Infectious Pancreatic Necrosis Virus: Quantification of Carriers in Lake Populations During a 6-Year Period

An infectious pancreatic necrosis (IPN) carrier population of brook trout (Salvelinus fontinalis) planted in an isolated natural lake was tested for virus over a period of 6 yr. The initial isolation rate of 90% had decreased to 50% during this period indicating that there is a prolonged persistence of virus in infected animals of this species. The ceca with the associated pancreatic tissue of these fish yielded virus most frequently during the early years but the virus then decreased to low levels. Kidneys were the most consistent organs to yield virus throughout the study period. The feces yielded little virus at first but later increased for several years and then decreased. The spleen and liver yielded virus infrequently. Surgically removed gonads were the least frequent yielders of virus. The low virus isolation rate from progeny of the carrier population and from other planted fish indicated that transmission in the wild is probably infrequent. Other lakes with normal angling that had plantings with IPN carrier fish but subsequently planted annually with disease-free fish had populations whose carrier rate had decreased to undetectable levels within a 6-yr period. These findings indicated that the practice of planting IPN-free fish into areas where carriers were previously introduced may lead eventually to a carrier-free state. Key words: infectious pancreatic necrosis virus, Salvelinus fontinalis, virus persistence, carrier fish, virus isolation rate