Sulfonamide Therapy of Furunculosis in Brown Trout

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Studies were made, both in the field and in the laboratory, to determine the effectiveness of sulfonamides in the treatment of the disease furunculosis in the brown trout (Salmo trutta). When sulfamerazine was fed at a level of 8 grams per 100 pounds of trout, the infected fish were not always rid of the bacterium which causes furunculosis. In the laboratory it was found that when virulent cultures of Bacterium salmonicida were injected into brown trout on the first day of the sulfamerazine feeding, several of the trout developed active infections after the treatment was discontinued. When virulent cultures were injected into trout on the fourth day of the sulfamerazine feeding, several “carriers” of furunculosis developed. A combination of sulfamerazine and sulfaguanidine was apparently effective in ridding experimentally infected trout of the bacterium of furunculosis. A bacteriological study was made of an outbreak of furunculosis which occurred in the brown trout fingerlings of a state hatchery. The epidemic was treated by the daily use of 8 grams of sulfamerazine per 100 pounds of trout. The sulfamerazine treatment produced a marked drop in the mortality rate of the fingerlings, but positive cultures were encountered in some treated fingerlings 8 days after the completion of the treatment. A secondary feeding of sulfamerazine at the 8-gram level was given to the hatchery trout in an effort to eliminate “holdovers” of the disease. Two positive cultures were obtained from live and apparently healthy brown trout after the secondary treatment, indicating that some of the treated hatchery trout were “carriers.” An outbreak of furunculosis, which occurred among the brood-stock brown trout of a state hatchery, was treated by the use of a combination of sulfamerazine and sulfaguanidine. The treatment produced a marked drop in the mortality rate. To date, the brown trout treated by this combination of sulfonamides have yielded negative bacteriological cultures.