WNV is our most recent experience with emerging
mosquito-borne disease. While new to the United States, it is not an emergent
disease outside the Western Hemisphere. The virus was initially isolated
from the blood of a woman in the West Nile District of Uganda in 1937,
and subsequently found in humans, birds, and mosquitoes during studies
conducted in Egypt throughout the 1950s. WNV is closely related to others
in the genus Flavivirus and family Flaviviridae responsible for Japanese
encephalitis, Murray Valley encephalitis and St. Louis encephalitis. Birds
are the natural reservoir host and serve in the amplification of the virus.
WNV is maintained in a mosquito-bird-mosquito epizootic transmission cycle,
while humans (and other mammals) are incidental, dead-end hosts. WNV is
now recognized as the most widespread of all the flaviviruses and has
been isolated in more than 40 species of mosquitoes in at least 30 countries
across Africa, Asia, Europe, and North America.
Since its discovery in 1999, WNV, an exotic species,
has firmly established itself in the continental United States: 44 states
and the District of Columbia report evidence of the virus. However, mosquito-borne
diseases are not new to the United States. There are records of malaria
and yellow fever epidemics as early as the 1690's, and the period between
1793 and 1806 is referred to as the "yellow fever era" in America.
Through persistent public health intervention neither disease is presently
endemic in the United States. Prior to WNV, the last major outbreak of
a mosquito-borne disease in the United States occurred in the Midwest
during 1975, with more than 2,000 documented cases of St. Louis encephalitis
and 142 deaths.
West Nile virus (WNV) poses a threat to wildlife
species as well. 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
species also appear highly susceptible to WNV. Thus far, WNV has been
detected in over 290 bird and 34 mammalian species, as well as two species
of captive reptiles. Some reports estimate the number of birds that potentially
die annually of WNV at well over a million. 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.