West Nile Virus
The West Nile virus is an arbovirus
(arthropod-borne virus). It is transmitted between susceptible hosts via
blood-feeding insect (arthropod) vectors such as ticks and mosquitoes. Like all
viruses, the West Nile virus requires a living host in order to replicate. The
primary hosts of the West Nile virus are wild birds.
This figure shows a surface-shaded
image of the West Nile virus particle produced by Purdue University biologists
using cryoelectron microscopy. The surface is composed of proteins that enable
the virus to bind with and invade a host cell. The particle is approximately 50
nanometers in diameter, or about 1/1000th of the width of a human hair. (Purdue
Department of Biological Sciences image)
The virus, first identified in 1937
in the West Nile region of Uganda, was detected in the United States two years
ago in New York State.
West Nile virus is a
mosquito-borne disease that can cause encephalitis, or a brain infection.
Mosquitoes acquire the virus from birds and pass it on to other birds, animals
and people. Mosquitoes spread this virus after they feed on infected birds and
then bite people, other birds and animals. It is not spread by person-to-person
contact and there is no evidence that people can get the virus by handling
infected animals. West Nile virus cases occur primarily in the late summer or
early Fall, although the mosquito season is April through October.
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. 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 is it
The West Nile virus is an
arborvirus (arthropod-borne virus), transmitted between hosts via a
blood-feeding insect (called the virus vector). The primary hosts of the West
Nile virus are wild birds, and the primary vectors are bird-feeding mosquitoes,
mainly of the Culex species. These organisms are vital for the virus'
transmission cycle: birds provide an environment in which the virus can
replicate at high levels - high enough that blood picked up by mosquitoes
contains virus particles. The mosquitoes are needed for the spread of the virus
particles to new hosts.
In some cases, animals other than
the primary hosts may become infected with the virus. The West Nile virus, for
example, can also infect humans. These alternative hosts are often referred to
as "dead end" hosts, as they are not part of the virus' normal
transmission cycle. These organisms do not usually produce enough virus
particles to allow them to become a reservoir for the virus. The infection thus
reaches a dead end in this host.
Mosquitoes are usually the
primary vectors for transmission of the West Nile virus. In Africa and Asia,
however, ticks have been found carrying the virus.
West Nile virus is only spread
via its insect vector; it is NOT transmitted person-to-person or bird-to-person.
In warmer climates, the virus can
be found year-round. In temperate climates, the virus usually occurs in late
summer or early fall (ie - in peak mosquito season). It is not known how the
virus returns at the end of the cold season, although there are a few theories:
The virus may:
- survive in
hibernating Culex mosquitoes
- be transmitted
to offspring by infected female Culex mosquitoes
- establish a
chronic infection in its hosts
reintroduced each spring by chronically infected migratory birds.
generally occur five to 15 days following the bite of an infected mosquito. Most
people who are infected have no symptoms or may experience a fever and headache
before fully recovering. People with mild infections may experience fever,
headache, body aches, skin rash and swollen lymph glands. More severe infections
result in high fever, headache, neck stiffness, stupor, disorientation, tremors,
occasional convulsions and paralysis. At its most serious, the infection can
result in coma, permanent neurological damage, and death. There is no vaccine
and treatment is directed at relieving symptoms. Because they are viral
infections, antibiotics are not effective and there is no specific treatment
Is West Nile Virus?
West Nile virus (WNV) is a potentially serious illness. Experts believe WNV is
established as a seasonal epidemic in North America that flares up in the
summer and continues into the fall. This fact sheet contains important
information that can help you recognize and prevent West Nile virus.
Are the Symptoms of WNV?
WNV affects the central nervous system. Symptoms vary.
Symptoms in Most People.
Approximately 80 percent of people who are infected with WNV will not show
any symptoms at all.
Symptoms in Some People.
Up to 20 percent of the people who become infected will display mild
symptoms, including fever, headache, and body aches, nausea, vomiting, and
sometimes swollen lymph glands or a skin rash on the chest, stomach and
back. Symptoms typically last a few days.
Symptoms in a Few People.
About one in 150 people infected with WNV will develop severe illness. The
severe symptoms can include high fever, headache, neck stiffness, stupor,
disorientation, coma, tremors, convulsions, muscle weakness, vision loss,
numbness and paralysis. These symptoms may last several weeks, and
neurological effects may be permanent.
Do Infected People Get Sick?
Generally, WNV is spread by the bite of an infected mosquito. Mosquitoes
are WNV carriers that become infected when they feed on infected birds.
Infected mosquitoes can then spread WNV to humans and other animals when
Transplants, and Mother-to-Child.
In a very small number of cases, WNV also has spread through blood
transfusions, organ transplants, breastfeeding and even during pregnancy
from mother to baby.
WNV is not spread through casual contact such as touching or kissing a
person with the virus.
People typically develop symptoms between 3 and 14 days after they are bitten by
the infected mosquito.
WNV Infection Treated?
There is no specific treatment for WNV infection. In cases with mild symptoms,
people experience symptoms such as fever and aches that pass on their own. In
more severe cases, people usually need to go to the hospital where they can
receive supportive treatment including intravenous fluids, help with breathing
and nursing care.
Should I Do if I Think I Have WNV?
Mild WNV illness improves on its own, and people do not necessarily need to seek
medical attention for this infection. If you develop symptoms of severe WNV
illness, such as unusually severe headaches or confusion, seek medical attention
immediately. Severe WNV illness usually requires hospitalization. Pregnant women
and nursing mothers are encouraged to talk to their doctor if they develop
symptoms that could be WNV.
Is the Risk of Catching WNV?
For most, risk is low. Less than 1 percent of people who are bitten by
mosquitoes develop any symptoms of the disease and relatively few mosquitoes
actually carry WNV.
risk for those outdoors a lot.
People who spend a lot of time outdoors are more likely to be bitten by an
infected mosquito. They should take special care to avoid mosquito bites.
over 50 can get sicker.
People over the age of 50 are more likely to develop serious symptoms of WNV
if they do get sick and should take special care to avoid mosquito bites.
through medical procedures is low.
The risk of getting WNV through blood transfusions and organ transplants is
very small, and should not prevent people who need surgery from having it. If
you have concerns, talk to your doctor before surgery.
and nursing do not increase risk of becoming infected with WNV.
Can I Do to Prevent WNV?
The easiest and best way to avoid WNV is to prevent mosquito bites.
you are outdoors, use insect repellents containing DEET (N,
N-diethyl-meta-toluamide). Follow the directions on the package.
mosquitoes are most active at dusk and dawn. Consider staying indoors
during these times or use insect repellent and wear long sleeves and
pants. Light-colored clothing can help you see mosquitoes that land on
sure you have good screens on your windows and doors to keep mosquitoes
of mosquito breeding sites by emptying standing water from flower pots,
buckets and barrels. Change the water in pet dishes and replace the water
in bird baths weekly. Drill drainage holes in tire swings so water drains
out. Keep children's wading pools empty and on their sides when they
aren't being used.
Is the CDC Doing About WNV?
CDC is working with state and local health departments, the Food and Drug
Administration and other government agencies, as well as private industry, to
prepare for and prevent new cases of WNV.
Some things CDC is doing include:
a nation-wide electronic database where states share information about WNV
states develop and carry out improved mosquito prevention and control
better, faster tests to detect and diagnose WNV
new education tools and programs for the media, the public, and health
new testing laboratories for WNV
Else Should I Know?
If you find a dead bird: Don't handle the body with your bare hands.
Contact your local health department for instructions on reporting and disposing
of the body.
Credit: CDC, Microbiology Department, Mount Sinai
Hospital, Toronto, ON, Canada, Purdue University