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Assessing the Evolution of Virulence in the Salmon Louse (Lepeophtheirus salmonis) in the Bay of Fundy

ABSTRACT: An unintended consequence of fish farming is the production of an environment which may be conducive for the evolution of virulence for pathogens interacting with the host in production. Rearing conditions used in salmon aquaculture such as high densities, compressed rearing cycles, limited genetic diversity of broodstock as well as practices applied to control infectious disease, may all lead to increased selection pressures for the evolution of virulence of pathogens. These selection pressures may also be occurring without the balancing of negative fitness costs of virulence due to a constant supply of naïve hosts entering the system every 18-24 months. Changes in virulence of Canadian Lepeophtheirus salmonis populations have not yet been assessed, despite having been identified as a crucial knowledge gap to understand the interactions between farmed and wild hosts as well as the impact of increased disease prevalence at sites which may harbour highly virulent strains. We developed a virulence challenge model by rearing lice originating from wild and farm sources for 6 generations and compared differences in virulence on their host, testing the hypothesis that lice originating from farms were more virulent than those from wild sources. RNA sequencing was completed for copepodid, chalimus II and adult stages of the louse as well as their respective attachment sites on the host to determine responses at the host-parasite interface. We describe the first instance of elevated lesion severity for host Atlantic salmon when infected with farm-sourced lice compared to their wild counterparts. However, this increase did not appear to translate to meaningful differences in host growth rates (e.g. change in lengths or weights). Approximately 900 transcripts had differential expression for at least one life stage between wild and farm sources of lice. Many of these transcripts had functions related to chitin- and heme-binding, serine-endopeptidase, metallo-endopeptidase, and trypsin-like activities.

ORGANISM(S): Salmo salar Lepeophtheirus salmonis

PROVIDER: GSE295072 | GEO | 2025/04/18

REPOSITORIES: GEO

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Project description:The salmon louse is an ectoparasitic copepod that causes major economic losses in the aquaculture industry of Atlantic salmon. This host displays a high level of susceptibility to lice which can be accounted for by several factors including stress. In addition, the parasite itself acts as a potent stressor of the host, and outcomes of infection can depend on biotic and abiotic factors that stimulate production of cortisol. Consequently, examination of responses to infection with this parasite, in addition to stress hormone regulation in Atlantic salmon, is vital for better understanding of the host pathogen interaction. Atlantic salmon post smolts were exposed to stress hormone cortisol, lice and their combination. The transcriptomic effects of hormone treatment in salmon skin were significantly greater than lice-infection induced changes. Cortisol stimulated expression of genes involved in metabolism of steroids and amino acids, chaperones, responses to oxidative stress and eicosanoid metabolism and suppressed genes related to antigen presentation, B and T cells, antiviral and inflammatory responses. Cortisol and liceBoth treatments equally down-regulated a large panel of motor proteins that can be important for wound contraction. Cortisol also suppressed multiple genes involved in wound healing, parts of which were activated by the parasite. Down-regulation of collagens and other structural proteins was in parallel with the induction of proteinases that degrade extracellular matrix (MMP9 and MMP13). Cortisol reduced expression of genes encoding proteins involved in formation of various tissue structures, regulators of cell differentiation and growth factors. Atlantic salmon post smolts were organised into four experimental groups: lice + cortisol, lice + placebo, no lice + cortisol, no lice + placebo. Infection levels were equal in both treatments upon termination of the experiment. Gene expression changes in skin were assessed with 21 k oligonucleotide microarray and qPCR at the chalimus stage 18 days post infection at 9oC.

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Assessing the Evolution of Virulence in the Salmon Louse (Lepeophtheirus salmonis) in the Bay of Fundy

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