Abstract
BackgroundMortality 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.ResultsJuvenile 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 °C than at 23 or 11 °C. Co-infection with Flavobacterium columnare at 28 °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 °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 °C apart, making their synergistic effects highly unlikely under field conditions.ConclusionsThe 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.
Highlights
Mortality episodes have affected young-of-year smallmouth bass (Micropterus dolomieu) in several river systems in Pennsylvania since 2005
Exposure of Smallmouth bass (SMB) to Largemouth bass virus (LMBV) Exposure of SMB to LMBV by IP resulted in morbidity and mortality in all LMBV infected groups with variable rates, the 13–295 Susquehanna LMBV and the 13–286 Juniata isolates being the most pathogenic to SMB
Mortality in SMB in the groups that received LMBV at a concentration 103–107 Tissue culture infectious dose (TCID50)/fish started at the 2nd day post-infection and reached a cumulative mortality of 100% within 14 days pi, except the fish exposed to 12–342 Allegheny and 14–204 Pine Creek strains
Summary
Mortality episodes have affected young-of-year smallmouth bass (Micropterus dolomieu) in several river systems in Pennsylvania since 2005. Since 2005, mortality episodes of young of the year (YOY) smallmouth bass (Micropterus dolomieu; SMB) have been consistently reported from a number of river systems in Pennsylvania, including the Susquehanna, Juniata, and Allegheny Rivers. These mortality events have been more persistent and severe in the Susquehanna and Juniata. River systems, occurring annually to varying degrees, than in the Allegheny and other drainages These SMB die-offs have created considerable concerns among the sportfishing industry, as well as in state and federal agencies, as decreases in relative abundance of YOY and adult SMB and shifts in size structure have concurrently been noticed [1]. Several fish-pathogenic bacteria and parasites were reported from fish collected during the course of mortality episode
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