Since the introduction of West Nile virus (WNV)
into the United States in 1999, public concern and media attention have
been focused primarily on the health threat of the disease to humans;
however, the potential impacts on vertebrate wildlife are also significant.
Although humans, horses, and other animals can become ill from WNV, birds
are the natural host for the virus. High avian mortality, not observed
in WNV outbreaks in the eastern hemisphere,is being observed in the United
States and Canada. Highest mortality has been observed in birds in the
corvid family (crows, jays, and related species). Some raptor species
also appear highly susceptible to WNV. The range of species affected,
and the extent to which WNV causes direct mortality, or increases susceptibility
to other stressors, remains unknown. Thus far, WNV has been detected in
over 290avian and 34 mammalian species, as well as two species of captive
reptiles. Since most surveillance programs to date have focused primarily
on corvid species and raptors, the list of affected species will likely
continue to increase as more species are tested.
Impacts of WNV on wildlife populations have not
been determined. In 2002, approximately 125,000 dead birds were reported
to public health and wildlife agencies. Over 30,000 of these birds were
tested for WNV, and about 16,000 were found positive for WNV. Because
of the large number of dead birds that are never found, these reports
represent only a fraction of the number of dead birds in the wild. Some
reports estimate the number of birds that potentially died of WNV at well
over a million. However, because birds tested for WNV were not evaluated
for other causes of death, the role of the virus as a source of mortality
is yet to be determined.
Some anecdotal reports suggest localized declines
in bird populations, but efforts to evaluate the impact of WNV are ongoing.
As the disease occurrence increases in the western United States, the
number of corvid and raptor species as well at threatened and endangered
species potentially at risk increases substantially. With limited management
options, some zoos have vaccinated rare birds with the equine WNV vaccine.
However, the efficacy of this vaccine appears minimal and no vaccines
are approved for use in birds. New vaccines are under development and
testing, but none are currently available for general use.
Monitoring bird populations and dead bird surveillance
are important wildlife management activities that can be performed by
NPS staff. Parks also offer excellent opportunities for use as outdoor
laboratories for cooperative research on WNV. Park staff are encouraged
to be observant for dead birds and to submit samples for diagnostic testing.
Possession of migratory birds for submission for diagnostic testing within
48 hours is permissible under the Migratory Bird Treaty Act. Because WNV
can be transmitted by infected tissues as well as the bite of infective
mosquitoes, personal protective equipment (e.g., gloves) should be used
when handling dead birds and diagnostic samples.
Much remains unknown about the ecology and epizootiology
of WNV in North America. Although the impacts of the disease remain unknown,
one thing that experts agree on is that WNV is here to stay.
Wildlife Health Center