Infection with parasitic copepod salmon louse Lepeophtheirus salmonis , represents one of the most important limitations to sustainable Atlantic salmon ( Salmo salar L.) farming today in the North Atlantic region. The parasite exerts negative impact on health, growth and welfare of farmed fish as well as impact on wild salmonid populations. It is therefore central to ensure continuous low level of salmon lice with the least possible handling of the salmon and drug use. To address this, vaccination is a cost-effective and environmentally friendly control approach. In this study, efficacy of a vaccine candidate, containing a peptide derived from ribosomal protein P0, was validated post infestation with L. salmonis , at the lab-scale. The sampling results showed good potential of the vaccine candidate when administered intraperitoneally in the host, in reducing the ectoparasite load, through reduction of adult female lice counts and fecundity and with greater presumptive effect in F1 lice generation. The sampling results correlated well with the differential modulation of pro-inflammatory, Th1, Th2 and T regulatory mediators at the transcript level at different lice stages. Overall, the results supports the effectiveness of the vaccine candidate in controlling salmon lice infestation load. However, further validation is necessary under field conditions.

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) is a multifunctional neuropeptide that is widely distributed and conserved across species. We have previously shown that in teleost fish, PACAP not only possesses direct antimicrobial properties but also immunomodulatory effects against the bacterial pathogens Flavobacterium psychrophilum and Pseudomonas aeruginosa using in vitro and in vivo experiments. These previous results suggest PACAP can be used as an alternative to antibiotics to prevent and/or treat bacterial infections in the aquaculture industry. To accomplish this goal, more studies are needed to better understand the effect of PACAP on pathogens affecting fish in live infections. In the present study, the transcripts PACAP, PRP/PACAP, and VPAC2 receptor were examined in rainbow trout (Oncorhynchus mykiss) naturally infected with Yersinia ruckeri, which exhibited an increase in their expression in the spleen when compared to healthy fish. Synthetic Clarias gariepinus PACAP-38 has direct antimicrobial activity on Y. ruckeri and inhibits up to 60% of the bacterial growth when the peptide is at concentrations between 50 and 100 µM in TSB. The growth inhibition increased up to 90% in the presence of 12.5 µM of PACAP-38 when salt-free LB broth was used instead of TSB. It was also found to inhibit Y. ruckeri growth in a dose-dependent manner when the rainbow trout monocyte/macrophage-like cell line (RTS11) was pre-treated with lower concentrations of the peptide (0.02 and 0.1 µM) before going through infection. Differential gene expression was analyzed in this in vitro model. Overall, the results revealed new evidence to support the role of PACAP as an antimicrobial and immunomodulatory peptide treatment in teleosts.

The global aquaculture industry has significant losses each year due to disease outbreaks. Antibiotics are one of the common methods to treat fish infections, but prolonged use can lead to the emergence of resistant strains. Aeromonas spp. Infections are a common and problematic disease in fish, and members of this genera can produce antibiotic resistant strains. Antimicrobial peptides (AMPs) have emerged as an alternative method to treat and prevent infections and pituitary adenylate cyclase activating polypeptide (PACAP) is a prominent member of this family. The objective of this research was to study PACAP's direct antimicrobial activity and its toxicity in fish cells. Four synthetic variants of the natural PACAP from Clarias gariepinus were tested in addition to the natural variant. The experimental results show a different antimicrobial activity against A. salmonicida and A. hydrophila of each PACAP variant, and for the first time show dependence on the culture broth used. Furthermore, the results suggest that the underlying mechanism of PACAP antimicrobial activity includes a bacterial membrane permeabilizing effect, classifying PACAP as a membrane disruptive AMP. This study also demonstrated that the five PACAP variants evaluated showed low toxicity in vitro, at concentrations relevant for in vivo applications. Therefore, PACAP could be a promising alternative to antibiotics in the aquaculture sector.

As aquaculture intensifies to satisfy the rising global demand for food, viral disease pressure is increasing in farmed fish. The variable efficacy of existing vaccines underscores the need for prophylactic strategies that confer broad antiviral protection. Long double-stranded RNA is a potent inducer of the type I interferon response in rainbow trout that can protect against a broad range of virus families. The current work investigates the effect of a commercially available viral dsRNA mimic, high molecular weight polyinosinic: polycytidylic acid (HMW poly I:C), complexed to a phytoglycogen nanoparticle (Nanodendrix, NDx), on primary gut sac preparations and peripheral blood leukocytes (PBLs). PBLs treated with NDx alone or HMW poly I:C + NDx induced significant increases in metabolism, which is an indicator of phagocyte proliferation and activation. The HMW poly I:C + NDx complex was able to enhance overall phagocytosis in adherent and non-adherent PBL populations while upregulating expression of the antiviral genes mx1 and vig-3 after 24hs while HMW poly I:C alone induced some phagocytosis and mx1 expression at 24h. Using a gut perfusion model, NDx suppressed vig-3 expression in the middle intestine and upregulated its expression in the distal intestine after 2hrs, while HMW poly I:C + NDx upregulated vig-3 expression in the middle intestine after 6hrs and HMW poly I:C alone had no effect. Overall, this preliminary translation of antiviral prophylactic dsRNA therapeutics to an ex vivo study of rainbow trout tissues demonstrates the potential for HMW poly I:C + NDx to be analyzed in vivo for the ability to protect rainbow trout against viral infection.