Viral hemorrhagic septicaemia (VHS) is a disease caused by a negative-sense single-stranded RNA virus of the genus Novirhabdovirus. Infected fish suffer from haemorrhaging of their internal organs, skin and muscles. Symptoms that can be observed from the outside includes reddened eyes, gills, skins and fin, opens sores, a bloated abdomen, and bulging eyes, but some fish show no outward signs at all.

The virus can spread through water transfer and through the consumption of infected eggs or fish, which means that baitfish can introduce the fish to new localities. A fish that manages to survive the disease can become a lifelong carrier of the virus, excreting it through its urine and sperm or ovarian fluids. In Europe, the gray heron is known to spread the virus without being infected; the virus appears to remain inactive as long as it resides in the digestive tract of the bird.

Different strains with different properties

Historically, VHS was associated with Western Europe where it was documented as a pathogenic disease among cultured salmonids as early as the 1950s. In 1963, the viral cause of the disease was discovered by M. H. Jenson. Until late 1988, VHSv Type I was the only known strain of the disease and it appeared to be contained within freshwater fish farms in continental Europe, affecting primarily rainbow trout and only occasionally brown trout and pike.

In 1988 the first case of VHS was reported from the United States and the culprit turned out to be a distinct, more marine-stable strain of VHSv than the European variant. The virus was present in salmon returning to Washington State from the Pacific Ocean. Today, we know of four different main strains and except for type IV, all of them are endemic to Europe.

The type IV virus can be divided into two subtypes: IV-a and IV-b. IV-a has been reported from marine fish living in the Northwest Pacific, along the North American north Atlantic coast, and along the shores of Japan and Korea. IV-b is the type causing problems for freshwater fish in the North American Great Lakes region.

The IV-b strain was first isolated from fish living off Canada’s Atlantic coast where it did not cause any high mortality rates. This strain is capable of infecting not only salmonids but a long row of warm-water freshwater species previously assumed to be resistant to VHS. The European strains are particularly deadly do rainbow trout, but the IV-b strain only have a mild affect on this species. It is on the other hand capable of killing fish such as chinook salmon, lake trout, steelhead trout, gobies, emerald shiners, yellow perch, walleye, muskies and whitefish.