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Natural Cycle and Transmission
West Nile virus (WNV) is a flavivirus first isolated in 1937 in Uganda. It has
since been seen in Africa, Asia, the middle east, the Mediterranean region of Europe
and recently in the United States (first isolated in New York in 1999).
Although WNV has been identified in many wild and domestic animals, including humans,
birds act as the natural host. WNV is spread through mosquito bites, therefore the
risk of transmission is greatest from May to October when mosquitoes are most active.
The virus is unable to adequately replicate in animals other than birds. When non-avian
animals are infected, it is a mistake in the lifecycle of the virus. They act as
a dead end host, meaning the virus is unable to infect a mosquito and continue on
to the next host. There is variability in the susceptibility of different avian
species to WNV. Some species such as corvids (crows, jays, ravens, magpies) and
raptors (hawks, owls, eagles) are more susceptible.
Clinical Signs
Infected animals, including birds, typically do not develop obvious disease associated
with WNV. In areas where WNV is most prevalent, disease and death are most commonly
seen in susceptible young birds while adults do not show signs of disease. In non-avian
species, when infection causes disease, clinical signs are usually limited to flu
like symptoms (lethargy, fever, malaise, inappetence, nausea, swollen lymph nodes).
Unfortunately most affected birds are found dead so symptoms are not often seen.
WNV can cause inflammation of the brain (encephalitis) and neurological signs. If
this occurs, disorientation, instability (ataxia), inability to rise, muscle tremors,
blindness, and seizures may be observed. Incubation time in birds is about 4 days.
Diagnosis
Exposure to WNV in mammals can be detected using a blood test, however this is
not yet available for birds. After death, the virus can be demonstrated in the brain
and spinal chord, however, animals suspected of having WNV should be necropsied (autopsied)
at the state laboratory.
Treatment
There is no specific treatment for WNV, but supportive therapy according to clinical
signs (often fluids and nutritional support) can be of significant benefit.
Prevention
The best method of prevention is to decrease an animal's exposure to mosquitoes
by limiting time outdoors between dusk and dawn when mosquitoes are most active and
reducing the mosquito population around your home. Eliminate standing water by disposing
of containers around property in which water may accumulate, cleaning gutters, changing
the water in aviary water vessels daily, stocking ponds with mosquito fish, and landscaping
to limit puddle accumulation. Window and door screens may also help limit your pet's
exposure to mosquitoes.
For More Information
Center of Disease Control: http://www.cdc.gov
CA WNV Surveillance Information Center: http://www.westnile.ca.gov
California Department of Health Services: http://www.dhs.cahwnet.gov
Reporting of dead corvids and birds of prey: (877)WNV-BIRD or (800)968-2473
Mosquito and vector control Association of CA: http://mvcac.org
Marin-Sonoma mosquito and Vector Control: (707)285-2200
Equine information: http://www.cdfa.ca.gov/ahfss/ah/wnv_info.htm
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