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What is it?
West Nile Virus (WNV) is a virus disease of humans
and animals that is usually transmitted by
mosquitoes.
Mosquitoes obtain the virus when they bite infected
birds, which is where the virus is maintained in
nature. WNV is
closely related to St. Louis encephalitis, but it
normally causes a milder disease in people.
Historical Background
Worldwide:
WNV was first identified in 1937
in a woman in Africa. The earliest epidemic of WNV
was reported from Israel in 1950. Since then, additional
epidemics have occurred in Europe, Africa, the
Middle East and North America. United States:
WNV was not known to occur in North America before
1999. It was apparently introduced into New York
City during the summer of that year. By the end of
1999, there had been 25 equine(horse) cases and 62
human cases, of which seven people died. The disease
has spread rather rapidly since the initial epidemic
in the New York City area. In 2000, there were
another 21 human cases from three northeastern
states and 60 more equine cases from seven
northeastern states. WNV-positive birds were
reported in 13 states, the southernmost being North
Carolina. In 2001, WNV spread throughout most of the
eastern half of the United States. WNV-positive
birds were reported from 28 states. There were 738
equine cases in 20 states and 66 human cases in 10
states. Southeastern states that reported human WNV
cases were Florida, Georgia, Alabama and Louisiana.
The continued expansion of WNV suggests that it is
firmly established in the United States.
Louisiana: WNV spread from New York to Louisiana
much faster than originally predicted. The first
sign of the disease was a WNV-positive blue jay
reported in July of 2001. Afterward, there were nine
equine cases from four southern parishes and one
human case in Jefferson Parish (New Orleans area).
As of April 1, there had already been one equine
case and one positive bird
reported in 2002. WNV is expected to remain in
Louisiana.
How did WNV spread so fast?
The rapid spread of WNV can probably be attributed
to infected migratory birds.
Which animals can be infected with
WNV?
Birds are the most commonly infected. Other
animals include equines (horses, mules, donkeys),
bats, chipmunks, skunks, squirrels and domestic
rabbits. From a veterinary perspective, horses are
the most at risk. WNV infections have occurred in
dogs and cats, but these cases are extremely rare,
so WNV is not a serious threat to dogs
and cats.
The Vector: How is the disease transmitted?
WNV is transmitted to people and animals by infected
mosquitoes. Although ticks have been reported to
transmit WNV, too, mosquitoes are by far the most
important vector (carrier).
Which mosquitoes can transmit WNV,
and are they present in Louisiana?
More than 25 species of mosquitoes in the United
States have been found infected with VVNV.
Mosquitoes of the genera Culex are
thought to be the most important group and
potentially the main vectors. Louisiana mosquitoes
that have been found infected with WNV either in
Louisiana or elsewhere are
Aedes albopictus,
Culex restuans, Culex quinquefasciatus, Culex
saliniarus, Culiseta melanura, Psorophora ferox,
Ochlerotatus triseriatus
and Aedes vexans. This
indicates that mosquitoes in Louisiana can transmit
WNV.
The normal disease cycle.
WNV is usually maintained in nature by birds and
mosquitoes. Mosquito-to-bird transmission and
bird-to-mosquito
transmission both occur when mosquitoes take a blood
meal from birds. Although many birds die from
the disease, they probably serve as the natural
reservoir for the virus. Humans and other animals
become involved in
the cycle when they are bitten by an infected
mosquito. They are, however, thought to be
dead-end hosts, meaning they cannot pass the virus
on to other mosquitoes, as birds do.
The Reservoir: How are birds affected?
WNV has been isolated in more than 70 species of birds
found dead in the United States. Certain bird
species, such as crows, appear to be much more
susceptible than others. Large die-offs of crows
have occurred in
some areas. Mortality is less frequent in other bird
species, and these birds probably serve as carriers
of the WNV. Most birds are thought to be infectious for
only four to five days, but some migratory birds can
travel several hundred miles in this short period.
Can people get WNV from birds?
There is no documented evidence that people can get
WNV from handling live or dead infected birds. VVNV
is
transmitted by infected mosquitoes.
The Disease: WNV in People
Most people
infected with WNV will not become ill or severely
ill. Less than 10% of the people who are
infected will develop serious illness. Healthy
children and young adults are in the low-risk
category, but people older
than 50 are considered to be at a higher risk to
develop a serious illness. Symptoms occur three to
15 days after the initial infection. Mild cases
usually result in slight fever and headaches. More
severe cases result in high fever,
head and body aches, disorientation, tremors and
possibly convulsions. Extreme cases can result in
paralysis or
death. Of those people who develop a serious
illness, the mortality rate ranges from 3% to 15%.
Is a human vaccine available?
No.
Can people get WNV from horses or
other infected animals?
No. People may become infected when an infected,
mosquito bites them. There is no documented evidence
of
animal-to-man transmission of WNV.
The Disease WNV in
horses.
Some horses that become infected do not show signs
of illness. Clinical signs of illness in horses may
include
weakness, staggering, mild tremors and a sleepy
appearance. The mortality rate appears to be
35% to 50%. In
reaching a diagnosis, a veterinarian must consider
several diseases including other types of virus
encephalitis: '' rabies and equine protozoal ''moldy
corn poisoning, myeloencephalitis. The veterinarian
can submit a blood
sample for an encephalitis test.
Is a WNV vaccine available to
protect horses?
Yes. Horses vaccinated for the first time must
receive two injection dosages at a three- to
four-week
interval. The two dose series is essential to obtain
an effective immune response. A booster
shot should be given in the spring each year
thereafter. Booster shots may be indicated at
six-month
intervals. Currently, WNV vaccine is available only
through veterinarians.
Will other encephalitis vaccines such
as Eastern Equine (EEE), Western Equine (WEE) and
Venezuelan Equine (VEE) protect my horse from WNV?
No. Your horse must be vaccinated with a WNV
vaccine. Currently, there is a five-in-one
equine vaccine which protects horses against five
different encephalitises, including West Nile
virus. Ask your veterinarian for details on
this vaccine.
What should I do if I suspect WNV
symptoms in my horse?
Contact a local veterinarian to determine the exact
cause of illness. WNV symptoms can mimic other types
of
equine illness. Some horses that become infected do
not show signs of illness. Therefore, the safest
course of action is
to have your horse vaccinated.
Can Mosquito Control (or other
government agency) diagnose my horse for free?
No.
Testing for West Nile Virus and other diseases is
considered part of a veterinarian's care.
Can a horse get WNV frvm another
infected horse?
There is no documented evidence of animal-to-animal
transmission. WNV is transmitted to horses by
infected mosquitoes.
Can people get WNV from caring for or
handling an infected horse?
There is no documented evidence of animal-to-people
transmission. WNV is transmitted to people by
infected mosquitoes.
How can people protect themselves
from WNV?
Since there is no approved vaccine, the best way to
protect yourself from WNV is to protect yourself
from
mosquito bites.
1.
Avoid outdoor activity during periods of heavy
mosquito activity, normally in the early morning and
late afternoon/evening hours.
2.
Use an effective repellent such as DEET to protect
exposed skin areas when working or playing outdoors.
3.
Eliminate standing pools of water and empty
containers that hold water around the home.
Mosquitoes breed in water.
4.
Approved, general use insecticides can be applied
around the perimeter of the house and the home to
eliminate resting areas for mosquitoes.
5.
Small, portable foggers are available to homeowners
for a reasonable price. They can be used to kill
live mosquitoes
around the home premises, but they provide no
residual control.
6.
Wear long sleeves and pants when working outdoors to
discourage mosquito bites.
How can people protect their horses
from WNV?
1.
Have your horse vaccinated with a WNV vaccine as
directed above.
2.
Insecticides will give some protection to horses
from mosquito bites, but this does not guarantee
protection
from WNV. Insecticides are not totally effective,
and they must be applied regularly at short
intervals. They do
not provide total protection, because horses are
constantly exposed to mosquito activity outdoors.
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