562 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



I 



Figure 2. — Sulfonamide-sensitivity test performed with 

 a sensitive culture of A. salmonicida using Trypticase 

 Soy agar (left) and Mueller Hinton agar (right). Lab- 

 oratory-prepared disks contained 0.1, 0.25, and 0.5 

 milligram of sodium sulfadiazine. 



of the most convenient method for the determina- 

 tion of the sensitivity of fisli-pathogenic bacteria 

 to sulfonamides, antibiotics, and perhaps, to other 

 therapeutic agents. 



DISCUSSION 



Micro-organisms appear to be capable of developing 

 resistance to the majority of drugs which are employed 

 clinically. In view of the cross-resistance shown between 

 different groups of drugs, it is obviously important that 

 the clinician should have information as to which groups of 

 drugs may be usefully tried in infections with resistant 

 strains (Work and Work, 1948). 



It has been found, as coukl be expected, that 

 with the wide use of sulfonamides for treatment of 

 fish diseases, there is an increasing number of 

 outbreaks of furunculosis which are refractory to 

 treatment with these drugs. Since "once resist- 

 ance is liighly developed it is apparently perma- 

 nent" (Northey 1948), it is increasingly important 

 to have means of rapid and reliable determination 

 of such resistance. 



In this paper, observations are described on the 

 treatment of experimentally induced furunculosis 

 employing strains of trout whicli are susceptible 

 or resistant to this disease. The outbreaks were 

 caused by strains of the pathogen which were found 

 to be either sensitive or resistant to sulfonamides. 



Wlien it was found that the disease was refractory 

 to treatment with sulfonamides, chloramphenicol 

 was used with good results (tables 1 to 3). Oxy- 

 tetracycline (terramycin), which is also effective 

 in the treatment of furunculosis and ulcer disease 

 (Snieszko et al., 1952), was not used in this investi- 

 gation because strains of A. salmonicida resistant 

 to this antibiotic have recently been isolated. 



It is evident from the results described here 

 that in addition to chloramphenicol and o.xytetra- 

 oycline other antibiotics and sulfonamides arrest 

 the growth of A. salmonicida in vitro. The results 

 of these tests caiuiot be applied indiscriminately 

 for therapeutic use. While some of the sulfon- 

 amides gave promising results in vitro, their use 

 in the treatment of fish diseases may be limited by 

 their to.xicity to some species of fish or by the 

 arresting of the fish's growth during treatment. 

 Other drugs may not be absorbed from the in- 

 testinal tract and are therefore useless in systemic 

 infections by micro-organisms sensitive to them 

 in vitro (Johnson and Brice, 1953; Snieszko and 

 Wood, 1955; Snieszko and Friddle, 1951; Snieszko 

 and Griffin, 1955). It is interesting that chlor- 

 tetracycline (aureomycin) which lias been found 

 to be effective in vitro against A. salmonicida and 

 Hemophilus piscium is entirely ineffective in the 

 treatment of trout suffering from furunculosis and 

 ulcer disease (Snieszko, Griffin, and Friddle, 1952). 

 Therefore, the in-vitro tests, while very convenient 

 because of the speed and ease witli which they can 

 be performed, must be supplemented by exact 

 treatment trials with several species of fishes, 

 before tiie therapeutic value of drugs can be 

 determined. 



The greatest value of the in-vitro test is in the 

 determination of the acquired drug resistance of 

 the pathogen concerned. This can be explained best 

 by using an example. It is known that furuncu- 

 losis usually can be effectively treated with stil- 

 fonamides. It has also been shown in this paper 

 that the growth of sensitive strains of A. salmoni- 

 cida can be arrested in vitro with tliese drugs. 

 Therefore, if it is found that a particular strain of 

 A. salmonicida is resistant to sulfonamides in vitro, 

 an outbreak of furunculosis caused by that strain 

 will most likely be refractory to a treatment with 

 sulfonamides. It does not mean, however, that 

 if the organism is found to be sensitive to a certain 

 drug in vitro, the treatment with that drug of fish 

 infected with such an organism will also be effec- 



