296 FISH HATCHERY MANAGEMENT 



petechiae may be present throughout the flesh. As with IPN virus, the 

 causative agent of VHS must be identified by serological methods involv- 

 ing cell cultures and immune serum specific for the virus. Fluorescent anti- 

 body procedures also have been developed and work well. 



There is evidence that resistance increases with age. Infections usually 

 are more severe in fingerlings and yearling fish, whereas fry and broodfish 

 appear to be less susceptible. Brook trout, brown trout, and Atlantic sal- 

 mon have been infected experimentally and grayling and whitefish were re- 

 ported to be susceptible. 



Natural transmission occurs through the water, suggesting that virus is 

 probably shed in feces or urine. There also is some evidence that the virus 

 can occur on eggs. Survivors of an epizootic become carriers of the virus. 

 This disease usually occurs during the winter and spring; as water tempera- 

 tures rise, epizootics subside. Sporadic outbreaks may occur in the summer 

 at water temperatures less than 68°F. 



Preventive measures against VHS in the United States consist largely of 

 preventing the introduction of the virus through importation of infected 

 eggs or fish. No salmonid eggs or fish may enter the United States legally 

 unless they have been thoroughly inspected and found free of VHS. 



As in the case of other viral infections of fish, chemotherapy of VHS is 

 unsuccessful. The only effective measure at present is avoidance, consisting 

 of propagating clean fish in clean hatcheries and controlling the access of 

 fish, personnel, animals, and equipment that might introduce the virus. 



INFECTIOUS HEMATOPOIETIC NECROSIS (IHN) 



Infectious hematopoietic necrosis, a viral disease of trout and salmon, first 

 was recognized in 1967. Recent findings show that the pathogenic agent 

 causing IHN disease is morphologically, serologically, physically, and 

 biochemically indistinguishable from those implicated in viral diseases of 

 sockeye and chinook salmon. Furthermore, clinical signs of the diseases 

 and the histopathological lesions are the same. Thus the descriptive name 

 infectious 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 hy- 

 peractivity. In chronic cases, abdominal swelling, exophthalmia, pale gills, 

 hemorrhages at the base of fins, and dark coloration are typical signs of the 

 disease. Internally, the liver, spleen, and kidneys usually are pale. The 

 stomach may be filled with a milk-like fluid and the intestine with a 

 watery, yellow fluid that sometimes includes blood. Pin-point hemorrhages 

 throughout the visceral fat tissue and mesenteries often can be seen. In oc- 

 casional cases, signs may be absent and fish die of no apparent cause. 



During the course of an epizootic, a generalized viremia occurs and the 

 virus can be isolated from almost any tissue for diagnostic purposes. After 



