USE OF DIQUAT DIBROMIDE (Reward®) FOR TREATMENT OF COLUMNARIS DISEASE IN SALMONIDS: A CASE REVIEW

In the fall of 2017 and winter of 2017/2018, Wells Fish Hatchery experienced unusually high pre-spawning mortality (upwards of 15%) in each of its summer Chinook (Oncorhynchus tshawytscha) and summer steelhead (Oncorhynchus mykiss) salmon broodstock populations due to Flavobacterium columnare (the causative agent of columnaris disease).  Diquat dibromide (Reward^®), an aquatic herbide available for use through AADAP's Investigational New Animal Drug (INAD) program, was considered as a treatment option because of its reported efficacy against external flavobacteriosis in freshwater-reared finfish and higher margin of safety compared to other oxidizing chemicals. Because of its cost prohibitive price-tag, information regarding its successful use in salmonid hatcheries within the state of Washington was limited. The promising outcome of our experimentation with Reward^® prompted the exclusive use of this product for treatment of summer Chinook for columnaris disease in the fall of 2018, this time before the progression of clinical symptoms had induced substantial morbidity. A summarization of the outcome of our treatments, general findings, and management considerations is offered to facilitate the discussion of control methods for this pathogen among fish health professionals.