West
Nile Virus Home
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Ecology and Virology > Vertebrate Ecology
Vertebrate
Ecology
Transmission
Cycle
West
Nile (WN) virus is amplified during periods of adult mosquito
blood-feeding by continuous transmission between mosquito vectors
and bird reservoir hosts. Infectious mosquitoes carry virus particles
in their salivary glands and infect susceptible bird species during
blood-meal feeding. Competent bird reservoirs will sustain an
infectious viremia (virus circulating in the bloodstream) for
1 to 4 days after exposure, after which these hosts develop life-long
immunity. A sufficient number of vectors must feed on an infectious
host to ensure that some survive long enough to feed again on
a susceptible reservoir host.
People, horses, and most other mammals are not known to develop
infectious-level viremias very often, and thus are probably "dead-end"
or incidental-hosts.
Birds
West
Nile virus has been detected in dead birds of at least 317 species.
Although birds, particularly crows and jays, infected with WN
virus can die or become ill, most infected birds do survive. Click
here for more information on species of dead birds in the U.S.
in which West Nile virus has been detected.
There
is no evidence that a person can get WN virus from handling live
or dead infected birds. Persons should avoid bare-handed contact
when handling any dead animals, and use gloves or double plastic
bags to place the bird carcass in a garbage bag or contact their
local health department for guidance.
Dogs and
Cats
West
Nile virus does not appear to cause extensive illness in dogs
or cats. There is a single published report of WN virus isolated
from a dog in southern Africa (Botswana) in 1982. West Nile virus
was isolated from a single dead cat in 1999. A serosurvey
in New York City of dogs in the 1999 epidemic area indicated that
dogs are frequently infected. Nonetheless, disease from WN virus
infection in dogs has yet to be documented.
There is no documented evidence of person-to-person or animal-to-person
transmission of WN virus. Because WN virus is transmitted by infectious
mosquitoes, dogs or cats could be exposed to the virus in the
same way humans become infected. Veterinarians should take normal
infection control precautions when caring for an animal suspected
to have this or any viral infection. It is possible that dogs
and cats could become infected by eating dead infected animals
such as birds, but this is undocumented.
There
is no reason to destroy an animal just because it has been infected
with WN virus. Full recovery from the infection is likely. Treatment
would be supportive and consistent with standard veterinary practices
for animals infected with a viral agent.
Horses
Cases
of WN virus disease in horses have been documented, either by
virus isolation or by detection of WN virus-neutralizing antibodies
in 1999, 2000, and 2001. Approximately 40% of equine WN virus
cases results in the death of the horse. Horses most likely become
infected with WN virus in the same way humans become infected,
by the bite of infectious mosquitoes.
In
locations where WN virus is circulating, horses should be protected
from mosquito bites as much as possible. Horses vaccinated against
eastern equine encephalitis (EEE), western equine encephalitis
(WEE), and Venezuelan equine encephalitis (VEE) are NOT protected
against WN virus infection. A West Nile virus vaccine for horses
was recently licensed, but its effectiveness is unknown. Horses
infected by WN virus develop a brief low-level viremia that is
rarely, if ever, infectious to mosquitoes. There is no reason
to destroy a horse just because it has been infected with WN 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.
Other Vertebrates
Through
December 2001, CDC has also received a small number of reports
of WN virus infection in bats, a chipmunk, a skunk, a squirrel,
and a domestic rabbit.
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