·
What is
West Nile Virus?
o
West
Nile Virus is a virus of the family Flaviviridae, specifically of the genus
Flavivirus Japanese Encephalitis Antigenic Complex. Flaviviruses are about 40-60nm in size,
enveloped and contain positive-sense single stranded RNA.
·
Where
does West Nile Virus occur?
o
West
Nile virus exists in Europe, Africa, the Middle East, west and central Oceania. West Nile Virus was first documented in the
United States in 1999 and since then, cases of West Nile Virus have been
documented in every state except Oregon, Hawaii, and Alaska.
 |
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West Nile virus (WNV) activity reported to
ArboNET, by state, United States, 2012 (as of December 11, 2012)
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·
How is
West Nile Virus transmitted?
o
There
are at least 64 different species of mosquitos that can carry West Nile Virus. Birds serve as the reservoir host for West
Nile Virus, which is then amplified by adult mosquitos following blood
feeding. Mosquitos serve as the primary
vector for transmission of West Nile Virus and carry the virus particles in
their salivary glands. When a mosquito
feeds on a bird, the virus is transmitted to the bird where it causes an
infectious viremia for 1-4 days. After
the viremia subsides in the bird, it develops life-long immunity. Among bird species, geese have been
documented as the only natural host of West Nile Virus, but house sparrows are
noted as the most important reservoir host given the high quantity of virus
they carry once infected and since they do not die from the viral infection like
some other bird species.
There are a very small number of
reported cases of West Nile Virus transmission through blood transfusion,
transplants, nursing, and pregnancies.
·
Who gets
West Nile Virus?
o
West
Nile Virus has been detected in more than 326 species of birds. Some birds, mainly crows and jays, will die
from West Nile Virus, but most birds do survive. People, horses, and most other
mammals are known to be dead-end hosts for West Nile Virus, meaning they can contract
the disease if bitten by an infected mosquito.
Serious disease has not been reported in dogs and cats, but could be
possible if the virus is transmitted from an infected mosquito. Horses are one of the most highly publicized
victims of West Nile Virus, with about 40% of infections in horses resulting in
death.
Humans can also contract West
Nile Virus from infectious mosquito bites.
In 2012, a total of 5,387 cases were reported with 283 deaths. Individuals over the age of 50 are at
increased risk for severe disease.
Infections in chipmunks, bats, squirrels,
skunks, llamas, and rabbits have also been reported infrequently.
·
What are
the clinical signs of West Nile Virus?
o
Clinical
signs of West Nile Virus vary with species affected, but the virus can cause
very serious illness and death. Please
see section on Clinical Presentation for more information.
·
How can
people get West Nile Virus?
o
There is
no evidence that humans can contract West Nile Virus from handling dead or
infected birds, but precautions such as avoiding direct contact with bare hands
should still be employed. There is also
no evidence of direct human to human transmission or animal to human
transmission of West Nile Virus, though blood transfusion/organ transplants and
mother to child transmissions have been reported rarely. The mosquito vector is therefore required to
obtain most active West Nile Virus infections.
·
How is
West Nile Virus diagnosed?
o
Clinical
signs of West Nile Virus allow for presumptive diagnosis. Collection of whole blood of suspected animals
during acute and convalescent stages of the disease and demonstration of a
substantial increase in antibody titers is best for definitive diagnosis of the
disease. Virus isolation, serology,
Real-Time PCR are all methods of diagnosis applicable to West Nile Virus.
·
How do
you treat West Nile Virus?
o
Supportive
care for clinical signs and fluid losses are the main treatments of choice. Virus-specific treatments do not exist for
West Nile Virus, so prevention is crucial.
·
How do
you prevent West Nile Virus?
o
Mosquito
control and the prevention of mosquito bites is the best means to prevent West
Nile Virus infection. Avoiding outdoor
exposures during dawn and dusk when mosquitos are most active, using insect
repellent/protective clothing, and eliminating common mosquito breeding areas
involving standing water are all valid means of decreasing mosquito exposure.
Vaccination for West Nile Virus
in horses in the United States is considered by the AAEP as a core viral
vaccine and should be used in all horses.
A DNA vaccine is licensed for use in horses as well as a canary poxvirus
recombinant vaccine. Regular vaccination of horses is recommended, but should
still be employed with appropriate mosquito control protocols.
There are currently no West Nile
Virus vaccines for humans, but a vaccine may become available in the future.
·
What is
being done to monitor West Nile Virus in the United States?
o
Any
horse that develops CNS signs with suspicion of West Nile Virus should be
quarantined and appropriate state veterinary officials should be notified.
Federal and State government agencies as well
as field veterinarians and physicians monitor and report any cases of West Nile
Virus (in any species) to information databases. The CDC alone has an active surveillance
program that provides weekly updates on reported West Nile Virus cases in all
species.
REFERENCES
Centers for Disease Control and
Prevention, (2012). West nile virus. Retrieved from website: http://www.cdc.gov/ncidod/dvbid/westnile/wnv_factsheet.htm
Subbiah, E. VM 8124 Course Notes, Equine Viruses
I. Spring 2013; Lecture 13.
