Aspects of infectious pancreatic necrosis (IPN) virus infections in farmed fish

A rainbow trout (Slamo gairdneri Richardson) population of IPN virus carriers was studied over a one-year period using both homogenisatian and co-cultivation for virus isolation. The percentage of virus-yielding fish was high between March and June but then declined. This was diametrically opposite to the trend in the serum antibody levels indicating that the marked humoral immune response resulted in a very significant reduction in the virus titres. The highest isolation rate was obtained from the kidneys after co-cultivation (from seventeen of the twenty-three virus-positive fish) underlining the very high sensitivity of this recently developed method for virus detection. Twelve of the twenty-three virus-positive fish yielded virus from the pyloric caeca after homogenisation. Virus was occasionally isolated from the faeces indicating that this may well be a possible avenue for horizontal transmission of the virus. No virus was ever detected in gonadal tissue. The virulence of the rainbow trout virus was enhanced in various ways and used to infect tilapia Oreochromis spilurus Gunther of different ages through a variety of routes. Fry infected by direct immersion, orally and by force feeding showed little or no signs of infection. Intraperitoneally and intramuscularly injected fingerling and adult fish developed marked haemorrhaging, severe loss of skin mucus and up to 50% mortalities were recorded. Gross pathology included enlarged and liquifying liver, gastroenteritis and mild brain haemorrhaging. Histopathologically there was extensive cellular vacuolation and degeneration as well as marked leucocytic infiltration in the liver, intestine and swimbladder. Eosinophilic granule cell infiltration of the intestinal wall was also very prominent. Virus was recovered from several organs and determination of virus titres revealed that active viral replication had occurred in the tilapia tissues, a finding further supported by electron microscopical evidence. The fish showed a clear humoral antibody response