Introduction: In the EU several drugs are registered for the control of fish lice (Lepeophtheirus salmonis) in atlantic salmon (Salmo salar) in marine cultures. However, no data are available regarding the safety and efficacy of such drugs for the treatment of carp louse (Argulus foliacaeus) infestation in carp (Cyprinus carpio). We therefore tested four ectoparasiticides registered for bathing treatment of atlantic salmon (deltamethrin, hydrogen peroxide) or for medication via feed (emamectin benzoate, teflubenzurone) for their suitability for carp louse control in carp-farming.
Methods: The treatments were carried out in flow-through water tanks with 20 carps per group. In the tolerance study every drug was used in four different doses in uninfested, healthy carps (the recommended dose and up to 25 times the recommended dose; hydrogen peroxide: maximal 7 times the recommended dose) and putative changes in behaviour were recorded. During the efficacy trials the carps were artificially infected with carp lice and subsequently treated with the recommended dosage or half of this dose. Results: The carps tolerated all drugs very well at doses recommended for salmon. After overdosing the carps showed dose-dependent symptoms of toxicity (e.g. apathy, gasping), in case of the oral formulations also denial of food at the highest dosages and in case of bathing treatment a 40% loss of fish. The histological examination of selected carps showed no pathological changes except major erosions of the gills in animals subjected to higher doses of hydrogen peroxide.
The efficacy study demonstrated a significant reduction of parasites in all treated groups in comparison to the untreated controls, and a complete loss of parasites after bathing treatment. Conclusion: Both trials confirmed the safety and efficacy of deltamethrin (Alpha Max®), hydrogen peroxide (the substance was taken), emamectin benzoate (Slice®) and teflubenzurone (Ektobann®) using the recommended dosages. The direct comparison revealed that the bath treatment showed better results. The oral administration, however, is probably easier to perform for the fish breeder and was much better tolerated by the fish. The next step is to find out if a combination of the two methods of treatments is more effective in the open field.