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Viral Hemorrhagic Septicemia (VHS)
Fish can suffer from two forms of this disease: acute (severe) or chronic (less severe). In the acute form of the disease, fish become listless, dark and anemic, with bulging eyes, congested kidneys, mottled liver, and bleeding in the eyes, skin, gills, fin bases, skeletal muscle and viscera. Mortality is very high and the disease is short-lived. In the chronic form of the disease, mortality is low and all the symptoms are similar to the acute form, except that bleeding is not common; instead, the liver, spleen and kidneys accumulate fluid. The body bloats and the liver and kidneys turn very light in color. Survivors of infection can be carriers of the virus throughout the rest of their lives.
VHS is indigenous to eastern and western Europe, Japan, and the Pacific Coast (from California to Alaska) and Atlantic Coast of North America. Evidence suggests that the European strains of VHS are native to the Atlantic Ocean. It is generally believed that all strains of VHS are derived from a common marine ancestor.
Great Lakes Range:
It is not known how VHS was introduced to the Great Lakes–St. Lawrence River system. Genetic evidence suggests that the virus originated from the Atlantic coast of North America, and was introduced possibly via transport in ballast water or infected migratory fishes.
VHS virus has been present in the Great Lakes since at least 2003. The North American strain of the virus was first isolated from muskellunge caught in the northwest part of Lake St. Clair, Michigan. In 2005, infected freshwater drum and round gobies were captured in the Bay of Quinte, Lake Ontario. In the spring and summer of 2006, VHS was detected in fishes in the Thousands Islands area of the St. Lawrence River and in Lake St. Clair, Lake Erie and Lake Ontario. The virus has also been detected in lake whitefish and chinook salmon from Lake Huron. All viruses from these areas belong to the same sub-grouping of the North American genotype.
VHS occurs in both marine and fresh water environments. Fish mortality from VHS is greatest at 38–54º F and is very rare above 60º F. The virus becomes inactive after 24 hours at 68ºF in water, but can persist for five days at 39º F in water. It requires an incubation period of approximately 7 to 15 days, depending on water temperature. It becomes inactivated in ether, chloroform, glycerol, formalin, sodium hypochlorite, sodium hydroxide, iodophors, UV radiation, or by desiccation, or exposure to pH levels lower than 2.4 or higher than 12.2.
Fishes are susceptible to infection at any age. VHS is transmitted to juvenile and adult fish most often via urine and sex products that enter a fish through their gills, or possibly through fin bases or via wounds; it cannot enter eggs and infect fish before hatching. Juvenile fish are generally more susceptible than adults. Experiments have recorded infection after contact with infected fish and after immersion in infected water; the virus can remain activated in water for several days. There is evidence of infections transferred between farmed and free-living fishes in European inland waters and coastal areas. The potential for transport with bait fish is demonstrated by the virus' recovery in cell culture from frozen Pacific herring after two freeze/thaw cycles in a conventional freezer. Waterfowl might also play a role in moving the virus, but it does not survive passage through the gut of birds. The mortality rate for infected fish varies between 20% and 80%, depending on environmental conditions, and has reached 100% in trout fry.
Impact of Introduction:
A) Actual: The North American VHS strain is less virulent to salmon and trout than the European strain and has not caused large fish kills of salmon and trout in the Great Lakes to date (2006). However, mortality of other species has been documented. In 2005, VHS apparently caused large die-offs of freshwater drum and round gobies in eastern Lake Ontario and muskellunge in Lake St. Clair. In the spring and summer of 2006, VHS was implicated as a cause of large die-offs of round gobies and muskellunge in the Thousands Islands area of the St. Lawrence River and die-offs of muskellunge, northern pike, gizzard shad, smallmouth bass, walleye and yellow perch in Lake St. Clair, Lake Erie and Lake Ontario.
B) Potential: Over 50 species of fish are known to be susceptible to VHS. The virus was first isolated from most of these species only within the past two decades. Susceptible fishes include several important species (e.g., lake trout, rainbow trout and brook trout) that have not yet been killed by the virus in the Great Lakes Basin. European freshwater-strain VHS infections usually impact salmonids, particularly rainbow trout, which suffer high mortality rates. Although the North American strain appears to be of low pathogenicity to salmonids, it has caused mass mortality in a variety of other marine fishes.
What Boaters and Anglers Can Do
This fact sheet was modified by Minnesota Sea Grant from the original written by:
Rebekah M. Kipp and Anthony Ricciardi
Citation for this information:
Rebekah M. Kipp and Anthony Ricciardi 2006. GLANSIS.
View the original fact sheet with references at: http://www.glerl.noaa.gov/res/Programs/ncrais/vhs_factsheet.html
Reprinted by permission of the Minnesota Sea Grant Institute. 2305 E 5th Street Duluth, MN 55805 (218) 726-8106 firstname.lastname@example.org
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