Here
in the Midwest we are blessed with more than 25% of the freshwater on the
entire planet – the Great Lakes and St. Lawrence River ecosystem. Human
exploitation of this resource, once deemed as vast as the endless forests
of North America, has introduced numerous invasive species to this system
that we now know is finite and fragile (e.g., the sea lamprey, zebra
mussel, round goby). The latest invader is the viral hemorrhagic
septicemia (VHS) virus, a dangerous pathogen of fresh and saltwater
fish. Different strains of the VHS virus occur in different regions of
the oceans and affect a variety of species; none appear to affect humans.
As the name implies, the virus causes internal hemorrhaging due to the
presence of the pathogenic virus in the blood of infected fish.
VHS has been known in
western Europe since the 1950s when it was documented among cultured
salmonids. By 1988 a marine-stable strain designated Type IV, distinct
from the freshwater European strain, had been found in migrating Pacific
salmon returning to Washington rivers. Subsequent studies continued to
find the Type IV virus around the circumference of the northern
hemisphere, including the northern Atlantic coast of North America
infecting a variety of species such as Atlantic herring, a baitfish, and
striped bass. Ongoing studies have found the virus in new areas and new
species of fish. The first freshwater detection of VHS in North America
(Type IVb) was made in Lake Ontario in 2005; this discovery prompted
scientists to examine archived tissue samples, revealing the presence of
the virus in the Great Lakes system as early as 2003 from a Lake St. Clair
musky sample. How the VNS virus spread to the Great Lakes is unknown, but
it has been documented throughout the system, with the exception of Lake
Superior. In 2006 and 2007 large die-offs of several species of fish,
including some of the most sought game fish, have made the news; the list
of infected freshwater species continues to grow: lake trout, steelhead,
Chinook salmon, yellow perch, muskellunge, smallmouth bass, northern pike,
yellow perch, walleye pike, black crappie, bluegill, rock bass, white
bass, redhorse and bluntnose suckers, drum, gizzard shad, round goby,
emerald shiner, and whitefish. It is quickly obvious that the VHS virus
is not a host-specific pathogen and has the potential to adversely impact
the freshwater food chain from bottom to top.
The course of VHS
infection can be either chronic or acute, with some carriers showing no
symptoms at all. The National Center for Research on Aquatic Invasive
Species describes the chronic infection as producing hyperactive, nervous
circular or corkscrew swimming behavior. However, fish acutely infected
are lethargic, dark colored, with bulging, hemorrhagic eyes and skin,
gill, muscle, fin and visceral bleeding. Organs usually affected are
liver, kidney, spleen, and skeletal muscle. Surviving fish can carry and
shed the virus for the remainder of their lives. Transmission is believed
to occur via waste and reproductive products that infect via gills,
wounds, or possibly fin bases. Because of optimum replication temperature
for the virus, fish mortality occurs between about 37 – 54 degrees F.
Scientists speculate that the virus may be widespread – many species are
affected and some individuals appear healthy – but the disease may
manifest itself in mortalities only when environmental conditions are
conducive. The virus has been demonstrated to survive freezing and
thawing, suggesting that both live and frozen bait can contribute to its
spread. Many of the Great Lakes states have enacted regulations
restricting the movement of bait fish and fish stocking to prevent the
spread of VHS. A U.S. Federal Order also restricts the interstate
movement of live, susceptible species:
http://www.aphis.usda.gov/vs/aqua/pdf/vhs-fed-order_odc-changes.pdf
To prevent the spread
of this virus, Midwest recreational boaters and fishermen should clean
fishing equipment, boats, and trailers with a 10% bleach solution before
entering a new body of water; this solution is also effective against
other aquatic nuisance species such as zebra mussels. Also, fish should
never be transferred from one body of water to another.
