Treatments for Ich Infestations in Channel Catfish Evaluated under Static and Flow-Through Water Conditions

Abstract

In response to producer reports of poor efficacy using published treatments against Ichthyophthirius multifiliis (ich) infestations of channel catfish Ictalurus punctatus, we initiated a preliminary study to see which of the compounds that are permissible in food-fish aquaculture were efficacious under what we judged to be ideal laboratory conditions. We planned to use the results of this work as a basis for further study in the field. In our studies we used fingerling catfish in glass aquariums with stable water quality and daily treatments in both static and flow-through water systems. In some experiments, infested fish and healthy fish were stocked together so that the ability of the treatment to eliminate preexisting infestations could be examined separately from the treatment's ability to block transmission. Malachite green and methylene blue were used as positive controls, and untreated fish were used as the negative control for efficacy. Treatments with sodium chloride, hydrogen peroxide, potassium permanganate, elevated temperatures, chloramine-T, povidone iodine, and high flow rates were ineffective under all conditions and concentrations tested. Alternate-day treatments with formalin at 25 and 50 mg/L in static water or 100 mg/L in flow-through water aquariums were not effective at eliminating infestations or preventing fish-to-fish transmission. Daily treatments with 50 mg/L formalin in static water or 100 mg/L formalin in flow-through water aquariums were toxic to channel catfish. Formalin at 25 mg/L in static water aquariums was effective at eliminating infestations and reducing fish-to-fish transmission when used as a daily treatment. Copper sulfate was effective in blocking the infestation of healthy fish, but it did not eliminate established infestations on channel catfish, even after 20 daily treatments under static conditions. Malachite green and methylene blue eliminated established infestations on fish, whereas copper only prevented transmission; this suggests that the dyes are able to penetrate deeply or act systemically, allowing them to kill subcutaneous stages of ich. Further field trials were not pursued because none of the legal or economically viable treatments showed efficacy under presumably ideal laboratory conditions.