A more sensitive technique for isolating infectious pancreatic necrosis virus from asymptomatic carrier rainbow trout, Salmo gairdneri Richardson

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Abstract
An improved technique for isolating infectious pancreatic necrosis virus (IPNV) is described. It involves trypsinization of the suspect tissue, separation of the trypsinized cells and their co‐cultivation with indicator RTG‐2 cells. This co‐cultivation method is compared with homogenization, followed by membrane filtration, a procedure routinely employed by most disease diagnostic laboratories. From the two IPN‐infected farms studied, IPNV could be isolated by homogenization from only 50 and 66%, respectively, of the fish from which virus was detected by co‐cultivation. This difference in sensitivity can be explained in two ways, viz: (a) some of the virus is being retained by the membrane filter during filtration to remove contaminants following homogenization; (b) the presence of neutralizing factors in the tissue homogenate are neutralizing the virus and thus no cytopathic effect ensues. No correlation between serum neutralizing antibody levels and the differences in sensitivity of the two methods was found. The co‐cultivation method is as practical as, and certainly no more time‐consuming than, the homogenization technique.