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The efficacy of copper sulphate (CuSO4) against the early stages of an experimental acute infection of Flavobacterium columnare in channel catfish (Ictalurus punctatus) was evaluated. Fish were challenged, by waterborne exposure to F. columnare in an ultra-low flow-through water delivery system, and treated with CuSO4 at 4.2 and 2.1 mg L−1 at 5.5 h post challenge for 4 h. Bacterial challenged non-treated fish acted as a positive control and non-challenged non-treated fish acted as a negative control. Fish challenged with F. columnare and treated with CuSO4 at 4.2 and 2.1 mg L−1 post challenge had mortalities of 6.4% and 18.3%, respectively, compared with the positive control, which had 90.2% mortality. The mortality of challenged fish treated with CuSO4 at 4.2 and 2.1 mg L−1 was significantly different from the positive control. There was no significant difference between the mortalities of the 4.2 and 2.1 mg L−1 treatments. The results suggest that CuSO4 has clear therapeutic value against columnaris infections.

The most common bacterial diseases in pond-raised channel catfish Ictalurus punctatus (Rafinesque) are enteric septicemia of catfish and columnaris, caused by Edwardsiella ictaluri and Flavobacterium columnare respectively. Medicated feed containing antibiotics is one management approach that catfish producers use in the treatment of bacterial diseases. However, the future use of all types of medicated feed in catfish aquaculture is uncertain. To discover effective alternatives to antibiotics, a rapid 96-well microplate bioassay utilizing E. ictaluri and F. columnare to evaluate natural compounds and extracts was developed. In this bioassay, bacterial growth is determined by absorbance measurements of microplate wells after 24 h incubation and then confirmed by detecting cell viability after the addition of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide with additional incubation for 24 h. The minimum inhibitory concentration, minimum bactericidal concentration and 50% inhibition concentration (IC50) are determined by graphing the absorbance data. The 24 h IC50 results of test compounds are compared with the 24 h IC50 results of the drug controls oxytetracycline and florfenicol. Among the antibiotics evaluated, doxycycline and tetracycline appear more effective against E. ictaluri and F. columnare than either drug control. This bioassay is rapid, reproducible and economical for evaluating a large number of compounds and extracts.

The efficacy of potassium permanganate (KMnO4) against the early stages of an experimental acute infection of Flavobacterium columnare in channel catfish, Ictalurus punctatus, was evaluated. Fish were experimentally challenged by waterborne exposure for 2 h to F. columnare after cutaneous abrasion, and treated with KMnO4 at 2.0 mg L−1 above the KMnO4 demand at 0, 1, 2 or 4 h postchallenge for 24 h. Challenged non-treated fish acted as a positive control and non-challenged non-treated fish acted as a negative control. Fish challenged and treated with KMnO4 at 0, 1, 2 or 4 h postchallenge had mortalities of 26%, 63%, 64% and 83% respectively. The mortality of challenged fish treated with KMnO4 at 0 h postchallenge (26%) was significantly less than the positive control (77%). The mortalities of challenged fish treated at 1, 2 or 4 h postchallenge were not significantly different from the positive control fish. The results suggest that KMnO4 has a clear therapeutic value in early stages of columnaris infection but limited therapeutic value once the infection has progressed.

Summary The alternative and classical complement pathways of channel catfish (Ictalurus punctatus), most importantly the classical complement pathway which requires antibody, demonstrated bactericidal activity against two Gram-negative bacterial fish pathogens, Flexibacter columnaris and Vibrio anguillarum. This shows the importance of antibody-mediated bactericidal immunity in fish by the classical complement pathway in providing protection following immunization or natural infections.