Susceptibility to the salmon louse is related to both host and parasite responses

Stable parasite-host relationships result from co-evolution in which the parasite employs strategies to minimize immune recognition while the host seeks to better recognize, reject or neutralize the parasite. The salmon louse Lepeophtheirus salmonis is an ectoparasite of anadromous salmonids and a pest of Atlantic salmon mariculture. Persistent infections and associated pathophysiology occur in susceptible species, whereas heightened inflammation in resistant species results in parasite rejection. Salmon louse secretions suppress host inflammatory reactions and aid in parasite feeding. Our research focuses on understanding the roles of structural, cytological and molecular processes in the skin in determining the resistant or susceptible phenotype. In all species, we observed an initial T_H1-type response including exaggerated expression of pro-inflammatory cytokines and acute phase proteins followed by a T_H2-type response permitting tissue remodeling and wound repair. At a systemic level, more resistant species respond by reducing circulating iron availability potentially indicating the use of nutritional immunity. Transcriptomic responses by the louse indicate a species-specific feeding response where lice feeding on susceptible Atlantic salmon overexpress virulence factors and digestive proteins. Concurrent transcriptomic and proteomic analyses of the parasite suggest an exaggerated expression of virulence factors and more robust secretory response by L. salmonis on more susceptible host species. Our work suggests that resistance to sea lice involves an enhanced host response coupled with a weakened louse response while susceptibility is due to a weakened host response and a more robust louse response.