Do We Or Do We Not Have Infectious Pancreatic Necrosis Virus in Indiana's Prize Skamania Steelhead and Rainbow Trout?

            This is the question that has been asked by many.  Unfortunately, there is no precise answer, but, the following information may shed some light on this issue.

            In late October, 1998, in order to comply with the Great Lakes Fish Health Committee's control policy and model program, Mixsawbah State Fish Hatchery in Walkerton, Indiana, of the Indiana's Department of Natural Resources, Fish and Wildlife Division, submitted tissue samples from winter run steelhead trout for annual fish health inspection evaluation to the Animal Disease Diagnostic Laboratory (ADDL), Purdue University, West Lafayette, Indiana.  Viruses isolated from four of twelve 5 fish kidney/spleen pools were identified as Infectious Pancreatic Necrosis Virus (IPNV) or an antigenically similar "aquatic birnavirus" which was cross-reactive with polyclonal IPNV anti-sera using fluorescent antibody (FA) techniques.

            Subsequently, a sample of this lot of fish were euthanized, necropsied and examined histologically by pathologists at the ADDL.  The results of these tests were equivocal for the diagnosis of Infectious Pancreatic Necrosis Virus infection.  However, kidney samples collected at necropsy and submitted for VI and FA techniques were positive for IPNV or an antigenically similar "aquatic birnavirus" which was cross-reactive with polyclonal IPNV anti-sera utilizing the same previous mentioned techniques for these fish.

            In early December, summer run skamania steelhead trout from this same facility also had six of twelve 5 fish kidney/spleen pools which tested positive for Infectious Pancreatic Necrosis virus or an antigenically similar "aquatic birnavirus" which was cross-reactive with polyclonal IPNV anti-sera utilizing the same previously mentioned techniques.  These fish had previously tested negative during a routine annual fish health inspection (August, 1998) using the same methods.

            Adult broodstock of "London strain" rainbow trout being kept at Curtis Creek Trout Rearing Station near Howe, Indiana for spawning during the winter months were sampled for their annual fish health inspection in December, 1998.  Four of fourteen 5 fish ovarian fluid pools were positive for Infectious Pancreatic Necrosis Virus or an antigenically similar "aquatic birnavirus" which was cross-reactive with polyclonal IPNV anti-sera utilizing the same previously mentioned techniques.

            Adult steelhead broodstock were sampled throughout the spawning season (January and February, 1999) at Bodine State fish Hatchery in Mishawaka, Indiana.  A total of seven out of forty-two 5 fish kidney/spleen pools (representing 210 fish from a total of 338 females that spawned) were positive for Infectious Pancreatic Necrosis Virus or an antigenically similar "aquatic birnavirus" which is cross-reactive with polyclonal IPNV anti-sera utilizing the same previously mentioned techniques.  To date, hatched offspring from both the steelhead from Bodine State Fish Hatchery and the rainbow trout from Curtis Creek Trout Rearing Station have not tested positive for the presence of this pathogen.  Additionally, no increased morbidities or mortalities or lesions indicative of IPN disease have been observed in any of the hatcheries where "IPN positive" fish have been identified.

            Based upon the importance of IPNV being listed as a restrictive disease agent, and the potential ramifications of large numbers of production fish which may need to be destroyed due to a lack of alternative stocking in non-Great Lakes watersheds and basins, coupled with the wording that "every effort should be made not to release these fish into waters of the Great Lakes basin" as stated in the model program, diagnostic assistance was requested from the Washington Animal Disease Diagnostic Laboratory in Pullman, Washington to confirm the identity of this viral isolate.  This laboratory was able to confirm each of these three initial IPNV isolates using very similar laboratory methods as those used in the virology laboratory of the ADDL.  Subsequently, the electron microscopy laboratory of the ADDL has confirmed this isolate as a birnavirus based upon its characteristic size and ultrastructural morphology.

            A management decision was made to destroy the entire year's stock of approximately 110,000 of the winter run steelhead trout in order to prevent violation of the control policy and model program of the Great Lakes Fish Health Committee.  Factors involved in this decision-making process included the short time period prior to stocking of these fish and the potential jeopardizing of good culture and husbandry practices for arriving chinook and coho salmon production fish in order to minimize exposure of this viral agent to other salmon stocks.

            These issues were discussed at great length by the Great Lakes Fish Health Committee at its annual meeting in March, 1999 in Winnipeg, Canada.  Several key questions were posed to this committee as asked by mutual agreement of agency fishery chiefs in Illinois, Indiana, Wisconsin and Michigan.  The outcome of the decision-making process at this meeting indicated that more research regarding the potential pathogenicity of this isolate needed to be done in order for proper risk assessment modeling procedures to be initiated to ensure the best possible outcome of these "IPN positive" fish.  It was also noted that the control policy and model program of this committee should be revised as soon as possible.  Much discussion centered around the issue of disease versus pathogen detection.

            Beginning in late 1999 and early 2000, Drs. White, Kanitz and Sakamoto in the School of Veterinary Medicine of Purdue University and at the Animal Disease Diagnostic Laboratory of Purdue University, including support staff trained in aquaculture species will conduct research to try to focus on the following issues regarding this isolate:

• Serotyping and its significance.
• Pathogenicity as determined by experi-mental infection of health fish.
• Virulence factors associated with disease propagation such as age susceptibility, water temperatures, etc.
• Improvement of current diagnostic testing procedures for this agent.

Once these issues are addressed, models for risk assessment can be more completely developed to understand the significance of this isolate.  This will allow fishery chiefs and hatchery managers within the Great Lakes basin to make educated, informed and science-based decisions regarding these current findings rather than having to destroy hundreds of thousands of fish in order to be in compliance with the current control policy and model program of the Great Lakes Fish Health Committee due to the presence of this isolate within seemingly otherwise health fish.

            So, the answer to this question regarding whether or not Indiana's salmonids have IPNV is currently unresolved.  However, one fact that is certain is that more research work needs to be done regarding this viral isolate to better answer this question.

- by M. Randy White, DVM, PhD