West Nile Virus and Mosquito-Borne
Viruses in Colorado
What are the mosquito-borne viruses in Colorado?
There are currently four viruses in Colorado that
are transmitted by mosquitoes.
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Western equine encephalitis (WEE) is
distributed across the central and western United States.
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St. Louis encephalitis (SLE) is found
throughout the continental United States.
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California encephalitis viruses are a group
of several viruses found throughout the U.S.
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West Nile (WN) virus historically occurred
in parts of Asia, Eastern Europe, Africa, and the Middle East.
This virus was first detected in the United States in 1999 during
an outbreak in New York City.
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How Are People and Animals Infected with these
Viruses?
These viruses are transmitted to people and animals
by bites from infected mosquitoes. Only certain species of
mosquitoes
carry the virus and very few mosquitoes actually are
infected.
In Colorado, these viruses are transmitted to
people by a species called Culex tarsalis, a
medium-sized mosquito that feeds in the few hours around dawn and
dusk. During the day they rest in shady, secluded
areas, such as under porches, roof overhangs, tall grass, shrubs, and
storm sewers.
They breed in almost any source of
standing water, including irrigated fields, old tires, hoof prints,
flowerpots, tree holes, or any puddle of water that
lasts for more than a few days.
These viruses are maintained in a bird-mosquito-bird
cycle. Mosquitoes are infected by feeding on a bird with virus
in its blood.
The virus is transmitted to a new host
in the mosquito’s saliva when the insect bites another person or
animal. Humans and horses are incidental dead-end
hosts in that they cannot infect other mosquitoes.
Person-to-person
transmission does not occur. These viruses are
prevalent from May to September when mosquitoes are most
abundant, but the risk to humans occurs primarily from
August through early September.
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What are the Symptoms?
Most people who are infected with mosquito-borne
viruses do not become ill and have no symptoms. For persons who do become ill, the time between the mosquito bite
and the onset of symptoms, known as the incubation period, ranges from 5-15 days.
Two clinically different types of disease occur in
humans: (1) viral fever syndrome, and (2) encephalitis, an
inflammation of the brain. Symptoms of the viral fever syndrome
include fever, headache, and malaise. These symptoms persist for a about 2-7 days.
In rare cases, the virus can cause a more serious
brain infection such as aseptic meningitis or encephalitis. These
infections begin with a sudden onset of high fever and a
headache, and then may progress to stiff neck, disorientation, tremors, and coma. Severe infections can result in
permanent brain damage or death. Most deaths occur in persons over 50 years of age. There is no specific treatment
for infection with these viruses except supportive care.
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Which animals get infected with
these viruses?
An infected mosquito can bite any animal, but not all
animals will become ill. As the reservoir host of these viruses, birds are most often infected, but other animals can
be infected and become ill as well.
Horses
Horses are susceptible to infection with WEE and WN
viruses, but not SLE. Another virus, eastern equine encephalitis
(EEE) is not found in Colorado, but could be a problem
if a horse travels to the eastern U.S. These diseases do not seem to be specific to a particular breed or age of
horse. Clinical signs in a horse can include lack of coordination or muscle control, weakness of limbs, inability to rise,
and death. Fever has been detected in less than one-quarter of all confirmed cases of WN virus. WEE, EEE, and WN virus
vaccines are available for horses through veterinarians.
Birds
Mosquitoes acquire the viruses from wild birds.
Infection has been reported in more than 70 bird species. With WEE and SLE, infected birds will not appear ill
or die. However, WN virus is new to this country and does cause
illness and death in native birds that have no natural
resistance to infection. The highest death rates are seen among birds in the corvid family, which includes
crows, magpies, ravens, and jays. American crows constitute the
majority of birds reported positive for WN virus.
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Components of the Colorado
Mosquito-Borne Virus Surveillance Program
Local health departments and the Colorado Department
of Public Health and Environment have conducted a statewide mosquito-borne encephalitis surveillance program since
1988 for WEE and SLE. In 2001, the program expanded to detect WN virus.
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Sentinel Chicken Testing
Chicken flocks are strategically placed throughout the
state and are tested bi-weekly during the mosquito season to detect evidence of infection with WEE, SLE, and WN
viruses.
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Mosquito Surveillance
Mosquitoes are collected to monitor the abundance and
type of mosquitoes in the area and some are sampled for the presence of WN, WEE, and SLE viruses.
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Dead Corvid Testing
Colorado began to test dead corvids for WN virus in
2001. Birds that have died within the previous two days are collected and submitted to the state laboratory by the local
health department or animal control agency. Persons finding a dead crow should use gloves when handling a carcass.
Information on the collection and submission of a dead corvid is available on the Colorado Department of Public
Health and Environment Web site.
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Equine Case Surveillance
Horses can become ill and die from infections with WEE
and WN. The occurrence of horse cases in an area indicates significant virus transmission and can indicate an
increased risk to humans. Veterinarians are required to report suspect
cases and are encouraged to submit blood samples for
testing.
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Human Case Surveillance
Cases of encephalitis suspected of being caused by
these viruses are physician- reportable conditions under Colorado
law. The Colorado Department of Public Health and
Environment lab offers testing for WEE, SLE, and WN virus on any suspected cases.
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Mosquito-Borne Virus Prevention
and Control
To decrease exposure to mosquitoes and the
viruses they may carry:
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Limit outside activity around dawn and dusk when Culex
tarsalis mosquitoes feed. This is particularly important
for elderly adults and small children.
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Wear protective clothing such as lightweight long
pants and long sleeve shirts when outside.
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Apply insect repellant to exposed skin when
outside. Repellents with DEET are effective, but should be applied
sparingly.
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Products with 10% or less of DEET are recommended for
children.
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Make sure that doors and windows have tight-
fitting screens. Repair or replace screens that have tears or holes in
them.
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Drain all standing water on private property, no
matter how small an amount.
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Stock permanent ponds or fountains with fish that
eat mosquito larvae.
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Change water in birdbaths or wading pools and empty
flowerpot saucers of standing water at least once a week.
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Check around faucets and air conditioner units and
repair leaks or puddles that remain for several days.
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Make sure roof gutters drain properly and remove
any standing water under or around structures or on flat roofs.
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Remove items that could collect water such as old
tires, buckets, empty cans, and food and beverage containers.
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Eliminate seepage and standing water from cisterns,
cesspools, septic tanks, and animal watering tanks.
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Do not over-water lawns and gardens to prevent
standing water.
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For additional information or to report a dead corvid,
contact Boulder County Health Department 3450 Broadway, Boulder, CO 80304 at 303-441-1564.
Colorado surveillance data for mosquito-borne viruses
is available on the Colorado
Department of Public Health and Environment Web site.
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