VHS is caused by an aquatic rhabdovirus. It has long been considered a serious disease of rainbow trout and a few other freshwater fish species raised for commercial aquaculture in Europe .   Known as Egtved virus in these fish populations, VHS virus causes high mortality and can have severe economic consequences.   For this reason, VHS is listed as a reportable disease by the World Organization for Animal Health (the OIE ─ formerly the Office International des Epizooties).   Related VHS virus strains have also been isolated from a variety of marine fishes in the North Pacific, North Atlantic , and seas around northern Europe and Japan .

Four genotypes of VHS virus have been identified, and appear to be distributed geographically, rather than by host or year of epizootic occurrence.   Genotypes I, II, and III are mainly found in Europe and Japan , while isolates of genotype IV have been recovered only from fish in North America , Japan and Korea . The pathogenicity of VHS virus varies by genotype and species affected.   The VHSV isolates originating from wild marine fish show no or low pathogenicity in rainbow trout and Atlantic salmon, although several are pathogenic to turbot.    The European/Asian freshwater isolates are highly pathogenic to rainbow trout.

VHS virus was first reported in the United States in 1988 in spawning salmon in the Pacific Northwest . The disease has been found in both wild and hatchery raised salmon in the Pacific Northwest .   VHS is now enzootic among Pacific herring and Pacific cod populations off the coast of Alaska , Canada , and Washington State .   In the Atlantic Ocean , the virus has been isolated from Atlantic herring and Greenland halibut.   Although the North American strain of VHS virus is moderately pathogenic to herring, causing occasional self-limiting epizootics, it is relatively avirulent for several species of marine salmonids .  

The number of wild fish species found to be susceptible to the North American genotype of the VHS virus is growing, with at least 40 different species (both freshwater and marine) testing positive for the virus.   Susceptible fish species are found among the Salmoniformes (salmon, trout), Esociformes (pike), Clupeiformes (herring, anchovy), Gadiformes (cod), Pleuronectiformes (flounders, soles, other flatfishes), Osmeriformes (smelt), Perciformes (perch, drum), Scorpaeniformes (rockfishes, sculpins ), Anguilliformes (eels), Cyprinodontiformes ( mummichog ) and Gasterosteiformes (sticklebacks).  

The European/Asian freshwater genotype of the VHS virus is readily transmissible to fish of all ages, primarily via the urine.   Survivors of infection can be lifelong carriers and shedders.   Once VHS virus is established in farmed fish stock, and in associated watersheds, the disease becomes enzootic due to carrier fish, including cultured and wild populations.   The virus has been isolated from feral fish in waters receiving hatchery effluent and can persist in water for several days.   Control methods for VHS currently rely on fish health surveillance programs and measures such as eradication and fallowing.   These procedures have been shown to be effective and have resulted in the elimination of VHS from several parts of Europe .

All VHS viruses can be recovered from homogenates of internal organs, sex products, or urine. Little virus can be recovered from feces.   Experimentally, fish can be infected by cohabitation, immersion, intraperitoneal and intramuscular injection, brushing virus on the gills, and feeding virally spiked food.

Fish-eating birds, such as the gray heron, can be mechanical vectors of VHS virus, but passage through the gastrointestinal tract of birds appears to inactivate the virus. The virus does not appear to be transmitted by parasitic vectors or to be capable of replication in insects. In the hatchery environment, mechanical transfer of VHS virus on the surface of animate or inanimate objects presents a substantial hazard.

Sources: International Database on Aquatic Animal Diseases (through OIE Collaborating Centre for Information on Aquatic Animal Diseases at the CEFAS Weymouth Laboratory , UK ).   Viral Hemorrhagic Septicemia of Fishes, Fish Disease Leaflet 83, U.S. Department of the Interior, Fish and Wildlife Services, 1990.   Viral haemorrhagic septicaemia virus in marine fish and its implications for fish farming – a review.   H.F. Skall , N.J. Olesen , and S. Mellergaard ; Journal of Fish Diseases, Volume 28 Page 509 - September 2005, doi:10.1111/j.1365-2761.2005.00654.x

In addition to the potential effects of VHS on cultured fish species, there are several species of fish which are commercially caught in the lower Great Lakes and which are susceptible or likely to be susceptible to VHS.   Total catch for these vulnerable species from lakes Erie, Ontario, and St. Clair amounted to about 1.3 million pounds during 1999 (Table 5).   Lake Erie yielded the vast majority of the catch and, by weight, yellow perch comprised 57% the species caught in the three affected bodies of water.

Table 5.   Commercial fisheries catch in the lower Great Lakes , by state and selected species, 1999 .

Source:   Great Lakes Fisheries Commission

According to Great Lakes Fisheries Commission data, the fish species represented on Table 6 accounted for about 36% of all species caught in the lower Great Lakes in 1999.   The number of different species that are known to be susceptible to the Great Lakes strain of VHS virus is increasing and detailed information on the status of other Great Lakes species of fish regarding their susceptibility to VHS is not known.  

The United States wild caught fish numbers show that over 3,000 tons of fish were caught in 2004, the majority being gizzard shad, yellow perch, and freshwater drum (Table 6).

Products from both aquacultured and wild caught fish totaled over 1 million tons in both 2003 and 2004 (Table 7).

Quantities of live fish (except for eel) are not reported through WTA, but values serve as a good representation of the magnitude of the volume of fish being exported.   The US exported $231,000 worth of live trout in 2005, and $137,000 from January to April 2006 (Table 8).   The NESOI category (not elsewhere specified or included) does not give specific species, however some of the species affected by VHS would be included in this category.

The US exported $16.6 million worth of live fish, NESOI in 2005 and nearly 8 million from January to April 2006.   The US also exported 501,442 KG (worth $5.6 million of live eel) in 2005, and 82,708 KG (worth $912,000) from January to April 2006.   Canada , Chile , South Korea , Belgium and China were the primary recipients of these live fish exports from January through April, 2006.   The U.S. exported nearly $200 million in whole fish or relevant fish products during 2005 and more than $90 million in fish or fish products during the first four months of 2006 (Table 8).   Canada , Germany and Japan were the primary recipients of these exported products.

Source: World Trade Atlas