Diquat dibromide is a pesticide (herbicide) active ingredient that has been in use since the 1960’s. It also has many formulations that are legal for aquatic use. Since at least the 1970’s it has also been used by some fish hatchery managers and fish farmers to control bacterial infections. Anecdotal observations by hatchery managers throughout the US indicate that treatments of 15-18 ppm for 1-3 hours up to three consecutive or alternating days is an effective dose range.
In 2007, a study protocol for an INAD (investigational new animal drug) was issued for a diquat formulation to determine the efficacy of diquat to control mortality caused by bacterial gill disease and external flavobacteriosis in a variety of fish species.
Two published studies are worth mentioning. In a 2004 paper, channel catfish fingerlings were challenged with 4 virulent isolates of Flavobacterium columnare and treated with potassium permanganate, chloramine-T, hydrogen peroxide, copper sulfate and diquat. At 20 hours, the fish were treated with indefinite bath treatments. The untreated controls suffered 100% mortality. The fish treated with diquat suffered no mortalities. Bath treatments with chloramine-T and potassium permanganate reduced mortality from 100% to 75% and 69% respectively, but the copper sulfate and hydrogen peroxide were not effective.
In a 2008 paper, channel catfish fingerlings were challenged by abrasion and waterborne exposure with Flavobacterium columnare and treated at 22-hr with four different concentrations (2.5, 5.0, 10.0, & 15ppm) of diquat for 6-hr. At the conclusion of the trial, 21-days post challenge, the untreated fish had experience 95% mortality. The 5.0, 10.0 and 15.0ppm treatments experience 68%, 59% and 49% mortality respectively.
Diquat will not have any effect on fish with internal infections. Only antibiotics could have any impact on this type of infection, but it does appear that diquat can be another “tool in the toolkit” to attempt to control or limit bacterial infections in trout raceways.
