in horses?
A. Following transmission by an infected mosquito, West
Nile virus multiplies in the horse's blood system, crosses
the blood brain barrier, and infects the brain. The virus
interferes with normal central nervous system functioning
and causes inflammation of the brain.
Q. Can I get infected
with West Nile virus by caring for an infected horse?
A. West Nile virus is transmitted by infectious
mosquitoes. There is no documented evidence of
person-to-person or animal-to-person transmission of West
Nile virus. Normal veterinary infection control precautions
should be followed when caring for a horse suspected to
have this or any viral infection.
Q. Can a horse infected
with West Nile virus infect horses in neighboring
stalls?
A. No. There is no documented evidence that West Nile
virus is transmitted between horses. However, horses with
suspected West Nile virus should be isolated from mosquito
bites, if at all possible.
Q. My horse is vaccinated
against eastern equine encephalitis (EEE), western equine
encephalitis (WEE), and Venezuelan equine encephalitis
(VEE). Will these vaccines protect my horse against West
Nile virus infection?
A. No. EEE, WEE, and VEE belong to another family of
viruses for which there is no cross-protection.
Q. Can I vaccinate my
horse against West Nile virus infection?
A. A West Nile virus vaccine for horses was recently
approved, but its effectiveness is unknown.
Q. How long will a horse
infected with West Nile virus be infectious?
A. We do not know if an infected horse can be
infectious (i.e., cause mosquitoes feeding on it to become
infected). However, previously published data suggest that
the virus is detectable in the blood for only a few
days.
Q. What is the treatment
for a horse infected with West Nile virus? Should it
be destroyed?
A. There is no reason to destroy a horse just because
it has been infected with West Nile virus. Data suggest
that most horses recover from the infection. Treatment
would be supportive and consistent with standard veterinary
practices for animals infected with a viral
agent.
Q. Where can I get more
information on horses and West Nile virus?
A. Visit the USDA website Animal
and Plant Health Inspection Service (APHIS)
West Nile Virus and Wild Game Hunters
Q. Are duck and other
wild game hunters at risk for West Nile virus
infection?
A. Because of their outdoor exposure, game hunters may
be at risk if they become bitten by mosquitoes in areas
with West Nile virus activity. The extent to which West
Nile virus may be present in wild game is unknown.
Surveillance studies are currently underway in
collaboration with the U.S. Geological Survey (USGS)
National Wildlife Health Center (in Madison, Wisconsin) and
with state and local wildlife biologists and naturalists to
answer this question.
Q. What should wild game
hunters do to protect against West Nile virus
infection?
A. Hunters should follow the usual precautions when
handling wild animals. If they anticipate being exposed to
mosquitoes, they should apply insect repellents to clothing
and skin, according to label instructions, to prevent
mosquito bites. Hunters should wear gloves when handling
and cleaning animals to prevent blood exposure to bare
hands and meat should be cooked thoroughly.
Q. Who should wild game
hunters contact for information about the risk for West
Nile virus infection in specific geographic areas?
A. Hunters should check with their local area
department of wildlife and naturalist resources, state
epidemiologist at the state health
department, or the U.S. Geological Survey (USGS)
National Wildlife Health Center, Madison, WI, 608-270-2400
for information on local area risk.
Additional Information About West Nile Virus