Clinical and pathological investigation of Psychrobacter immobilis infection in rainbow trout (Oncorhynchus mykiss, Walbaum)

The pathogenicity of Psychrobacter immobilis, isolated from naturally infected rainbow trout Oncorhynchus mykiss, was investigated. In experimentally-infected trout, no mortality was recorded during 35 days but the following external and internal symptoms were observed. Externally, skin was darkened, gills were pale and swimming was abnormal. Internally, there were dilatations of the vascular structures on cross-sections of the liver; vascular congestion, inflammatory mononuclear cell infiltration and degeneration in the gills; interstitial inflammatory mononuclear cell infiltration and free bleeding in patches of the posterior kidney; vascular congestion and free bleeding in the spleen; degeneration, atrophy, polymorphonuclear leukocyte infiltration and liquefaction necrosis in muscle fibers; edema, inflammatory mononuclear cell infiltration and basophilic degeneration among muscle fibers of the heart; and congestion in vascular structures on cross-sections of the brain tissue. It is suggested that P. immobilis is an opportunist pathogen that causes secondary infections.