Exxon Valdez oil spill long-term herring research and monitoring program final report

This study includes annual field surveys of Ichthyophonus, viral hemorrhagic
septicemia virus, and erythrocytic necrosis virus in adult and juvenile Pacific herring (Clupea
pallasii) in Prince William Sound, Alaska and several reference populations in Alaska, British
Columbia, and Washington. Results from controlled experimental studies with Ichthyophonus
indicated that:

• Pacific herring could become infected after repeated feedings on simulated infected offal,
• Gross external signs of ichthyophoniasis can persist for extended periods without causing
direct host mortality,
• A novel tool (chromogenic in situ hybridization) was developed to confirm the presence
of Ichthyophonus in histological sections,
• A circulating stage of Ichthyophonus was detected in the blood of infected hosts,
• Ichthyophonus remains viable in a fish carcass for up to 4 weeks and remains infectious
for at least 5 days post mortem,
• Six distinct genetic types of Ichthyophonus were identified in different hosts throughout
the world,
• Tissue explant culture was confirmed to be more sensitive than qPCR for detecting low -
intensity Ichthyophonus infections directly from fish tissues.

Results from controlled experimental studies with viral hemorrhagic septicemia virus indicated
that:
• Cooler temperatures are more conducive to viral hemorrhagic septicemia epizootics in
Pacific herring,
• A blocking ELISA was developed to detect fish antibodies to viral hemorrhagic
septicemia virus,
• A more sensitive plaque neutralization test was optimized to detect herring neutralizing
antibodies to viral hemorrhagic septicemia virus,
• The relative susceptibility of Pacific herring to other viral hemorrhagic septicemia virus
genotypes was assessed, experimental spill-over, amplification, and spill-back was
demonstrated between Atlantic salmon (Salmo salar) and Pacific herring,
• The efficacy of homologous and heterologous DNA vaccines against viral hemorrhagic
septicemia virus was demonstrated in Pacific herring.

Controlled experimental studies with erythrocytic necrosis virus resulted in the development of a
conventional PCR technique that is capable of the virus in the blood and the development of a
quantitative PCR technique that is capable of detecting the virus in any herring tissues.