FISH HEALTH MANAGEMENT 299 



Infected fry become lethargic; some swim erratically and are hyperactive, 

 apparently losing motor control during the terminal stages. Exophthalmia 

 is pronounced and abdominal darkening is common. Hemorrhage may be 

 seen in the eyes of fish with exophthalmia. Abdominal distension is com- 

 mon and gills are abnormally pale. 



Internally, ascitic fluid is abundant, and anemia and edema may be evi- 

 dent in the visceral mass. The liver, spleen, and digestive tract are flaccid 

 and the vascular organs are mottled with areas of hyperemia. The kidneys 

 are pale, though not necessarily swollen. The digestive tract is void of food. 



Presently, specific immune antiserum has not been developed for defini- 

 tive identification of the virus. Diagnosis, therefore, must be based on clini- 

 cal signs of the disease, histopathological changes, and presumptive tests of 

 the agent itself. This requires the services of a pathologist at a well- 

 equipped laboratory. 



Fish- to- fish transmission is assumed, because the virus can be isolated 

 from ovarian fluid, and eggs must be considered contaminated if they come 

 from an infected source. Rainbow trout and landlocked sockeye salmon 

 thus far are the only known susceptible species. Atlantic salmon, brown 

 trout, and brook trout tested experimentally were refractory. Other species 

 of salmon have not been tested. 



To date, reports of herpesvirus disease have been scattered and efforts 

 should be made to prevent the spread of this potentially damaging disease. 

 Avoidance is the only certain method of control. Chemotheraphy is ineffec- 

 tive. 



LYMPHOCYSTIS DISEASE 



Lymphocystis disease, although rarely lethal, is of special interest because 

 of its wide range of occurrence and presence in so many propagated and 

 free-ranging fish species. Marine as well as freshwater fishes are suscepti- 

 ble, but the disease has not been reported among salmonids. Among the 

 propagated freshwater fishes, walleyes and most centrarchids are suscepti- 

 ble. 



Lymphocystis is a chronic virus-caused disease causing generally granu- 

 lar, wart- like or nodular tissue lesions composed of greatly enlarged host 

 cells and their covering membrane. Cells of infected tissue may attain a 

 size of a millimeter or more and resemble a spattering of sand-like granules 

 or, when larger, a raspberry- like appearance (Figure 79). 



The causative agent of the disease is a virus maintained in susceptible 

 host fishes. Healthy fish may be exposed when infected cells burst and the 

 virus particles are released. This can occur intermittently through the dura- 

 tion of infection, or it can be massive upon death and decomposition of in- 

 fected fish. Lymphocystis lesions are persistent and commonly remain for 

 several months; some may continue for a year or more. 



