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Viral Hemorrhagic Septicemia Arrives in the Midwest

By Dave Votaw

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.

Additional information on VHS is available here and here.

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