Farm and Home Biosecurity
Consumer/General Public and Livestock/Row Crop Producer
West Nile Virus
West Nile (W-N) virus has emerged in recent years in
temperate regions of Europe and North America, presenting a threat to public,
equine, and animal health. The most serious manifestation of W-N virus infection
is fatal encephalitis (inflammation of the brain) in humans and horses, as well
as mortality in certain domestic and wild birds.
West Nile virus was first isolated from a febrile
(feverish) adult woman in the West Nile District of Uganda in 1937. The ecology
was characterized in Egypt in the 1950s. The virus became recognized as a cause
of severe human meningoencephalitis (inflammation of the spinal cord and brain)
in elderly patients during an outbreak in Israel in 1957.
Equine disease was first noted in Egypt and France in the
early 1960s. The appearance of W-N virus in North America in 1999, with
encephalitis reported in humans and horses, may be an important milestone in the
evolving history of this virus.
Persons become infected from the bite of mosquitoes
infected with West Nile virus. 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
Human illness from West Nile virus remains rare in areas
where it has been reported, and the chance that any one person is going to
become ill from a mosquito bite is low.
year WNV has been detected over a much wider range-likely spread by migrating
birds and by transport of materials such as used tires that harbor infected
mosquitoes. By mid-October 2001 WNV had been confirmed in 27 states in the US,
from the Northeast and mid-Atlantic regions to the Mid-West and along the Gulf
Coast as well as in Washington D.C and southern Canada (in Alabama, Arkansas,
Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky,
Louisiana, Maine, Maryland, Massachusetts, Michigan, Mississippi, Missouri, New
Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode
Island, Tennessee, Virginia, and Wisconsin).
WNV has already shown that it can survive winters in the Northeast US, there is
some question about how it will fare in harsher years. Concern is greater that
it will more easily become permanently established in the milder sections of the
in the Gulf Coast States.
spread of WNV is tracked by surveillance of mosquitoes and sentinel birds.
Crows have become the primary sentinel species because they seem to be
particularly susceptible to WNV-caused fatalities. The numbers and locations of
dead crows are closely monitored and some are tested for the disease. Since WNV is primarily transmitted between birds and mosquitoes (with mammals
only occasionally becoming infected if bitten by an infected mosquito), WNV-positive
birds are almost always detected in an area well before the disease presents a
risk to the local human population.
people now know that the virus hits many types of birds particularly hard and
that dead birds in a neighborhood may mean that mosquitoes carrying the virus
are in the area. Most of the time, the bird's death was not caused by the virus.
However, if you see a dead bird, you should tell your local or state health
department. They may choose to pick up and test the bird for the virus.
the most part, people are tested for WNV infection only if they are severely ill
with central nervous system infection (showing symptoms of encephalitis or
meningitis, often accompanied by muscle weakness, fever, headache and
disorientation). Or if they are participating
in one of the several epidemiological studies to determine such things as the
spread of WNV, percentage of people infected in "hotspot" localities,
and percentage of the infected who show mild flu-like symptoms or the more
results of tests for WNV infection are important to scientists and medical
doctors studying the disease, they are not particularly useful to the
individuals who don't feel well or who are concerned because they have been
bitten by a mosquito. This is because;
is no particular cure for West Nile Virus--so a sick person does not become
better off by being tested for WNV infection. (If you become seriously ill, you
should see your doctor, no matter what the cause of the illness. If you show
signs of WNV, you will be treated for relief of your symptoms.
(2) Being bitten
by a mosquito should not cause undue alarm because most mosquitoes are not
infected with WNV. Most of the people who are bitten by an infected mosquito are
protected by their immune system and will not get sick. A 1999 study in Queens,
NY found that about 30% of people infected reported minor "flu-like"
symptoms. Several studies have indicated that about 1 infected person in 150
becomes seriously ill. Older people are at higher risk for becoming ill, but
children are not in a higher risk group. During the past three years, 62 people
were hospitalized with WNV in the US in 1999 (7 died), 20 were hospitalized in
2000 (2 died) and,
as of September 25, 2001, 29 people had been hospitalized. One has died from the
virus and another died from other causes. The ill are concentrated in the
initial NYC metropolitan area epicenter and in two areas of Florida.
of Nov 15, 2001 47 people have been hospitalized, including 4 who have died, in
2001 due to WNV. A fifth person with WNV died due to other causes.]
has also been confirmed in about 100 horses, most from Northern Florida and
nearby areas in Georgia, as well as in 6 other mammal species, nearly 80 bird
species and about a dozen species of mosquitoes. [As of Oct 31 USDA APHIS
reports 344 infected horses, about 20% of which have died or been euthanized.]
the Northeast, WNV has been most closely associated with the Culex pipiens
pipiens mosquito-a species that breeds in standing water, especially in water
polluted with decaying plants, animal wastes or other organic matter. While this
species primarily bite birds, it is thought that people and domestic animals are
either more likely to be bitten by C. p. pipiens if breeding sites are available
near homes and animal enclosures, and/or that infection will spread into
mosquito species that do "prefer" to bite mammals. Thus it is a good
idea to heed the oft-cited precautions about eliminating or refreshing standing
water to prevent mosquito breeding and to avoid places where and when mosquitoes
Reducing Personal and Community Risk from West Nile Virus
hygiene" includes source reduction of mosquito breeding sites and avoidance
of biting mosquitoes--Both are key to reducing risk from WNV. Eliminating
mosquito breeding sites is also key to reducing risk of pesticide
exposure--because most communities will not consider applying mosquito
adulticides unless WNV is detected in local
bird or mosquito populations!
breed in wet areas, and Culex are found particularly where there is decaying
organic matter (e.g., leaves, grass clippings, animal wastes). There does not
have to be much water and the water does not have to be left standing for very
long--Some species can reproduce within a week! Thus, throughout the mosquito
breeding season in your area (and especially after each rainstorm, drizzle,
watering of the garden or washing the car):
Eliminate or empty the "artificial water-collecting containers" that
are prime breeding spots for the mosquito species implicated in transmission of
West Nile Virus
Clean out rain gutters
Aerate swimming pools and ponds (and perhaps stock with mosquito-eating fish)
Empty unused buckets, water troughs, etc.
Keep unused tires under cover so they do not collect water
Drill drainage holes in tires and other containers used in construction sites,
farms, gardens and play areas
Clean bird baths and animal water bowls at least once a week
Avoid mosquito bites by wearing long clothes and/or by using insect repellent
when out after dusk or in shaded areas (such as woods) during the daytime. This
is when and where most vector species are more likely to bite. People should be
especially careful when in "mass gatherings" where the CO2 given off
by the crowd attracts more mosquitoes from greater distance.
West Nile Virus as a Health Risk for People
Birds are far more likely to become infected than people (or other mammals)
Birds are far more likely than people to become sickened by West Nile Virus
Most mosquitoes are not infected with West Nile Virus
Most mosquito bites will not lead to a West Nile Virus infection
If past outbreaks in the U.S. are any guide, West Nile Virus in the Northeast
U.S. is most likely to bridge into the human population in August and September
The virus incubation period is 1-6 days after being bitten by an infected
Most people who become infected do not get sick
About 30% of people infected in 1999 reported minor "flu-like"
In U.S. outbreaks, fewer than 1% of infected people became seriously ill, with
central nervous system infection (encephalitis and/or meningitis). About 1
infected person in 150 became seriously ill
Older people and the immuno-compromised are at greater risk of becoming
seriously ill from West Nile Virus
Children are not in a higher risk group
Symptoms of serious illness: fever, disorientation, muscle weakness, neck
stiffness, headache, nausea
Whether or not these serious symptoms are caused by West Nile Virus, they should
be treated. If you or someone you care about becomes seriously ill, go to your
There is no vaccine or medication specific to West Nile Virus -- Treat the
Of 62 hospitalized in 1999 with West Nile Virus, 7 died. In 2000, of 20
hospitalized with West Nile Virus, 2 died. One person has died thus far in 2001.
One "model" for predicting a WNV outbreak in the human population uses
1.5 WNV-positive dead crows per square mile as the threshold number indicating
likelihood of a human outbreak. Another model suggests that hot, dry summers
with few birds are the best indicator of greater human risk.
NPIC's West Nile Virus Resource Guide -
National Pesticide Information Center (NPIC) provides access to
information about pesticides and toxicology; mosquitoes and West Nile virus;
effects of WNV on humans, animals, and wildlife; contacts to local, state, and
federal organizations, maps and statistics.
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