<title>Abstract</title> Salmon louse (<italic>Lepeophtheirus salmonis</italic>) poses significant risks to both wild and farmed salmon populations. Atlantic Salmon (Salmo salar) are highly susceptible to the parasite, while Pacific salmon species such as Pink (<italic>Oncorhynchus gorbuscha</italic>), Coho (<italic>Oncorhynchus kisutch</italic>), and Chum (<italic>Oncorhynchus keta</italic>) salmon exhibit varying levels of resistance. This study explored L. salmonis infestation dynamics in four salmonid species (Atlantic, Pink, Coho, and Chum Salmon) across three challenge trials using the same fish cohort. Infestation was only successful in the third trial, where sedation facilitated parasite attachment. Skin and fin samples were collected at 12, 24, 36, 48, 60, and 168 hours post-infestation (hpi) during the early infestation phase. Results showed low L. salmonis settlement on Chum and Pink Salmon, while Coho Salmon initially had the highest lice counts on fins, followed by Atlantic Salmon. A strong correlation was observed between mucous cell area and susceptibility, with Chum Salmon showing 30-40% mucous cell coverage compared to 10% in Atlantic Salmon. Coho Salmon successfully rejected copepodids after two days, which was linked to an influx of polymorphonuclear cells at attachment sites. This immune response, highlighted by spatial transcriptomics, showed upregulation of host immune markers at the louse-host interface, suggesting that macrophages and neutrophils are key to copepodid rejection. The findings suggest that Coho Salmon’s rapid inflammatory response is crucial in resisting lice and differs significantly from the response in Atlantic Salmon.

Local inflammation at the salmon louse (Lepeophtheirus salmonis) attachment site drives copepodid rejection in Coho Salmon (Oncorhynchus kisutch)