A review of the use of ivermectin as a treatment for sea lice [Lepeophtheirus salmonis (Kroyer) and Caligus elongatus Nordmann] infestation in farmedAtlantic salmon (Salmo salar L.)

Ectoparasitic sea lice, Lepeophtheirus salmonis (Kroyer) and Caligus elonga tus (Nordmann) browse on the skin of farmed Atlantic salmon, Salmo salar L. . The resulting lesions cause stress and increase the susceptibility of the fish to secondary infections. In extreme infestations, fish can suffer fro m osmoregulatory failure and death. The most immediate treatment for the re lief of sea lice infestations at fish farms is the use of chemotherapeutant s, either by bath or oral administration. One compound which has been inves tigated for use as a chemotherapeutant is ivermectin. Ivermectin is a membe r of the avermectin group of macrocyclic lactones. The avermectins are neur otoxins, which have been used successfully in the treatment of helminthic p arasitic infections in a number of terrestrial farm animals and also in the treatment of river blindness in humans. Owing to the low solubility of the compound, ivermectin has been administered as an oral treatment with the f eed and has been found to be effective for the treatment of sea lice on Atl antic salmon. Ivermectin is poorly absorbed by fish with a high percentage of the administered dose being excreted in the faeces. The highest concentr ations of the absorbed ivermectin were found in lipid-rich organs. The iver mectin remained in the tissues of the treated fish for a prolonged period o f time and was excreted mainly in the unchanged form. Ivermectin can reach the marine environment via excretion from the bile, unabsorbed via the fish faeces and by uneaten food pellets and has a strong affinity to lipid, soi l and organic matter. Risk assessments have shown that ivermectin is likely to accumulate in the sediments and that the species therein would be more at risk than the species in the pelagic environment. Ivermectin has been sh own to be toxic to some benthic infaunal species in single species tests, b ut there is no evidence that treatment of fish with ivermectin has affected multispecies benthic communities in the field situation. This review paper ends with details of the development and the status of the use of ivermect in in the treatment of sea lice infestations at Scottish fish farms.