Biology Reference
In-Depth Information
The rabies virus and West Nile virus, disease agents for both animals and
humans, are two examples of agents that have invaded new geographical
areas and infected new hosts and vectors.
Rabies and related diseases are caused by single-stranded RNA viruses
of the genus
Lyssavirus
(Krebs et al. 1995). Rabies in terrestrial mammals,
occasionally including humans, is caused by a strain designated Geno-
type 1.This strain was introduced to Africa and the Americas by infected
dogs transported from Europe by early colonists. Because of vaccination
programs for domestic animals, however, rabies disease reservoirs in most
regions are now in wild mammals.
In North America, raccoons (
Procyon lotor
), foxes (
Vulpes
spp.), skunks
(
Mephitis
spp.), and coyotes (
Canis latrans
) are the main terrestrial reser-
voirs for rabies. These species tend to possess distinct genetic variants of
the virus (Krebs et al. 1995). Different variants exist in populations of
skunks in north-central and south-central regions of the United States.
These strains differ from those in raccoons in eastern states. Strains slightly
different from each other are found in foxes in Alaska,Texas, and Arizona.
Thus, some evolutionary specialization has occurred since introduction of
the virus to North America.
West Nile virus, which appeared in New York City in 1999, is proba-
bly also undergoing adaptation to bird species that are vectors for the
virus in North America. One of the early signs of the presence of this
virus was mortality of large numbers of American crows (
Corvus
brachyrhynchos
) and other birds (Rappole et al. 2000). In October 2002, for
example, surveys in Lake County, Illinois, where human cases of this viral
disease were frequent, revealed greatly reduced populations of crows, jays,
and some smaller songbirds (Bonter and Hochachka 2003). In the Old
World, heavy mortality of adult birds does not usually accompany out-
breaks of West Nile virus infection in humans. North American birds do
not appear to be as adapted to the virus as are Old World species. The
house sparrow (
Passer domesticus
), an introduced alien native to Europe,
may, in fact, be one of the species that has helped disperse the disease
because it can carry active virus for a prolonged period (Rappole and
Hubálek 2003). Nevertheless, the appearance of the disease in a human
resident of Cayman Brac, in the Caribbean Sea, indicates that some long-
distance migrant bird is a carrier. Severe declines of wintering North
American migrant birds in Costa Rica in winter 2002-03 suggested that
these birds may not only have introduced the virus to the neotropics but
also suffered heavy mortality (Causey et al. 2003).
