Bath Treatments With Emamectin Benzoate Control Dactylogyrideans Infecting <i>Colossoma macropomum</i> Gills Without Altering Physiology of This Host Fish

Intensification of fish aquaculture worldwide has led to severe problems of diseases caused by parasitic dactylogyrideans. In this study, the anti-dactylogyridean efficacy of treating Colossoma macropomum with baths of emamectin benzoate (EMB) was investigated for the first time, along with the effects of this treatment on the haematology and histology of its gills. Thirty-nine fish (three replicates of 13 each) received four consecutive daily baths with EMB at a concentration of 0.5 g L^-1 and one control group of 39 fish (three replicates of 13 each) only received water from the cultivation tank. The EMB treatments showed effectiveness of 50.2% against Anacanthorus spathulatus, Notozothecium janauachensis and Mymarothecium boegeri infecting the gills of C. macropomum. Fish treated with EMB only caused decreases in the total red blood cell, total thrombocytes, leukocytes, monocytes and neutrophils numbers, while the mean corpuscular haemoglobin concentration (MCHC) increased. Aneurysm was the gill structural alteration caused by EMB treatment, and mean assessment values (MAV) indicated moderate to severe damage due to this irreversible gill alteration. Our results lay a safe foundation for the application of EMB as a potential chemotherapeutic agent for dactylogyridean control in C. macropomum aquaculture, because this drug should be used cautiously for anthelminthic treatment in fish.