Larisa A. Ford, MS; Ronald L. Thune, PhD
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Johnson (unpublished data) have demonstrated that a similar protein is produced in strains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.
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virulence factors of Aeromonas hydrophila in fish infection. Can. J. Micro.
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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.
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groupings of motile Aeromonas species isolated from healthy and moribund fish . Appl.
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catfish. Southern Cooperative Series No. 225, Alabama Agricultural Experiment Station, Auburn
University, Auburn, AL. 92 pp.
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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.
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Aeromonas hydrophila B-haemolysin: purification and examination of its role in virulence
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