[3] Since 2005, massive die-offs have occurred among a wide variety of freshwater species in the Great Lakes region of North America.

VHSV is a negative-sense single-stranded RNA virus of the order Mononegavirales, family Rhabdoviridae, and genus Novirhabdovirus.

[9] Reverse genetics is a powerful tool to study and characterize the previously unknown viral genes.

[10][11] A vaccinia virus free reverse genetic system for Great Lakes VHSV (Genotype IVb) was developed by a research group from the USA.

[11] This system allows the investigators to explore the functional properties of individual viral genes of VHSV in detail.

A new role of NV protein has been discovered and demonstrated that it inhibits apoptosis at the early stage of viral infection.

[21] In 1988, the first marine strain of VHSV, now designated type IV, was found in normal-appearing salmon returning from the Pacific to rivers of Washington State.

[22] 1996 saw the first VHSV in Japan, among Japanese flounder farmed in the Seto Inland Sea,[23] and different genotypes have appeared in different areas since then.

[13] Type IV was later found off North America's northern Atlantic coast, in Atlantic herring (Clupea harengus)[24] mummichog (Fundulus heteroclitus), stickleback (Gasterosteus aculeatus aculeatus), brown trout (Salmo trutta), and striped bass (Morone saxatilis),[17] as well as dozens of freshwater species in the Great Lakes.

This is thought to represent both the spread of the virus into new areas, as with VHSV egg and live fish transfers from North America to Asia, or feeding of raw marine fish to inland farmed trout in Finland,[15] as well as discovery of existing populations, as with an apparently well established marine reservoir in the Black Sea.

[1] Originally found off the Atlantic coast of Canada, it was considered a low mortality marine strain.

While the European strain of VHSV is particularly deadly to rainbow trout, the Great Lakes region variant affects the species only mildly, as is typical with primarily marine genotypes.

Before the Type IVb die-offs in the Great Lakes, few states regulated transfer of live species other than for salmonids.

As of July 13, 2007, new rules have been enacted in the Canadian province of Ontario,[37] and US states of Michigan, New York, Ohio, Pennsylvania, and Wisconsin, while they are currently being drafted in Illinois, Indiana, and Minnesota.

[38] Additionally, the USDA's Animal and Plant Health Inspection Service issued a federal order in the fall of 2006 barring the transfer of all live susceptible species from the eight states bordering the lower Great Lakes, as well as importing such species from the Canadian provinces of Ontario and Quebec.

[41] Survivors of the disease can become lifelong carriers of the virus, contaminating water with urine, sperm, and ovarian fluids.

[1] The virus has been shown to survive two freeze/thaw cycles in a conventional freezer, suggesting both live and frozen bait could be a transmission vector.

[43] Living fish afflicted with VHS may appear listless or limp, hang just beneath the surface, or swim very abnormally, such as constant flashing circling due to the tropism of the virus for the brain.

Internal hemorrhaging can be observed as red spots inside a dead fish, particularly around the kidney, spleen, and intestines, as well as the swim bladder, which would normally have a clear membrane.

Most tissue changes can be observed as minor to major necrosis (cell death) in the liver, kidneys, spleen, and skeletal muscle.

In most cases, cell culturization is recommended for surveillance, with antibody tests and reverse transcription polymerase chain reaction (RT-PCR) and genetic sequencing and comparison for definitive confirmation and genotype classification.