Introduction
West Nile Virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in North America. The virus can infect humans, birds, mosquitoes, horses and some other mammals.
West Nile fever is a case of mild disease in people, characterized by flu-like symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.
More severe disease due to a person being infected with this virus can be West Nile encephalitis, West Nile meningitis or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.
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How is West Nile virus transmitted?
The principal
transmission cycle of West Nile virus involves several species of
mosquitoes and various species of birds. Mosquitoes become infected with
West Nile virus when they feed on a bird carrying the virus in its blood.
After 10 to 14 days, the virus can be transmitted to another bird, person,
or other animal that the mosquito bites. During blood feeding the mosquito
injects the virus, contained in its saliva, into the bird, animal, or
person – where the virus replicates and may cause illness.
Although humans and
some other animals may get sick when infected with West Nile virus, it is
thought that they usually do not develop enough virus in the bloodstream
to infect mosquitoes. For this reason, humans and animals such as horses
are referred to as incidental hosts.
West Nile virus is not spread
from person to person.
The main role that
people play in contributing to the continuing cycle of West Nile virus is
by maintaining environments (especially standing water) in which
mosquitoes can lay eggs.
What are the symptoms of West Nile Virus infection?
Most people who are
infected with West Nile virus will not develop symptoms. Some people may
become ill 3 to 15 days after the bite of an infected mosquito. Evidence
suggests that a minority of infected persons will develop a mild illness
with fever, headache, body aches, and sometimes skin rash and swollen
glands. There are no known long-term effects due to mild illness.
West Nile virus
infection infrequently results in a severe illness, however, known as West
Nile encephalitis. Encephalitis is an inflammation of the brain that may
be marked by headache, high fever, stiff neck, stupor, disorientation,
coma, tremors, convulsions, muscle weakness, and paralysis. A small number
of cases have been fatal.
Who is at risk for West Nile virus infection?
People are only at
risk for becoming infected with West Nile virus when in areas where the
virus is circulating in nature. In northern states, this is during the
summer months, but in southern states transmission may occur year-round.
The risk of West Nile encephalitis (the severe form of disease) is higher
for persons 50 years of age and older.
Even in areas where
West Nile virus transmission is occurring, infection of humans is still
relatively rare. A study carried out in 1999 among residents in the most
affected areas of New York City showed that 2.6% had been infected with
West Nile virus. Studies elsewhere have shown a lower infection rate. As
noted earlier, most infected persons have no or only mild symptoms.
There is no
documented evidence that a pregnancy is at risk due to a mother’s
infection with West Nile virus.
How is West Nile virus infection treated?
No specific therapy
is available for infection with West Nile virus. In the case of West Nile
encephalitis, intensive supportive therapy may be required–such as
hospitalization, intravenous fluids, respiratory support (ventilator),
prevention of secondary infections (such as pneumonia, urinary tract
infections, etc.) and good nursing care.
Prevention & Control
Avoid mosquito bites
- Apply insect
repellent to exposed skin when you go outdoors. The most effective
repellents contain DEET (N,N-diethyl-m-toluamide). The more DEET a
repellent contains, the longer time it gives protection from mosquito
bites. DEET concentrations higher than 50% do not increase the length of
protection.
- Use care in
applying repellent to children: don't put repellent on their hands, and
be careful to avoid their mouths and eyes. Products containing 10% or
less DEET are the most appropriate for children aged 2 - 12
years.
- Whenever possible,
wear protective clothing such as long sleeves, long pants, and socks
while outdoors.
- Spray clothing
with repellents containing DEET or permethrin, because mosquitoes may
bite through thin clothing.
- Consider staying
indoors between dusk and dawn, which is peak mosquito biting time. Avoid
activities in areas where mosquitoes are plentiful.
- Fix or install
window and door screens to keep mosquitoes out of buildings.
Always
read and follow the product directions when using any insect repellent or
insecticide!
Avoid breeding mosquitoes
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Mosquitoes lay their
eggs in standing water. To avoid helping mosquitoes breed in your
environment, drain standing water. Routinely empty water from flower pots,
pet bowls, clogged rain gutters, swimming pool covers, discarded tires,
buckets, barrels, cans, and other items that collect water in which
mosquitoes can lay eggs.
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