Tenacibaculum maritimum is a major etiological agent of tenacibaculosis in marine fish and represents an increasing concern for Chilean salmon aquaculture; however, updated information on the phenotypic and molecular diversity of circulating isolates is limited. This study characterized 40 isolates recovered from Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), and red cusk eel (Genypterus chilensis) obtained from outbreaks between 2020 and 2024. Isolates were analyzed using biochemical and phenotypic assays, multiplex PCR targeting the O-antigen gene cluster (O-AGC), REP-PCR-based genetic fingerprinting, and experimental bath challenges in Atlantic salmon. Phenotypic characterization revealed species-consistent traits but variable gelatin and starch hydrolysis and differences in seawater tolerance. O-AGC typing identified four molecular serotypes (1-0, 3-1, 3-2, and 4-0), with serotypes 1-0 and 3-2 detected for the first time in Chilean salmonids. Genetic fingerprinting distinguished previously described profiles and two novel REP patterns (REP6 and REP7), indicating additional genomic heterogeneity within dominant serotypes. Virulence assays showed cumulative mortality ranging from 0% to 100%, with serotype 3-1 isolates generally associated with higher mortality and serotype 4-0 displaying broad intra-serotype variability. These findings document substantial phenotypic, antigenic, and genetic diversity among Chilean T. maritimum isolates and provide epidemiologically relevant information for disease surveillance and vaccine design in salmon aquaculture.

Sea lice (Caligus rogercresseyi) are major ectoparasites in Chilean salmon aquaculture, causing economic losses and increasing fish susceptibility to secondary infections. Tenacibaculosis has recently emerged as the second leading cause of mortality in farmed salmon. Understanding whether sea lice may harbor or transmit bacterial pathogens is therefore highly relevant. A sea lice survey was conducted at an Atlantic salmon (Salmo salar) farm in the Los Lagos Region, Chile. Twenty adult parasites (10 ovigerous females and 10 males) were analyzed using an integrated approach combining immunohistochemistry (IHC) with specific antibodies and conventional PCR with appropriate controls. IHC revealed positive signals for Tenacibaculum maritimum in the gastrointestinal tract and cuticle, and for Tenacibacuum dicentrarchi in the cuticle and ovigerous sac of C. rogercresseyi. A specific PCR test confirmed the presence of both pathogens, yielding amplification products of 1088 bp for T. maritimum and 284 bp for T. dicentrarchi. This study provides the first dual evidence, by PCR and IHC, of Tenacibaculum species in C. rogercresseyi. These findings suggest that sea lice may play a role in pathogen dynamics in salmon farming, highlighting the need for further research on their contribution to disease transmission within aquaculture systems.