Fish Hatchery Management - fisheries & aquaculture

FISH HEALTH MANAGEMENT 299Infected fry become lethargic; some swim erratically and are hyperactive,apparently losing motor control during the terminal stages. Exophthalmiais pronounced and abdominal darkening is common. Hemorrhage may beseen in the eyes of fish with exophthalmia. Abdominal distension is commonand gills are abnormally pale.Internally, ascitic fluid is abundant, and anemia and edema may be evidentin the visceral mass. The liver, spleen, and digestive tract are flaccidand the vascular organs are mottled with areas of hyperemia. The kidneysare pale, though not necessarily swollen. The digestive tract is void of food.Presently, specific immune antiserum has not been developed for definitiveidentification of the virus. Diagnosis, therefore, must be based on clinicalsigns of the disease, histopathological changes, and presumptive tests ofthe agent itself. This requires the services of a pathologist at a wellequippedlaboratory.Fish-to-fish transmission is assumed, because the virus can be isolatedfrom ovarian fluid, and eggs must be considered contaminated if they comefrom an infected source. Rainbow trout and landlocked sockeye salmonthus far are the only known susceptible species. Atlantic salmon, browntrout, and brook trout tested experimentally were refractory. Other speciesof salmon have not been tested.To date, reports of herpesvirus disease have been scattered and effortsshould be made to prevent the spread of this potentially damaging disease.Avoidance is the only certain method of control. Chemotheraphy is ineffective.LYMPHOCYSTIS DISEASELymphocystis disease, although rarely lethal, is of special interest becauseof its wide range of occurrence and presence in so many propagated andfree-ranging fish species. Marine as well as freshwater fishes are susceptible,but the disease has not been reported among salmonids. Among thepropagated freshwater fishes, walleyes and most centrarchids are susceptible.Lymphocystis is a chronic virus-caused disease causing generally granular,wart-like or nodular tissue lesions composed of greatly enlarged hostcells and their covering membrane. Cells of infected tissue may attain asize of a millimeter or more and resemble a spattering of sand-like granulesor, when larger, a raspberry-like appearance (Figure 79).The causative agent of the disease is a virus maintained in susceptiblehost fishes. Healthy fish may be exposed when infected cells burst and thevirus particles are released. This can occur intermittently through the durationof infection, or it can be massive upon death and decomposition of infectedfish. Lymphocystis lesions are persistent and commonly remain forseveral months; some may continue for a year or more.