Laboratory investigation into the role of largemouth bass virus (Ranavirus, Iridoviridae) in smallmouth bass mortality events in Pennsylvania rivers

Background: Mortality episodes have affected young-of-year smallmouth bass (Micropterus dolomieu) in several river systems in Pennsylvania since 2005. A series of laboratory experiments were performed to determine the potential role of largemouth bass virus (Ranavirus, Iridoviridae) in causing these events. Results: Juvenile smallmouth bass experimentally infected with the largemouth bass virus exhibited internal and external clinical signs and mortality consistent with those observed during die-offs. Microscopically, infected fish developed multifocal necrosis in the mesenteric fat, liver, spleen and kidneys. Fish challenged by immersion also developed severe ulcerative dermatitis and necrotizing myositis and rarely panuveitis and keratitis. Largemouth bass virus-challenged smallmouth bass experienced greater mortality at 28 degrees C than at 23 or 11 degrees C. Co-infection with Flavobacterium columnare at 28 degrees C resulted in significant increase in mortality of smallmouth bass previously infected with largemouth bass virus. Aeromonas salmonicida seems to be very pathogenic to fish at water temperatures < 23 degrees C. While co-infection of smallmouth bass by both A. salmonicida and largemouth bass virus can be devastating to juvenile smallmouth bass, the optimal temperatures of each pathogen are 7-10 degrees C apart, making their synergistic effects highly unlikely under field conditions. Conclusions: The sum of our data generated in this study suggests that largemouth bass virus can be the causative agent of young-of-year smallmouth bass mortality episodes observed at relatively high water temperature.