Aeromonas hydrophila and A. sobria have been identified as major causes of mortalities among commercially raised channel catfish. According to the Fish Disease Committee of the Southern Division of AFS, Aeromonas sp. infections have accounted for 21% of the total bacterial cases since 1982. Mortality rates in MAS epizootics range from chronic to acute. At present, antibiotic therapy is the only prescribed treatment for A. hydrophila (Plumb, 1979). However, antibiotic therapy is expensive and often not effective due to the development of antibiotic resistant strains. Therefore, development of immunoprophylatic procedures for the prevention, rather than the treatment, of MAS is indicated. Several studies have demonstrated that the somatic antigens of A. hydrophila are complex and that somatic-antigen vaccine preparations do not confer protection to heterologous strains of A. hydrophila (Takahaski and Kusuda, 1977; Fliermans and Hazen, 1980; Thune, 1980; Leblanc et al., 1981; Dooley et al., 1986). Thus, another common antigen needs to be identified for the successful development of an A. hydrophila vaccine.

Evidence suggests that strains of the A. hydrophila complex have differential virulence (DeFigueido and Plumb, 1977). Extracellular products of A. hydrophila have been demonstrated to play a role on the virulence of the organism. Hemolysins (Allan and Stephenson, 1981; Thune et al., 1986; Charaborty et al., 1987) and proteases (Thune et al., 1982) have been implicated in the pathogenicity of motile aeromonad strains. Also, recent studies have indicated that various surface components play a role in the pathogenicity of the strains. Dooley and Trust (1988) have shown that members of an A. hydrophila serogroup that are virulent for trout also produce a S-layer protein. Thune and Johnson (unpublished data) have demonstrated that a similar protein is produced in strains isolated from a variety of sources and that production of this protein correlates to virulence in channel catfish fingerlings. Transblot techniques also indicated antigenic homogeneity of the protein within strains.

The objective of this study was to determine the role of A. hydrophila virulence factors in the pathogenicity of MAS epizootics in commercial catfish ponds. Bacterial isolates collected from channel catfish during MAS epizootics were screened for production of hemolysins, proteases, and the surface protein. Data indicates that the majority of isolates from fish showing clinical signs of MAS also produce the protein. Hemolysin and protease production were more variable among the isolates. Studies to determine protection conferred to channel catfish vaccinated with these virulence factors are in progress.

References

1.  Allan, B.J. and R.M.W. Stephenson. 1981. Extracellular virulence factors of Aeromonas hydrophila in fish infection. Can. J. Micro. 27:1114-1122.

2.  Charaborty, T., B. Huhle, H. Hof, H. Bergbauer and W. Goebel. 1987. Marker exchange mutagenesis of aerolysin determinant in Aeromonas hydrophila demonstrates the role of aerolysin in A. hydrophila-associated systemic infections. Infect. Immun. 55: 2274-2280.

3.  DeFigueido, J. and J.A. Plumb. 1977. Virulence of different isolates of Aeromonas hydrophila in channel catfish. Aquaculture. 11: 349-354.

4.  Dooley, J.S.G., R. Lallier and T.J. of Aeromonas hydrophila. Vet. 344. Trust. 1986. Antigenic structure Immunol. Immunopathol. 12:339.

5.  Dooley, J.S.G. and T.J. Trust. 1988. Surface protein composition of Aeromonas hydrophila strains virulent for fish: identification of a surface array protein. J. Bacteriol.

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7.  Fliermans, C.B. and T.C. Hazen. 1980. Immunoflourescence of Aeromonas hydrophila as measured by fluorescence photometric microscopy. Can. J. Micro. 26: 161-168.

8.  Leblanc, D., K.R. Mittal , G. Olivier and R. Lallier.1981. Sero groupings of motile Aeromonas species isolated from healthy and moribund fish . Appl. Environ. Microbiol. 42: 56-60.

9.  Plumb, J.A., ed. 1979. Principal diseases of farmed raised catfish. Southern Cooperative Series No. 225, Alabama Agricultural Experiment Station, Auburn University, Auburn, AL. 92 pp.

10. Takahaski, Y. and R. Kusuda. 1977. Studies on the scale protrusion disease of carp fishes. III. Serological properties of Aeromonas liquefaciens isolated form diseased fishes. Fish. Path. 12: 15-19.

11. Thune, R.L. 1980. Immunization of channel catfish (Ictalurus punctatus) against Aeromonas hydrophila via hyperosmotic infiltration. Dissertation completed at Auburn University, Auburn, AL. 46 pp.

12. Thune, R.L., T.E. Graham, L.M. Riddle and R.L. Amborski. 1982. Extracellular proteases from Aeromonas hydrophila: partial purification and effects on age-) channel catfish. Trans. Amer. Fish. Soc. 111: 749-754.

13. Thune, R.L., M.C. Johnson, T.E. Graham and R.L. Amborski. 1986. Aeromonas hydrophila B-haemolysin: purification and examination of its role in virulence in O-group channel catfish, Ictaluras punctatus (Rafinesque). J. Fish Dis. 9: 55-61.