TOXICITY AND PATHOLOGICAL EFFECTS OF ORALLY-ADMINISTERED IVERMECTIN IN ATLANTIC, CHINOOK, AND COHO SALMON AND STEELHEAD TROUT

Because ivermectin (22,23-Dihydroavermectin B1) has been proposed as a n oral treatment against sea lice infections of farmed salmonids, we i nvestigated the toxicity and pathological effects of various doses of ivermectin administered orally every second day to steelhead trout Onc orhynchus mykiss, coho salmon Oncorhynchus kisutch, chinook salmon Onc orhynchus tshawytscha and Atlantic salmon Salmo salar under laboratory conditions. These species differed in their ability to tolerate iverm ectin, with coho salmon the most tolerant followed by chinook, then At lantic salmon. Unequivocal results for steelhead trout were not obtain ed due to a secondary disease outbreak (vibriosis). Histological exami nation of the major organ systems revealed no pathological changes tha t could be associated with ivermectin toxicity in any of the salmon sp ecies tested.