Fish Hatchery Management - fisheries & aquaculture

296 FISH HATCHERY MANAGEMENTpetechiae may be present throughout the flesh. As with IPN virus, thecausative agent of VHS must be identified by serological methods involvingcell cultures and immune serum specific for the virus. Fluorescent antibodyprocedures also have been developed and work well.There is evidence that resistance increases with age. Infections usuallyare more severe in fingerlings and yearling fish, whereas fry and broodfishappear to be less susceptible. Brook trout, brown trout, and Atlantic salmonhave been infected experimentally and grayling and whitefish were reportedto be susceptible.Natural transmission occurs through the water, suggesting that virus isprobably shed in feces or urine. There also is some evidence that the viruscan occur on eggs. Survivors of an epizootic become carriers of the virus.This disease usually occurs during the winter and spring; as water temperaturesrise, epizootics subside. Sporadic outbreaks may occur in the summerat water temperatures less than 68'F.Preventive measures against VHS in the United States consist largely ofpreventing the introduction of the virus through importation of infectedeggs or fish. No salmonid eggs or fish may enter the United States legallyunless they have been thoroughly inspected and found free of VHS.As in the case of other viral infections of fish, chemotherapy of VHS isunsuccessful. The only effective measure at present is avoidance, consistingof propagating clean fish in clean hatcheries and controlling the access offish, personnel, animals, and equipment that might introduce the virus.INFECTIOUS HEMATOPOIETIC NECROSIS (IHN)Infectious hematopoietic necrosis, a viral disease of trout and salmon, firstwas recognized in 1967. Recent findings show that the pathogenic agentcausing IHNdisease is morphologically, serologically, physically, andbiochemically indistinguishable from those implicated in viral diseases ofsockeye and chinook salmon. Furthermore, clinical signs of the diseasesand the histopathological lesions are the same. Thus the descriptive nameinfectious hematopoietic necrosis (IHN) disease has been given to all.Diseased fish are lethargic but, as in the case of many viral infections,some individuals will display sporadic whirling or other evidence of hyperactivity.In chronic cases, abdominal swelling, exophthalmia, pale gills,hemorrhages at the base of fins, and dark coloration are typical signs of thedisease. Internally, the liver, spleen, and kidneys usually are pale. Thestomach may be filled with a milk-like fluid and the intestine with awatery, yellow fluid that sometimes includes blood. Pin-point hemorrhagesthroughout the visceral fat tissue and mesenteries often can be seen. In occasionalcases, signs may be absent and fish die of no apparent cause.During the course of an epizootic, a generalized viremia occurs and thevirus can be isolated from almost any tissue for diagnostic purposes. After