294 FISH HATCHERY MANAGEMENT 



Diseases of Fish 



Viral Diseases 



INFECTIOUS PANCREATIC NECROSIS (IPN) 



Infectious pancreatic necrosis is a viral disease of salmonids found 

 throughout the world. The disease is common in North America and has 

 been spread to other countries, probably via contaminated egg and fish 

 shipments. It has been reported in all species of trout and salmon. As a 

 rule, susceptibility decreases with age. High losses occur in young finger- 

 lings but few deaths or signs appear in fish longer than 6 inches. Some evi- 

 dence suggests that well-fed, rapidly growing fish are more vulnerable to 

 the disease than those less well- nourished. 



In an IPN epizootic, the first sign usually seen is a sudden increase in 

 mortality. The largest and best appearing fingerlings typically are affected 

 first. Spiraling along the long body axis is a common behavior of fish in 

 lots having high death rates. The spiraling may vary from slow and feeble 

 to rapid and frantic. Convulsive behavior may alternate with periods of 

 quiescence during which victims may lie on the bottom and respire weakly. 

 Death usually occurs shortly after the spiraling behavior develops. 



Signs include overall darkening of the body, protruding eyes, abdominal 

 swelling, and (at times) hemorrhages in ventral areas including the bases of 

 fins. Multiple petechiae occur in the pyloric caecal area, and the liver and 

 spleen are pale in color. The digestive tract almost always is void of food 

 and has a whitish appearance. Clear to milky mucoid material occurs in the 

 stomach and anterior intestine and provides a key sign in the presumptive 

 identification of IPN disease. Spiraling behavior, a mucus plug in the intes- 

 tine, and a lack of active feeding strongly suggest IPN disease. However, a 

 definitive diagnosis requires isolation and identification of the causal agent. 

 This requires isolation of the virus in tissue culture combined with a serum 

 neutralization test with specific immune serum. A positive diagnosis usually 

 can be obtained within 24 to 48 hours in cases where large die-offs occur. 



Infectious pancreatic necrosis cannot be treated effectively and avoidance 

 presents the only effective control measure. This consists of hatching and pro- 

 pagating IPN virus-free fish stocks in uncontaminated water supplies. Care 

 must be given to exclude sources of contamination such as egg cases, transport 

 vehicles from other hatcheries, and eggs and fish from uncertified sources. 



Some hatcheries are forced to operate with water from sources containing 

 IPN virus carriers. In these cases, extra eggs should be started to allow for 

 high production losses. When an IPN outbreak occurs, strict sanitation can 

 prevent the spread of the disease to fish in other holding units. If water is 

 reused, susceptible fish elsewhere in the system usually will contract the in- 

 fection. Survivors must be considered to be carriers of the virus. 



