DRUG RESISTANCE OF FURtTXCULOSIS IX TROUT 



563 



live. Tlic experience with cliloitetracycline pre- 

 viously cited may serve as an example. It is well 

 to remember the limitations of the in-vitro test 

 for a correct interpretation of the results. 



The results of our studies show that in the in-vitro 

 test sulfadiazine, sulfamerazine, and sulfisoxazole, 

 and probably other sulfonamides, can be used 

 equally well to determine sulfonamide sensitivity 

 of ^4. salmonicida. In the therapeutic and feeding 

 experiments, however, sulfadiazine has been found 

 to be more toxic to the fish than has sulfamerazine, 

 and sulfisoxazole has been found to be free from 

 any detectable untoward effects even to brown 

 trout (Snieszko and Wood, 1955). Unfortunately, 

 the experiment on the use of sulfisoxazole for the 

 treatment of furunculosis was unsuccessful because 

 the strains of A. salmonicida isolated from the 

 outbreak were resistant to sulfonamides ^ (tables 

 1 to 3). 



Testing of the sensitivity of A. salmonicu/a to 

 sulfonamides and antibiotics can be carried out 

 m several ways. The most practical method 

 seems to be the use of drug-containing, filter-paper 

 disks. Selection of a proper medium is very im- 

 portant when testing the sensitivity of the organ- 

 ism to sulfonamides. Trypticase Soy agar, special 

 furunculosis agar medium buffered and alkalized, 

 and Mueller Hinton agar, are especially recom- 

 mended. In the first two media, the brown pig- 

 ment so characteristic of A. salmonicida is pro- 

 duced within 48 hours and the paraphenylenedia- 

 mine test of Griffin (1952) can be performed as 

 soon as growth is visible to the unaided eye. With 

 the first two media, disks should contain 0.5 milli- 

 gram of the drug; with Mueller Hinton medium, 

 only 0.1 milligram. The test can be equally well 

 performed using either petri plates or agar slants — 

 the latter are more suitable for fieldwork. Also 

 media in which sulfonamides have been incorpo- 

 rated may be used provided that the control 

 medium is free of sulfonamides and that both 

 sulfonamide-resistant and sulfonamide-sensitive 

 strains oi A. salmonicida are available for controls. 



RECOMMENDATIONS 



On the basis of the results reported in this paper 

 and past experience in the treatmetit of furuncu- 

 losis, we wish to make the following recommenda- 

 tions : 



^ One experiment of this series with sulfisoxazole was performed at Leetowii 

 in 1955 by Bo Svenonius, a visiting fishery biologist from Sweden. 



1. As soon as a disease suspected to be furuncu- 

 losis breaks out, start treatment with sulfona- 

 mides. Sulfamerazine is still cotisidered the drug 

 of clioice, but sulfametlmzitie or a combination of 

 sulfamerazine with sulfaguanidine can also be 

 used (Snieszko 1954a). Sulfisoxazole is a drug 

 of great promise, but it still has to be evaluated 

 experimentally. 



2. The diagnosis of furunculosis should be con- 

 firmed as soon as possible by bacteriological 

 examination. This should include determination 

 in vitro of the sensitivity of the pathogen to the 

 drug. 



3. The dosage of sulfonamides is 8 to 10 grams 

 per 100 pounds of fish per day. Treatment should 

 last for 10 to 20 days and should result in a com- 

 plete stoppage of mortalities due to furunculosis. 

 Recurrences are particularly likely in disease- 

 susceptible strains of trout. 



4. Sulfonamides should never be used at lower 

 levels or treatments repeated at short intervals. 

 Such practice is the surest way to produce sulfa- 

 resistant strains of the pathogen. 



5. If the response to the treatment with sulfona- 

 mides is not rapid, if recent experience has shown 

 that treatment with sulfonamides is not effective, 

 or if the results of laboratory examination show 

 that the organism is sulfa-resistant but sensitive 

 to oxA'tetracycline or chloramphenicol, either 

 antibiotic should be used at a rate of 2.5 to 3.0 

 grams (of the antibiotic activity) per 100 pounds 

 of fish per day. Chloramphenicol is somewhat 

 better than oxytetracycline. 



6. One should never rely on any drug for a 

 permanent control of furunculosis, or of atiy other 

 disease. There is no drug in existence which would 

 permit elimination of any animal or plant disease. 

 The best a drug can do is to reduce losses tempo- 

 rarily. Long-lasting control of furunculosis, or 

 any other infectious disease, is only possible by 

 elimination of the source of infection, good sani- 

 tation, and introduction or development by selec- 

 tive breeding, of a strain of fish with greater 

 disease resistance. (iood hatchery practices, 

 avoidance of crowding, and balancetl nutrition 

 are very important factors in keeping fish healthy. 



SUMMARY 



Therapeutic studies were performed with three 

 sulfoiuunides and one antibiotic (chloramphenicol) 

 with two strains of fingerling brook trout — one 



