The potential for infectious salmon anemia virus (ISAV)—an internationally regulated pathogen of salmon—to transmit vertically from parent to offspring is currently unclear. While the highly virulent ISAV phenotype known as ISAV-HPRΔ has been observed intra-ova, evidence for vertical transmission of the avirulent ISAV phenotype known as ISAV-HPR0 is lacking. In this study, we identified ISAV-HPR0-infected Atlantic salmon broodstock during spawning within a government research recirculating aquaculture facility using qPCR. Eggs and milt from infected brood were used to initiate 16 unique family dam-sire crosses from which 29–60 fertilized eggs per cross were screened for ISAV using qPCR (limit of detection ~100 virus genome copies/egg). A portion of eggs (~300) from one family cross was hatched and further reared in biosecure containment and periodically screened for ISAV by gill clipping over a 2-year period. ISAV was not detected in any of the 781 eggs screened from 16 family crosses generated by infected brood, nor in 870 gill clips periodically sampled from the single-family cohort raised for 2 years in biocontainment. Based on these findings, we conclude that ISAV-HPR0 has a limited likelihood for vertical parent-to-offspring transmission in cultured Atlantic salmon.

Preliminary evidence has showed an emergent serotype O3 (SO3) strain of Vibrio anguillarum to cause mortality in pre-smolt Atlantic salmon (Salmo salar) by injection with >10⁵ colony forming units (cfus). Here, we sought to identify the susceptibility of Atlantic salmon post-smolts to this emergent strain by both injection and cohabitation to better understand transmission risk within cultured salmon and possibly between salmon and Atlantic menhaden (Brevoortia tyrannus), where this strain was identified. We identified that although mortality could be induced with a high-dose (>10⁶ cfus) intraperitoneal injection of the emergent SO3 strain (cumulative mortality of 40%), post-smolt Atlantic salmon were highly refractory to a low dose (<10⁶ cfus; cumulative mortality of 3%) or cohabitation exposure (no mortality). A qPCR assay targeting this strain was developed and analytically validated, revealing the limited presence of bacterial DNA in the spleen of low-dose-injected fish (2/36) and no detections in sampled cohabitants (0/70) across three timepoints during the 27-day challenge. These results suggest that although Atlantic salmon can succumb to high-dose artificial infections with V. anguillarum SO3, the risk of natural transmissibility and susceptibility of Atlantic salmon to this emergent strain is anticipated to be low.

Sea lice have been a persistent pest of the salmon farming industry for more than 50 years. In this study, we aimed to identify if different strains of Atlantic salmon with discrete long-term lice exposure histories had variable resistance to copepodid attachment and/or different attachment-specific transcriptome patterns. We additionally sought to characterize lice distributions on fins, head, and skin and identify if attachment location influenced transcriptomic profiles of lice. Lice counts were correlated with body size and highest on St. John River (SJR; open ocean-run) relative to Grand Lakes Stream (GLS; 200-year restricted ocean-run) or Sebago Lake (CAS; ~11,000 years landlocked) Atlantic salmon. However, lice density was similar between strains. Skin and fins had expectedly different transcriptomic profiles; however, notable differences were not observed between salmon strains. Variance in lice transcriptomes was minimally affected by attachment location even though lice strongly preferred fins relative to head or body. Attached lice did have different transcriptomic profiles on GLS relative to CAS or SJR. This study cumulatively identified a minimal host evolutionary component for sea lice attachment resistance, although lice behavior post-attachment appeared somewhat affected by strain. Non-uniform settlement distributions and tank-specific variability in lice attachment were observed across populations.