Fish Hatchery Management - fisheries & aquaculture

FISH HEALTH MANAGEMENT 295VIRAL HEMORRHAGIC SEPTICEMIA (VHS)Viral hemorrhagic septicemia, also known as Egtved disease, has not beenfound in North America but is a serious hatchery problem in several Europeancountries. Epizootics have been reported in brown trout but VHS primarilyis a disease of rainbow trout. It causes major losses among catchableor marketable trout but seldom is a problem among young fingerlings orbroodfish. The disease spreads from fish to fish through the water supply.Over the years, the disease has been given numerous names by variousGerman, French, and Danish workers. For simplification, the name ViralHemorrhagic Septicemia has been recommended and the abbreviationVHS appears frequently in the literature. In North America, VHS is consideredan exotic disease that, if introduced, would cause severe problemsin American culture of salmonids.Epizootics are characterized by a significant increase in mortality' Affectedfish become lethargic, swim listlessly, avoid water current, and seek theedges of the holding unit. Some individuals drop to the bottom and are reluctantto swim even though they retain their normal upright position. Justprior to death, affected fish behave in a frenzied manner and often swim intight circles along planes that vary from horizontal to vertical. Hyperactivitymay persist for a minute or more, then the fish drop motionless to thebottom. Most die, but others may resume a degree of normal activity for ashort time. Affected trout generally do not eat, although a few fish in aninfected population will feed.Trout with typical VHS become noticably darker as the diseaseprogresses. Exophthalmia can develop to an extreme stage' and the orbitfrequently becomes surrounded by hemorrhagic tissue. Such hemorrhagingis visible externally or may be seen during examination of the roof of themouth. Characteristically,the gills are very pale and show focal hemorrhages.On occasion, the base of ventral fins show hemorrhages. The dorsalfin may be eroded and thickened, but this also is a common feature amonghealthy rainbow trout under crowded conditions so its significance in VHSis not known. There is no food in the gastrointestinal tract and the liver ischaracteristically pale withhyperemic areas. Hemorrhages may occurthroughout the visceral mass, especially around the pyloric caeca. Thespleen becomes hyperemic and considerably swollen. One of the more commonsigns is extensive hemorrhages in swim bladder tissue. Kidneys of affectedfishes show a variable response. During the peak of acute epizootics,the kidneys usually have normal morphology but they may show hyperemia.Occasionally, the kidneys become grossly swollen and posteriorportions may show corrugation. It is not known whether this is a resPonseto the virus or to other complicating factors. Body musculature also showsa variable response; in some fish it appears to be normal but in others