I TREATMENT OF SULFONAMIDE-RESISTANT FURUNCULOSIS IN TROUT 

 AND DETERMINATION OF DRUG SENSITIVITY 



By S. F. SNIESZKO, Bacteriologist, and G. L. Bullock, Fishery Aid 



During tlic past 2 years, tlie Microbiological 

 Laboratory, at Leetown, W. Va.,' has received 

 an increasing number of reports of outbreaks of 

 furunculosis that failed to respond to treatment 

 with sulfonamides. Tlie problems resulting from 

 furunculosis, a widespread, bacterial disease of 

 salmonids, and control of the disease have been 

 reviewed in detail by McCraw (1952) and Snieszko 

 (1954a). The latter investigator recommended the 

 use of antibiotics for diseased fish that failed to 

 respond to treatment with sulfonamides. In some 

 of the recent incidences of furunculosis at the Lee- 

 town station the disease did not respond to sulfon- 

 amide therapy, but control was effected with 

 chloramphenicol. Failure of sulfonamides to con- 

 trol furunculosis suggested that the causal organ- 

 ism, Aeninwnas mlinoiiicida, was resistant to the 

 drugs usetl. 



Several methods are widely used in the routine 

 determination in vitro of the sensitivity of bac- 

 teria to sulfonamides and antibiotics. The results 

 obtained with antibiotics are generally accepted 

 as reliable aids in the selection of the most prom- 

 ising treatment. Such, however, is not the case 

 with sulfonamides, ami "The relation of in vitro 

 sensitivity test to clinical effectiveness is still con- 

 troversial" (Burdette, Plank, and Clapper, 1955). 



The experiments presented in this paper followed 

 three lines of endeavor: (1) A comparison of the 

 effectiveness of sulfonamide and antibiotic therapy 

 in strains of trout which were either susceptible or 

 resistant to furunculosis. (2) Comparison of the 

 therapy of furunculosis with sulfonamides and 

 chloramphenicol in trout wliich were suffering 

 from furunculosis caused by sulfonamide-resistant, 

 but chloramphenicol-sensitive, strains of A. sal- 

 monicida. (3) The development of a standard 

 method for laboratory and field use of determin- 

 ing in vitro the sensitivity of A. salmonicida to 

 sulfonamides and antibiotics. 



1 Po.st Office, Kearnevsville, W. Va. 



Note— Approved for publication, March 12, 1957. Fishery Bulletin 125. 



Many factors are capable of affecting in-vitro 

 tests of microbial sensitivity to sulfonamides; 

 therefore, correct interpretation and application 

 of the results of such tests will be reliable if a 

 proved method of obtaining reproducible results 

 is used. 



As the result of the findings presented in this 

 study and practical experience gained in the 

 treatment of this disease in trout hatcheries over 

 the entire country, a revised procedure has been 

 evolved and describetl for the treatment of this 

 disease. 



The authors wish to express their thanks to Dr. 

 K. E. Wolf for his assistance during the prepara- 

 tion of the manuscript and for furnishing the 

 strains of ^1. salmonicida isolated in Iowa, Utah, 

 Minnesota, and Wisconsin; and to Dr. R. E. 

 Lennon for supplying the fingerling brook trout 

 from Erwin, Tenn. 



MATERIALS AND METHODS 



Treatment with sulfonamides and chloramphenicol 



Experiments on tlie therapy of furunculosis were 

 carried out with two strains of fingerling brook 

 trout {Salrelinux fontinalis) and a strain of brown 

 trout [Salmo trutta). A strain of brook trout 

 from Bellefonte, Pa., was selected because it was 

 known to be resistant to furunculosis (Wolf 1954; 

 Snieszko 1954b), and a strain from Erwin, Tenn., 

 was used because it was suspected to be suscep- 

 tible. The brown trout were from Cortland, X. Y. 

 In general, brown trout are known to be more 

 resistant to furunculosis than are most strains of 

 brook trout (McCraw 1952; Wales and Berrian, 

 1937). 



Equal weights of the trout were distributed 

 among stainless steel troughs in which the water 

 was maintained at temperatures of 12° to 13° C. 

 (54°-55° F.). Infection of the fish was initiated 

 by adding fresh cultures of AeromonaK salmonicida 

 to the diet, and treatment was started when the 

 first mortality due to furunculosis occurred. 



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