FISH HEALTH MANAGEMENT 295 



VIRAL HEMORRHAGIC SEPTICEMIA (VHS) 



Viral hemorrhagic septicemia, also known as Egtved disease, has not been 

 found in North America but is a serious hatchery problem in several Euro- 

 pean countries. Epizootics have been reported in brown trout but VHS pri- 

 marily is a disease of rainbow trout. It causes major losses among catchable 

 or marketable trout but seldom is a problem among young fingerlings or 

 broodfish. The disease spreads from fish to fish through the water supply. 



Over the years, the disease has been given numerous names by various 

 German, French, and Danish workers. For simplification, the name Viral 

 Hemorrhagic Septicemia has been recommended and the abbreviation 

 VHS appears frequently in the literature. In North America, VHS is con- 

 sidered an exotic disease that, if introduced, would cause severe problems 

 in American culture of salmonids. 



Epizootics are characterized by a significant increase in mortality. Affect- 

 ed fish become lethargic, swim listlessly, avoid water current, and seek the 

 edges of the holding unit. Some individuals drop to the bottom and are re- 

 luctant to swim even though they retain their normal upright position. Just 

 prior to death, affected fish behave in a frenzied manner and often swim in 

 tight circles along planes that vary from horizontal to vertical. Hyperactivi- 

 ty may persist for a minute or more, then the fish drop motionless to the 

 bottom. Most die, but others may resume a degree of normal activity for a 

 short time. Affected trout generally do not eat, although a few fish in an 

 infected population will feed. 



Trout with typical VHS become noticably darker as the disease 

 progresses. Exophthalmia can develop to an extreme stage, and the orbit 

 frequently becomes surrounded by hemorrhagic tissue. Such hemorrhaging 

 is visible externally or may be seen during examination of the roof of the 

 mouth. Characteristically, the gills are very pale and show focal hemor- 

 rhages. On occasion, the base of ventral fins show hemorrhages. The dorsal 

 fin may be eroded and thickened, but this also is a common feature among 

 healthy rainbow trout under crowded conditions so its significance in VHS 

 is not known. There is no food in the gastrointestinal tract and the liver is 

 characteristically pale with hyperemic areas. Hemorrhages may occur 

 throughout the visceral mass, especially around the pyloric caeca. The 

 spleen becomes hyperemic and considerably swollen. One of the more com- 

 mon signs is extensive hemorrhages in swim bladder tissue. Kidneys of af- 

 fected fishes show a variable response. During the peak of acute epizootics, 

 the kidneys usually have normal morphology but they may show hy- 

 peremia. Occasionally, the kidneys become grossly swollen and posterior 

 portions may show corrugation. It is not known whether this is a response 

 to the virus or to other complicating factors. Body musculature also shows 

 a variable response; in some fish it appears to be normal but in others 



