Effects of Temperature on the Susceptibility of Largemouth Bass to Largemouth Bass Virus

—Temperature is an environmental variable thought to influence the susceptibility of fish to infectious diseases. This study demonstrated that juvenile largemouth bass Micropterus salmoides that were experimentally infected with largemouth bass virus (LMBV; family Iridoviridae) experienced greater mortality at 308C than at 258C. Juvenile largemouth bass were exposed to equal doses of LMBV and held at three temperatures: 25, 30, and 358C. Fish held at 308C suffered mortality at a higher rate than fish held at 258C and had higher viral loads (viral genomes per gram of tissue) at the time of death. The LMBV-injected fish held at 358C suffered mortality at rates equal to those of sham-injected controls, suggesting that stressful manipulation at temperatures approaching the upper lethal limit can induce mortality in fish independently of viral infection. These results suggest that temperature is an important determinant of host survival and viral replication in the LMBV system. Largemouth bass virus (LMBV) is a recently discovered pathogen that has become widely distributed throughout the southeastern and midwestern United States (Plumb et al. 1996; Goldberg 2002). This virus infects several warm water fish species, including bluegill Lepomis macrochirus, striped bass Morone saxatilis, spotted bass Micropterus punctulatus, smallmouth bass M. dolomieu, and most notably, the northern and Florida strains of largemouth bass M. salmoides (Goldberg 2002). Although LMBV has been associated with large-scale fish kills in bass, it has caused no known episodes of epidemic mortality in other species (Plumb and Zilberg 1999a; Goldberg 2002). The fact that some infected populations of largemouth bass do not suffer mortality suggests that environmental factors play a role in the pathogenesis of LMBV. It is becoming apparent that LMBV * Corresponding author: [email protected] Received February 24, 2003; accepted October 29, 2003 tends to strike during the warmest summer months (Goldberg 2002). Physiological stressors that come into play during the summer months include elevated water temperature, low dissolved oxygen, and increased angling pressure. The interaction of such environmental stressors with viral infection may precipitate fish kills. Environmental stressors may reduce the immune capacity of fish and increase the potential for the virus to replicate. Identifying the specific hostand pathogen-related factors that precipitate LMBV fish kills is critical to understanding and managing the disease. Temperatures outside of normal thermal regimes have been documented to have immunosuppressive effects in fish (Bly et al. 1997; LaPatra 1998). The average habitat temperature range for largemouth bass is from 108C to approximately 348C (Davis and Lock 1997). The critical thermal maxima (the temperatures at which death occurs when the environmental temperature is raised 0.28C/ min) for northern and Florida largemouth bass acclimated to 248C are 36.5 6 0.58C (mean 6 SD) and 37.5 6 0.68C, respectively (Fields et al. 1987).