What is VHS?
VHS is viral hemorrhagic septicemia, a viral fish disease that has caused large scale mortalities in rainbow trout and turbot aquaculture operations in Europe and in Pacific herring and pilchard populations along the Pacific Coast of North America. The disease is caused by a rhabdovirus, Viral Hemorrhagic Septicemia Virus (VHSv). This virus has a number of identified isolates (unique genetic types) grouped in four types; three from Europe and one from North America. Each appears to have unique effects with specific pathogenicity on certain species. The isolate found in the Great Lakes Basin here is most similar to the VHS strain previously isolated from the Atlantic Coast in Eastern North America.What does it do to fish?
VHSv types I-III is known to cause mortalities in short periods of time, particularly in rainbow trout. The virus is more active in colder water (< 10 C) which is why mortalities are often seen in the spring. Fish exhibit hemorrhaging in the skin including large red patches, particularly in on sides and anterior portion of the head. However, infected fish will sometimes exhibit very minor external hemorrhaging (pin-point spots called petichia) or no external signs at all. Internally, all organs are often congested with multiple hemorrhages in the liver, spleen, and intestines. The swim bladders are also often extremely congested with hemorrhages, giving the otherwise transparent membrane a mottled appearance. Sick fish will often appear listless, swim in circles, or hang just below the surface based on staff observations made this past spring.How is VHS transmitted?
VHS can be transmitted by urine, feces and sexual fluids. Reservoirs include clinically ill and carrier fish that do not show signs of the infection. The virus can be found on the surface of the salmonid eggs during spawning of infected female broodstock (sometimes at very high levels) and is capable of persisting for a sufficient time period to result in vertical (actually egg-associated) transmission between generations (adult to progeny). It is also likely to enter the body through the gills or through wounds, although oral transmission is unlikely. Experiments showed that blood suckinarthropods and leeches can transmit the infection physically.When did VHS get here?
The earliest confirmed report is 2003 in a Great Lakes muskellunge from Lake St. Clair so it is likely to have been introduced here in 2002 or 2003. At the time of collection, the virus was initially classified as an unknown rhabdovirus and recently confirmed in 2006 as VHSv Type 4.Source: Michigan Department of Natural Resource Viral Hemorrhagic Septicemia (VHS) Briefing Paper