Experimental challenge of Chinook salmon withTenacibaculum maritimumandTenacibaculum dicentrarchifulfils Koch’s postulates

Abstract

The bacterial skin disease tenacibaculosis, caused byTenacibaculumspecies, can compromise numerous species of economically important marine fish, including salmonids. While tenacibaculosis is a known threat to Atlantic salmon (Salmo salar) aquaculture, the pathogenesis ofTenacibaculum maritimumandTenacibaculum dicentrarchion Chinook salmon (Oncorhynchus tshawytscha) has not yet been investigated. In this study, three molecular O-AGC types ofT. maritimum(O-AGC Type 3-0, Type 2-1 and Type 3-2) andT. dicentrarchiisolated during a disease outbreak of farmed Chinook salmon in Aotearoa New Zealand were assessed for their ability to induce tenacibaculosis in salmon smolts under controlled conditions. Naive Chinook salmon were exposed toT. maritimumorT. dicentrarchiby immersion. Clinical signs of tenacibaculosis were apparent post-exposure and observed in 100% of all three molecular O-AGC types ofT.-maritimum-challenged fish, with 100% morbidity in O-AGC Type 2-1 and Type 3-2 and 60% in O-AGC Type 3-0. Chinook salmon exposed toT. dicentrarchishowed characteristic clinical signs of disease in 51% of the challenged population, with 28% morbidity. Common gross pathological signs observed for bothTenacibaculumspecies were congruent with observations on farmed fish in the field, including scale loss, erythematous skin lesion, skin ulcers, fin necrosis, mouth erosion and gill ulceration. Exophthalmia was observed only inT. maritimum-challenged fish, while skin ulcers appeared grossly more severe with exposed musculature inT. dicentrarchi-challenged fish. PureT. maritimumandT. dicentrarchicultures were reisolated from the skin and gills of the challenged fish and their identity was confirmed by species-specific PCR and molecular O-AGC typing. Challenge experiments and associated field surveillance (forT. maritimum) did not show the presence of culturableT. maritimumcells in the anterior kidney. This provides compelling evidence that tenacibaculosis in farmed Chinook salmon is an external infectious disease, and thatTenacibaculumis a marine obligate organism that is unable to survive in fish body fluids and does not cause septicaemia. This has repercussions for approaches to experimental challenges withTenacibaculumspecies, which must occur by immersion rather than intraperitoneal or intramuscular inoculation, to replicate the natural transmission pathway and to ensure a successful challenge model. This study fulfilled modernised Koch’s postulates for the three molecular O-AGC types ofT. maritimumand single strain ofT. dicentrarchias aetiological agents of tenacibaculosis in Chinook salmon that cause mortalities with considerable external abnormalities.Author summaryChinook salmon,Oncorhynchus tshawytscha, is the most significant species of Pacific salmon for its large size and nutritional content which makes it a premium choice for aquaculture. In Aotearoa|New Zealand, Chinook salmon is the only marine salmon species farmed. For a decade, the industry was impacted by an undiagnosed skin disease resulting in high mortalities. Disease susceptibility in Chinook salmon is scarcely studied and added to the challenge for a timely diagnosis. This novel research provides insight on disease susceptibility of Chinook salmon and confirmsTenacibaculumspecies identified in New Zealand pose a high threat to the aquaculture industry. This research has global implications and contributes valuable insights and approaches to disease management that can be applied in British Columbia and Canada where Chinook salmon populations are in decline.