Abstract
Investigations of interspecific horizontal transmission of Tenacibaculosis induced by Tenacibaculum maritimum from Atlantic salmon (Salmo salar L.) to Coho (Oncorhynchus kisutch W.) and Chinook salmon (Oncorhynchus tshawytscha W.).
Tenacibaculosis is a disease caused by several Tenacibaculum species globally. In British Columbia (BC, Canada), the disease is described as ‘yellow mouth’ or ‘mouthrot’ due to its regional clinical presentation in cultured Atlantic salmon, where infected fish present with degradation of tissue on the jaws with intralesional yellow plaques of bacteria. Mouthrot is an issue for BC Atlantic salmon production due an increased number of outbreaks during a single production cycle and concerns over the potential for disease transfer from farmed to wild salmonid populations migrating past marine netpen sites. Currently, there is no direct evidence that substantiates such transmission of disease, though there is evidence of the presence T. maritimum on various Pacific salmon species. The purpose of this research was to test the potential for Atlantic salmon infected with T. maritimum to transmit mouthrot to naïve Pacific salmon and conspecifics in a controlled laboratory exposure trial.For this study, Atlantic salmon were bath-exposed to an isolate of T. maritimum at 106 and 108 CFU/mL and grouped with naïve Chinook (trial 1), or Coho salmon smolts (trial 2), and monitored for 26 and 30 days respectively. Fish were sampled pre-exposure, post-exposure, and post-trial, targeting blood and tissues for bacteriology, histological examination, qPCR detection of T. maritimum , and microbiome inferences using Oxford Nanopores MinION platform. Overall, naïve Atlantic salmon smolts developed clinical and histopathological signs of mouthrot, had T. maritimum bacteria cultured from all diseased fish, and were qPCR positive for the bacteria from jaw tissues with cycle quotients between 15-25.Contrarily, neither of the Pacific salmon presented with any clinical signs of disease, though qPCR and bacterial culture identified the presence of T. maritimum . Subsequent microbiome comparisons revealed a drastic dysbioitic event in exposed and naïve conspecific Atlantic salmon, with T. maritimum as a dominant species, while T. maritimum comprised a greatly reduced relative abundance in Pacific salmon. Such results suggest that isolates of T. maritimum transmitted from infected Atlantic salmon have minimal influences on the survival of Pacific salmon in BC. However, a greater diversity of bacterial isolates and species, host families, and temporal scales need to be investigated. Results from this work will be of value for current aquaculture practices, regulatory decision-making and conservation strategies, informing how best to manage and mitigate Tenacibaculosis outbreaks in BC marine waters.