Toxicity and pathological effects of orally administered ivermectin in Atlantic, chinook, and coho salmon and steelhead trout

Because ivermectin (22,23-Dihydroavermectin B,) has been proposed as an oral treatment against sea lice infections of farmed salmonids, we investigated the toxicity and pathological effects of various doses of ivermectin administered orally every second day to steelhead trout Oncorhynchus mykiss, coho salmon Oncorhynchus kjsutch, chinook salmon Oncorhynchus tshawytscha and Atlantic salmon Salmo salar under laboratory conditions. These species differed in their ability to tolerate ivermectin, with coho salmon the most tolerant followed by chinook, then Atlantic salmon. Unequivocal results for steelhead trout were not obtained due to a secondary disease outbreak (vibriosis). Histological examination of the major organ systems revealc?d no pathological changes that could be associated with ivermectin toxicity in any of the salmon species tested.