potentiated sulphonamide and oxytetracycline are generally effective against

Potentiated sulphonamide and oxytetracycline are

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potentiated sulphonamide and oxytetracycline are generally effective against the pathogen, regardless of the disease manifestation and assuming that treatment begins at an early stage in the disease cycle (Gayer et al, 1980). Unfortunately, despite its comparatively recent arrival in the armoury of fisheries chemotherapeutants, resist- ance to amoxycillin has been documented in Scotland (Barnes et ai, 1994). For the future, florfenicol, dosed at lOmg/kg body weight of fish/day for 10 days, offers promise, insofar as it has already been used with some success against furunculosis in Norway (Nordmo et al, 1994; Samuelsen et al, 1998). Since the initial work with oxolinic acid (Endo et ai, 1973; Austin et ai, 1983b), it is apparent that substantial quantities have been used in aquaculture within many countries. Consequently, it is hardly surprising that resistant strains of Aer. salmon- icida have emerged (Hastings and McKay, 1987; Tsoumas et al., 1989; Barnes et al., 1990a). Yet, the widespread usefulness of the compound at controlling furunculosis has prompted a search for other related compounds. The fruits of this research may be illustrated by the apparent success of 4-quinolones/fluoroquinolones at inhibiting the pathogen (Barnes et al., 1990b; 1991b; Bowser and House, 1990; Lewin and Hastings, 1990; Martinsen et al., 1991; Inghs and Richards, 1991; Stoffregen et al., 1993; Elston et al., 1995). Thus, enrofloxacin and sarofloxacin have been found to be more effective than oxoHnic acid, in terms of MIC, at inactivating Aer. salmonicida. The effectiveness of enrofloxacin at lOmg/kg body weight offish/day for 10 days has been attested by field trials with lake trout (Salvelinus namaycush) (Hsu et al., 1995). In addition, enrofloxacin has been effective in controlHng atypical Aer. salmonicida in tom cod, insofar as a single injection with 5 mg of enrofloxacin/kg of fish stopped furunculosis (WilHams et al, 1997). Difloxacin, dosed at 5 or 10 days at 5 mg/kg body weight, and 10 days at 1.25 or 2.5 mg/kg body weight resulted in significantly lowered mortalities compared with controls following i.m. injection with a virulent culture of Aer. salmonicida. Of relevance, there was little difference in the results between 5 and 10 day treatment regimes with 5 mg/kg body weight (Elston et al., 1995). With the realisation that Aer. salmonicida occurs on the external surfaces, i.e. gills and mucus, attempts have been made at disinfection. Cipriano et al. (1996c, d) evaluated chloramine T, dosed at 15 mg/1 for 60 min on three consecutive days, but the infection was not controlled. Success occurred with 77 mg of oxytetracycline/kg
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392 Bacterial Fish Pathogens body weight offish/day for 10 days with mortahties stopping within 4 days of starting treatment (Cipriano et ai, 1996c). There can be no dispute that chemotherapeutic agents are (and will continue to be) invaluable for preventing heavy mortalities during outbreaks of furunculosis.
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