Secretions are central to the ability of the salmon louse Lepeophtheirus salmonis to parasitize salmonid hosts by promoting immunomodulation and feeding. Previous characterizations of secretory and excretory products (SEPs) have relied on pooled individuals, preventing assessment of inter-individual variation and responses to host-derived cues. A novel SEP collection method was developed to obtain samples from individual adult female L. salmonis , enabling evaluation of secretory profiles following exposure to conditioned seawater from a susceptible host, Atlantic salmon ( Salmo salar ), or a resistant host, coho salmon ( Oncorhynchus kisutch ). SEP concentrations ranged from 328 to 1597 µg mL⁻¹, with no effects of host species or conditioning treatment on protein yield. Across 16 individuals, a mean of 101.4 proteins were detected per replicate, including 61.2 secretory proteins, indicating substantial inter-individual variation. Conditioning did not alter protein richness, yet conditioned treatments showed clear qualitative differences in composition. S. salar –conditioned lice uniquely secreted 40 proteins, including proteases, protease inhibitors, C-type lectins, gamma crystallins, and labial gland factors, whereas controls yielded a single unique protein. O. kisutch conditioning was further associated with additional proteases, protease inhibitors, epidermal growth factor–like proteins, and other putative virulence factors. Detection of chitin deacetylase-7 and an LY6/uPAR domain protein across multiple conditions highlights previously uncharacterised candidates relevant to louse–host interactions. In total, these findings establish individual-level SEP collection as a robust and sensitive approach for resolving secretomic diversity in L. salmonis and detecting host-associated modulation of parasite secretory activity obscured by pooled analyses. • Novel method for secretory/excretory protein extraction from individual L. salmonis • Substantial inter-individual variability detected in adult female secretomes • Protein yields unaffected by host species or conditioned seawater treatments • S. salar conditioning induced 40 unique secreted proteins, incl. virulence factors • O. kisutch conditioning enriched proteases, inhibitors, and immune-related proteins
Dindial et al. (Thu,) studied this question.