Marine mammals are natural hosts of Oceanivirga salmonicida, a bacterial pathogen of Atlantic salmon.

During 1992 and 1993, a bacterial disease occurred in a seawater Atlantic salmon Salmo salar farm, causing serious mortalities. The causative agent was subsequently named as Oceanivirga salmonicida, a member of the Leptotrichiaceae. Searches of 16S rRNA gene sequence databases have shown sequence similarities between O. salmonicida and uncultured bacterial clones from the digestive tracts of marine mammals. In the current study, oral samples were taken from stranded dolphins (common dolphin Delphinus delphis, striped dolphin Stenella coeruleoalba) and healthy harbour seals Phoca vitulina. A bacterium with growth characteristics consistent with O. salmonicida was isolated from a common dolphin. The isolate was confirmed as O. salmonicida, by comparisons to the type strain, using 16S rRNA gene, gyrB, groEL, and recA sequence analyses, average nucleotide identity analysis, and MALDI-TOF mass spectrometry. Metagenomic analysis indicated that the genus Oceanivirga represented a significant component of the oral bacterial microbiomes of the dolphins and seals. However, sequences consistent with O. salmonicida were only found in the dolphin samples. Analyses of marine mammal microbiome studies in the NCBI databases showed sequences consistent with O. salmonicida from the common dolphin, striped dolphin, bottlenose dolphin Tursiops truncatus, humpback whale Megaptera novaeangliae, and harbour seal. Sequences from marine environmental studies in the NCBI databases showed no sequences consistent with O. salmonicida. The findings suggest that several species of marine mammals are natural hosts of O. salmonicida.