Control of sea lice, Lepeophtheirus salmonis, on farmed Atlantic salmon, Salmo salar, relies heavily on chemotherapeutants. However, reduced efficacy of many treatments and need for integrated sea lice management plans require innovative strategies. Resistance to emamectin benzoate (EMB), a major sea lice parasiticide, has been linked with P-glycoprotein (P-gp) expression. We hypothesized that host immunostimulation would complement EMB treatment outcome. Lepeophtheirus salmonis-infected Atlantic salmon were fed immunostimulatory or control feeds. Sea lice were collected for 24-h EMB bioassays 1 and 2 weeks prior to commencement of EMB treatment of the fish. Two weeks after cessation of immunostimulant-treated feed, EMB was administered at 150 μg kg(-1) fish biomass for 7 days. The bioassay revealed stage, gender and immunostimulant-related differences in EMB EC(50) . Sea lice attached to salmon with a history of immunostimulation exhibited significantly greater survival than those on control feeds, despite similar levels of EMB in host tissues. Lepeophtheirus salmonis from salmon with a history of immunostimulation also exhibited higher P-gp mRNA expression as well as greater survivability compared to controls. Administration of immunostimulants prior to EMB treatment caused increased expression of P-gp mRNA which could have consequently caused decreased efficacy of the parasiticide.

Several immunostimulatory feed additives have shown the ability to induce protective responses in Atlantic salmon to infection with Lepeophtheirus salmonis. However, even the most encouraging results rarely surpass a 50% protective index in the host. That fact coupled with the well-documented limitations of single-therapy strategies in the effective management of parasitic infections generally make it imperative to identify therapies that can be combined in an integrated pest management approach for sea lice. With this in mind, we hypothesized that immunostimulatory feeds could enhance the protection provided by SLICE® emamectin benzoate (EMB). To test this hypothesis, Atlantic salmon were fed one of two different immunostimulatory feeds (CpG ODN or Aquate®) for c. 7 weeks, challenged with L. salmonis copepodids early within that immunostimulatory feed period and then placed on a triple-dose (150 μg kg(-1) ) feed of SLICE® for 1 week following the completion of the immunostimulatory feeding period. CpG ODN (2 mg kg(-1) ) and the commercial yeast extract (Aquate® 0.2%) inclusion in feeds were able to successfully induce inflammatory gene expression (interleukin-1β) in the head kidneys of infected fish at 13 and 26 days post-exposure (DPE), and 13 DPE, respectively. Lice burdens were lower on fish fed CpG ODN (18%) or Aquate® (19%) diets; however, due to variability, these were not statistically significant over time. Despite no statistically significant reductions in lice numbers, by 33 DPE fish on immunostimulatory feeds had significantly reduced cortisol levels when compared to infected fish on control diet. Cortisol levels in fish receiving an immunostimulatory diet were no different from initial baseline levels prior to infection, whereas the levels in control diet fish were significantly elevated from all other time points. Despite the positive effects on infection of fish fed immunostimulatory feeds, no synergism was observed with follow-up treatment with SLICE® . In fact, highest survival of lice was observed in fish with prior immunostimulation.

Lepeophtheirus salmonis is a common parasite of salmonid fish and has a significant economic impact on Atlantic salmon (Salmo salar) fish farms. Over time L. salmonis has developed resistance to a number of chemotherapeutants, making the discovery of new treatments important to maintain a profitable farming industry. Determining processes in both L. salmonis and Atlantic salmon important to host selection and colonization may provide new targets for treatment development. During a two week cohabitation of L. salmonis infected and uninfected Atlantic salmon, we were able to collect information on the ability of L. salmonis to switch hosts, and preference for infected or uninfected fish. Whole L. salmonis and Atlantic salmon tissues were collected at 2 and 14 days post cohabitation to determine if differential gene expression was occurring during this process. At 2 days post cohabitation there was no significant difference in the number of male lice on the initially infected and uninfected fish. Eight L. salmonis genes putatively associated with various facets of lice survival (CYP18 A1-like, cytochrome p450 Isoform 1-like protein, glycene receptor α-2-like, leukocyte receptor cluster member 9-like, nicotinic acetylcholine receptor subunit-like, tissue plasminogen activator precursor-like, peroxinectin-like, and Trypsin-1) were analysed in both adult male and female lice, as well as five genes indicating immune status in Atlantic salmon. Comparisons were made to look for differential gene regulation as well as correlation between expression of L. salmonis genes and Atlantic salmon genes. Only MMP9 expression in salmon spleen was differentially regulated during the study period, however, correlations between the expression of several louse and salmon genes were found. Notably, the expression of a peroxinectin-like gene in male and female L. salmonis was correlated with the expression of IL-1, IL-12, IgT and matrix metalloproteinase 9 intermittently in salmon. This paper provides new insight into the interactions between L. salmonis and S. salar during infection.