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West Nile Virus
in the United States
as of January 21, 2004

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 Web site Animal and Plant Health Inspection Service (APHIS).

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west nile virus

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West Nile Virus
Questions and Answers

What is West Nile Virus?
West Nile Virus (WNV) is one of several viruses spread by mosquitoes.  It is closely related to St. Louis encephalitis (SLE) virus. SLE virus is most prevalent in the Southeast and Midwest U.S.   WNV is also found in Africa, Europe, and Asia (primarily in countries bordering the Mediterranean). Infection with this virus does not always result in clinical disease. Studies have shown that normally only a small percentage of humans infected with the virus will show symptoms of disease and even fewer will develop any serious complications.

Should I be worried about West Nile Virus?
No.  Studies have shown that normally only a small percentage of humans infected with the virus will show symptoms of disease and even fewer will develop any serious complications.  The disease caused by West Nile Virus is very similar to St. Louis Encephalitis (SLE), but is generally milder. SLE has been in the US for several years. Human infections with these mosquito-borne viruses are very rare and people can further reduce the risk by taking measures to avoid mosquito bites.

Is there a treatment for West Nile Virus for humans?
There is no specific treatment, medication, or cure for illnesses caused by West Nile Virus
However, the symptoms and complications of the disease can be treated. Most people who get the illness recover from it.

Is there a vaccine to protect humans from West Nile virus?
There is not a vaccine to protect humans against West Nile virus

Should I keep my children indoors?
No. The risks of acquiring a mosquito-borne disease are so low that staying indoors is unnecessary. However, if you are in an area with a lot of mosquitoes, you can take some precautions while you are outside, such as wearing long sleeves and long pants and applying an insect repellent (containing DEET) according to label directions.

A mosquito bit my child. Should he be tested for West Nile Virus?
Even if you live in an area where mosquitoes are known to carry West Nile Virus
or other viruses, very few mosquitoes will actually be infected and capable of transmitting the viruses to humans.  Even if an infected mosquito bites you, chances of becoming ill are very low.  If an illness does occur after a mosquito bite, particularly with fever, confusion, muscle weakness, or severe headaches, or if your eyes become unusually sensitive to light, you should consult your physician. Your health care provider will determine what kind of treatment you require and whether or not you should have any specific laboratory tests performed.

Why isn't my neighborhood being sprayed for mosquitoes?
Health officials in your county will likely compare the risks of West Nile Virus
infection and the benefits of applying mosquito-killing sprays in your community before providing a recommendation.  The most effective means of prevention is the abatement of mosquito breeding areas and larvicidal applications, when appropriate.  Broad applications of pesticides by aerial spraying are not encouraged because it´s neither the most beneficial (because of it´s short duration) nor the most cost-effective way to control mosquito populations or mosquito-borne diseases.  It can also harm beneficial insects.  There are also concerns about people with chemical sensitivities and lung problems. 

I have seen dead birds. Should I report them?
If you find dead birds, contact your county health department and ask them if they need them for WNV testing.  Health officials are testing crows, blue jays, and raptors (hawks) to determine where the virus
may have spread. They may ask you to bring them in or they may have all the samples from your area they need for that week.  Remember that birds may die for many reasons. Some birds die of old age, some are hit by cars or run into power lines, and some die of other causes. 

Will the dead birds in my area make me sick from WNV?
No. The viral infection is spread from live birds to people by the bite of a mosquito.

I sometimes set my pet canary outside while I am working in my garden.  Is it susceptible?
Yes.  It may be best to keep birds indoors during peak mosquito feeding times (dusk) and to check the integrity of window screens.  If you want your pet bird to have some fresh air, keep it in a screened-in area or consider mosquito netting over its enclosure. 

What about chickens?
Although wild birds have been killed by WNV, it is unlikely the disease would seriously impact the poultry industry. The disease has not proven to be a significant disease in poultry, and since most of the U.S. chickens are housed inside, the risk is even less.

Can West Nile Virus cause illness in my cat or dog?
This is very, very unlikely.  There are no confirmed clinical cases of WNV in dogs.  Although two cats from New Jersey were reported with the disease (one in 1999 and one in 2000), more research is needed to determine if WNV is of significant risk to cats.  With the virus
first appearing two years ago in New York City and nearby areas with high dog and cat populations, there likely would be more clinical evidence of disease if dogs and cats were susceptible. 

Should I use insect repellent on my dog?
There are insect repellents available for pets.  Be sure to follow the label directions.  Products intended for humans can be toxic to pets, however. Therefore, human repellants should not be used on animals without a veterinarian's recommendation. 

Can West Nile Virus affect horses?
Yes.  Horses can become sick and recover or possibly die from WNV.  It is likely that many horses become infected with the virus but recover without showing any clinical signs of the disease.

Is there a vaccine to protect horses from WNV?
The U.S. Department of Agriculture has approved a conditional license for a vaccine for horses against WNV.   It is uncertain how effective this new vaccine is.  Even if your horses are vaccinated with the new vaccine, we advise horse owners to keep all their other vaccinations up to date and practice mosquito control around their barns and stables.  (See our press release about mosquito control for horse owners.)

Where can I get this vaccine?
The vaccine will be available through your veterinarian.

Will vaccinating my horse against Eastern or Western Equine Encephalitis protect against West Nile Virus?
No. The equine encephalitis viruses and WNV belong to different families, so the vaccines are not expected to provide cross-protection.

Will other states quarantine against my horse if it is infected?
Infected horses do not appear to be carriers of WNV so a quarantine is not helpful in controlling the disease.

Will boarding an infected horse next to uninfected horses cause them to get the disease as well?  Can a mosquito bite an infected horse and spread the disease to other horses or to humans?
Boarding an uninfected horse with an infected one will not lead to the other one becoming infected.  A mosquito that is not already carrying West Nile Virus
cannot pick up the virus by biting a horse or a human.  The virus is not concentrated enough in the blood of humans or horses to be spread this way.

Will West Nile Virus affect my cattle?

Will an electric ``bug zapper" help control mosquitoes
in my yard or barn?
No. These are a waste of money.  In fact, bug zappers actually make things worse by attracting more mosquitoes to the area, and they end up killing thousands of beneficial insects that do not bother people.  Another drawback is the electric grid that kills the insects can cause the trapped insect to explode, spraying harmful bacteria into the surrounding area.  Not a pleasant thought as you bite into that hamburger you grilled near your bug zapper.

What about machines that emit ultrasound to keep mosquitoes away?
They are a waste of money.

I saw an advertisement for a type of scented geranium that would help repel mosquitoes.  Will this work? 
The oils in some scented geraniums may have some mild mosquito-repellent properties if you rub them on your skin.  The ads are totally false that claim if you grow these ``mosquito plants" they will repel mosquitoes.  Planting a whole garden of them would not deter even one mosquito. 

Are citronella candles and coils effective at repelling mosquitoes?
Their effectiveness is limited to small areas (a few feet) and windless conditions. 

I would like to put up a bat house for bats to roost in.  I hear they eat lots of mosquitoes.  Do you have any information?
can eat a lot of mosquitoes and other insects as well.  There are estimates that some species can capture 500 to 1,000 mosquitoes per hour.  In many areas bat populations have been decimated by loss of habitat and ignorant destruction.  Installing a bat house is a way to help these misunderstood and maligned creatures make a comeback.   But will installing a bat house solve your mosquito problems?  It’s a first step in the right direction. First of all, there have to be bats
in your area looking for a place to roost and your bat house must meet all the requirements as to design and location for the bats to take up long-term residence there.  If bats move into your bat house, do not expect them to stay within your property during their nightly forays and feed only on mosquitoes that are trying to feed on you.

I hear purple martins eat a lot of mosquitoes. Is this true?
Attracting a colony of purple martins with gourds or a purple martin house is a good way to enjoy the song and aerial acrobatics of these beautiful birds.  They eat many flying insects but not the large numbers of mosquitoes so often claimed by companies that manufacture purple martin housing.  For more information about these birds, contact the Purple Martin Conservation Association, Edinboro University of Pennsylvania, Edinboro, PA, 16444 (Telephone: 814-734-4420)

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