297
the infection was not controlled. Success occurred with 77 mg of oxytetracycline/kg
body weight of fi sh/day for 10 days with mortalities stopping within 4 days of starting treatment (Cipriano et al. 1996c ).
There can be no dispute that chemotherapeutic agents are (and will continue to
be) invaluable for preventing heavy mortalities during outbreaks of furunculosis.
Nonetheless, there are substantial reasons for avoiding total reliance upon such
compounds. For instance, the development of resistance by the pathogens to some
widely used drugs is cause for serious concern (Wood et al. 1986 ). Thus, it is disquieting that plasmids carrying antibiotic resistance factors (R factors) have been isolated from Aer. salmonicida strains (Aoki et al. 1971 ). Indeed, a strain resistant to
sulphathiazole and tetracycline was recovered as early as 1959. Moreover, Snieszko
and Bullock ( 1957 ) reported the occurrence of cultures that were resistant to sulphonamides, although at the time the mechanism of resistance was not known. Now,
it seems that resistance may refl ect alterations in the outer membrane of Aer. salmonicida (Barnes et al. 1992 ). Aoki et al. ( 1983 ) examined 175 isolates, which had
been isolated from cultured and wild salmonids in Japan, for susceptibility to a wide
range of antimicrobial agents. They noted that 96 % of the isolates from cultured
fi sh were resistant to at least one, and up to six, of the drugs, particularly nalidixic
acid and nitrofuran derivatives. In addition, transferable R plasmids, coding for
resistance to chloramphenicol, streptomycin and sulphonamides, and nontransferrable plasmids conveying resistance against tetracycline, were found in several strains. It was concluded from these results that drug resistant strains of Aer.
salmonicida had increased in direct proportion to the enhanced use of antimicrobial
compounds in fi sh culture. This was particularly evident in view of the observation
that few isolates recovered from wild salmonids exhibited drug resistance. Toranzo
et al. ( 1983a , b ) characterised the plasmids, determining that the organism frequently possessed more than one plasmid. In particular, fi ve strains possessed six
plasmids of varying molecular weights. However, these workers did not observe any
correlation between loss of plasmids, or changes in plasmid mobilities, and the loss
of resistance to sulphadiazine, in the case of one strain that was studied in detail.
Nevertheless, the molecular genetic studies of Mitoma et al. ( 1984 ) identifi ed gene
sequences in R plasmids coding for either chloramphenicol or tetracycline resistance. Hedges et al. ( 1985 ), using aeromonad isolates obtained from France, Ireland,
Japan and the UK, determined that plasmids from Aer. hydrophila and Aer. salmonicida were similar. These workers reported that some plasmids were transmissible to
Esch. coli, whereas others were unstable in this recipient organism. The R plasmids
of Aer. salmonicida were considered to confer upon the pathogen the potential to
withstand the onslaught of a wide variety of inhibitory agents, thus diminishing the
effectiveness of chemotherapy. It may be hoped that the development of new compounds, particularly synthetic or semi-synthetic molecules, and the strict rotation in
the use of currently available drugs may assist with the problem of resistance in the
pathogen. Obviously, a constant awareness of the problem must be maintained.
Another aspect of chemotherapy concerns the presence and retention of the compounds in fi sh tissues. In fact, diffi culties of this nature were recognised as early as
1951 with a report by Snieszko and Friddle who expressed concern with tissue
Aeromonas salmonicida
the infection was not controlled. Success occurred with 77 mg of oxytetracycline/kg
body weight of fi sh/day for 10 days with mortalities stopping within 4 days of starting treatment (Cipriano et al. 1996c ).
There can be no dispute that chemotherapeutic agents are (and will continue to
be) invaluable for preventing heavy mortalities during outbreaks of furunculosis.
Nonetheless, there are substantial reasons for avoiding total reliance upon such
compounds. For instance, the development of resistance by the pathogens to some
widely used drugs is cause for serious concern (Wood et al. 1986 ). Thus, it is disquieting that plasmids carrying antibiotic resistance factors (R factors) have been isolated from Aer. salmonicida strains (Aoki et al. 1971 ). Indeed, a strain resistant to
sulphathiazole and tetracycline was recovered as early as 1959. Moreover, Snieszko
and Bullock ( 1957 ) reported the occurrence of cultures that were resistant to sulphonamides, although at the time the mechanism of resistance was not known. Now,
it seems that resistance may refl ect alterations in the outer membrane of Aer. salmonicida (Barnes et al. 1992 ). Aoki et al. ( 1983 ) examined 175 isolates, which had
been isolated from cultured and wild salmonids in Japan, for susceptibility to a wide
range of antimicrobial agents. They noted that 96 % of the isolates from cultured
fi sh were resistant to at least one, and up to six, of the drugs, particularly nalidixic
acid and nitrofuran derivatives. In addition, transferable R plasmids, coding for
resistance to chloramphenicol, streptomycin and sulphonamides, and nontransferrable plasmids conveying resistance against tetracycline, were found in several strains. It was concluded from these results that drug resistant strains of Aer.
salmonicida had increased in direct proportion to the enhanced use of antimicrobial
compounds in fi sh culture. This was particularly evident in view of the observation
that few isolates recovered from wild salmonids exhibited drug resistance. Toranzo
et al. ( 1983a , b ) characterised the plasmids, determining that the organism frequently possessed more than one plasmid. In particular, fi ve strains possessed six
plasmids of varying molecular weights. However, these workers did not observe any
correlation between loss of plasmids, or changes in plasmid mobilities, and the loss
of resistance to sulphadiazine, in the case of one strain that was studied in detail.
Nevertheless, the molecular genetic studies of Mitoma et al. ( 1984 ) identifi ed gene
sequences in R plasmids coding for either chloramphenicol or tetracycline resistance. Hedges et al. ( 1985 ), using aeromonad isolates obtained from France, Ireland,
Japan and the UK, determined that plasmids from Aer. hydrophila and Aer. salmonicida were similar. These workers reported that some plasmids were transmissible to
Esch. coli, whereas others were unstable in this recipient organism. The R plasmids
of Aer. salmonicida were considered to confer upon the pathogen the potential to
withstand the onslaught of a wide variety of inhibitory agents, thus diminishing the
effectiveness of chemotherapy. It may be hoped that the development of new compounds, particularly synthetic or semi-synthetic molecules, and the strict rotation in
the use of currently available drugs may assist with the problem of resistance in the
pathogen. Obviously, a constant awareness of the problem must be maintained.
Another aspect of chemotherapy concerns the presence and retention of the compounds in fi sh tissues. In fact, diffi culties of this nature were recognised as early as
1951 with a report by Snieszko and Friddle who expressed concern with tissue
Aeromonas salmonicida