Drug resistance of atypical Aeromonas salmonicida from Atlantic salmon and rainbow trout in Newfoundland

Antimicrobial drug resistance patterns of atypical isolates of Aeromonas salmonicida from Atlantic salmon Salmo salar, or rainbow trout Oncorhynchus mykiss, from sites in Newfoundland were determined by disk diffusion assay. There was no resistance against chloramphenicol, kanamycin, nalidixic acid, neomycin or nitrofurantoin, but isolates resistant to erythromycin were found at 3 sites. Oxytetracycline-resistant strains reached 61% in 1995. Resistance to sulphamethoxazole-trimethoprim reached 52%, and a similar trend occurred with ormetoprim-sulfadimethoxine. Simultaneous resistance to the potentiated sulphonamides and oxytetracycline was common at some sites. Enrofloxacin appears to be the most promising drug, since all 72 isolates tested during 1995 were fully susceptible to the drug.