**The title, authors, and abstract for this completion report are provided below. For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209**
Virulence assessments of isolates of infectious pancreatic necrosis virus (IPNV) endemic and exotic to the Great Lakes basin
Phillip E. McAllister1
1 U.S. Department of the Interior
USGS, Leetown Science Center
National Fish Health Research Laboratory,11649
Leetown Road, Kearneysville, WV 25430
The following is a summary of results by study objective for project “Virulence assessments of isolates of infectious pancreatic necrosis virus (IPNV) endemic and exotic to the Great Lakes basin.”
1. Determine the virulence of IPNV isolates endemic and exotic to the Great Lakes basin.
Eight species of salmonid fishes (brook trout, Salvelinus fontinalis; brown trout, Salmo trutta; lake trout, Salvelinus namaycush; rainbow trout, Oncorhynchus mykiss; steelhead trout O. mykiss; Atlantic salmon, Salmo salar; Chinook salmon, O. tshawytscha; and coho salmon, O. kisutch) were each exposed to 40 IPNV isolates endemic and exotic to the Great Lakes basin. The challenges revealed a spectrum of virulence between and within Birnadent monoclonal antibody epitope groups, genotype groups, and species of fish. Challenge survivors were held in culture for 1-2 years and applied as IPNV carrier stock for chronic low level IPNV exposure trials.
2. Determine the potential for infection and disease when fish experience chronic exposure to low virus concentration.
Two scenarios were used to model the potential for infection and disease in brook trout exposed to IPNV-laden culture effluent. Scenario 1 presented chronic low-level exposure to calibrated IPNV-laden ambient water, and Scenario 2 posed transmission of IPNV via excretion-contaminated effluents from IPNV-carrier fish. Virus transmission, overt disease, and virus carrier state occurred under both scenarios and were a function of ambient virus concentration, population density, and excretion augmentation of ambient virus burden.
3. Determine early life susceptibility of salmonid species to IPNV.
Susceptibility of fish to IPNV from emergence through sac fry and subsequent early fingerling life stages was charted using rainbow trout as a model species. The salient finding is that rainbow trout are susceptible to IPNV infection essentially on emergence from the egg. This finding is probably applicable to other salmonid species. While fish may become infected with IPNV early in life, presentation of infection as mortality involves a constellation of interactions that include alevin/fingerling physiological state, ambient virus burden, and dynamics of the population in association with natural and husbandry environmental conditions.
4. Determine age-related susceptibility and immune response of fry and fingerlings to IPNV infection.
This study initially assessed the age related susceptibility of fry and fingerling brook trout, lake trout, and Atlantic salmon to selected isolates of IPNV and subsequently assessed the persistence of IPNV infection and prevalence of IPNV infectivity-neutralizing immune response in near-challenge and yearling survivors. Challenges showed that each species presented variable sensitivity to infection reflected as mortality yet a similar response to infection reflected in persistence of and immune response to IPNV. Brook trout showed the greatest propensity for lethal IPNV infection followed by lake trout and Atlantic salmon, respectively. When survivors of challenge were held in culture and assayed at 1 month post challenge and at near yearling life stage (9-13 months post hatch), IPNV infection persisted in each isolate population for all species and a high proportion from each species developed significant IPNV infectivity-neutralizing immune response.
5. Develop a protocol for rescue of gametes compromised by exposure to IPNV.
Having found IPNV to be associated with, but not integral to, fish gametes (both eggs and sperm), a methodology was developed for rescue of gametes compromised by association with IPNV. Eggs and sperm were washed to reduce virus burden and subsequently treated with polyclonal antibody to neutralize residual IPNV infectivity. Egg viability and sperm motility were preserved, hatch levels were comparable between treated and untreated lots of eggs, and IPNV infection was not detected in subsequent progeny.