Detection of piscine orthoreovirus (PRV) and Heart and Skeletal Muscle Inflammation (HSMI) in farmed Atlantic Salmon in the Broughton Archipelago, British Columbia, Canada, through a multi-farm longitudinal study

Piscine Orthoreovirus (PRV) is a viral pathogen that causes disease outbreaks in cultured salmon and trout farms worldwide, and the cause of heart and skeletal muscle inflammation (HSMI) in Atlantic salmon. We present results from a longitudinal study in the Broughton Archipelago (British Columbia, Canada) incorporating molecular, histological, and environmental RNA (eRNA) data from seven Atlantic salmon farms subject to regulatory oversight by the Mamalilikulla, ‘Na̱mg̱is, and Kwikwasut'inuxw Haxwa'mis First Nations as part of the Broughton Aquaculture Transition Initiative (BATI). PRV was commonly detected on farms, and a portion of fish on five of the seven farms were diagnosed as having HSMI via histological evaluation of tissue samples, and this result was confirmed by three independent pathologists. Out of 749 Atlantic salmon analyzed for histology, 50 were diagnosed with unambiguous HSMI, with the disease evident for several months on multiple farms. Moreover, the eRNA findings suggest that PRV abundance in the cultured population is a better predictor of PRV in the water column around active farms than is the proportion of the sampled population with HSMI. The findings of this study suggest that HSMI is more common in British Columbia than previously described and that even if farms are able to control HSMI, the disease, they will not protect wild salmon from farm to wild transmission of PRV, unless they can also control replication of PRV.