Virulence of 'atypical' Aeromonas salmonicida isolated from ulcerated flounder Platichthys flesus

Experiments were conducted to assess the pathogenicity of 2 'atypical' Aerornonas salmon~cida strains solated from ulcerated flounders Platichthys flesus for this species, rainbow trout Oncorhynchus mykiss, roach Rutilus rutilus, and bleak Alburnus alburnus. The pathogenicity of the strains was evaluated by 3 challenge methods: intraperitonea] (i-p.) injection, subdermal (S d . ) injection, and bath exposure. The bath exposure was performed with and without prior abrasion of the skin of the tested flounders. The i.p. injection experiments showed that the tested strain was weakly pathogenic for flounders but non-pathogenic for rainbow trout. The experiments with roach and bleak did not yield conclusive results. The s.d. injection experiment showed that the tested strain induced skin ulcerations in flounders but not in rainbow trout. The bath exposure experiment indicated that the tested strain was able to penetrate the defence mechanisms present in the skin of the flounders only if the skin was abraded prior to the exposure. It was concluded that the 'atypical' A. salrnonicida isolated from flounders will not represent any threat to rainbow trout farms in Finnish coastal waters.