The study, published in the scientific journal PLOS ONE, is the first to find heart and skeletal muscle inflammation, or HSMI, in salmon at B.C. fish farms. The disease can wipe out up to 20 per cent of a farmed population.
Researchers also showed a correlation between HSMI and piscine reo-virus, suggesting the two might be linked.
Emiliano Di Cicco, co-author of the study, said the findings could have negative consequences for B.C.’s salmon farming industry, which had an estimated net value after marketing costs of $476 million in 2013.
“[HSMI] is the No. 3 cause of mortality and economic loss in the salmon-farming industry,” Di Cicco said. “But the other issue we need to care about is the same virus [PRV] could infect Pacific salmon in B.C.”
For some scientists, the study provides the missing link showing that farmed salmon pose a threat to wild stocks.
PRV is highly contagious, and some scientists have argued it can spread from Atlantic salmon in open-net pen farms to wild salmon that swim past.
According to the Ministry of Agriculture, the virus affects up to 80 per cent of farmed salmon in B.C.
Pacific wild salmon have tested positive for the virus, but its prevalence in that population is unknown.
The study was co-funded by the Department of Fisheries and Oceans, conservation charity the Pacific Salmon Foundation, and Genome British Columbia, which promotes genomics innovation.
Stan Proboszcz, science adviser for conservation charity Watershed Watch Salmon Society, said the study is significant for breaking from positions held by the DFO and industry-funded studies.
“DFO has clung to the conclusion that there is no link between the virus and disease, hence no risk to wild fish that we know of,” Proboszcz said.
“But now this paper establishes a high association between the virus and the disease.”
DFO says the study’s findings are consistent with lesions observed through its audit program between 2013 and 2015.
However, a 2015 DFO report — used to justify industry operations and expansion — said there was no evidence showing PRV was associated with any disease, including HSMI.
Proboszcz said the new study proves that wrong.
Di Cicco said connecting the virus and disease wasn’t a goal, and emphasized that the study identifies only a statistical correlation — it doesn’t establish that the virus causes the disease.
Scientists tested 500 samples from four Atlantic salmon farms in B.C. for 44 microbes with pathogenic potential. HSMI was diagnosed at one of the farms.
“Our study wasn’t originally designed to find any association with any agent,” he said.
“We tested 44 agents known to cause disease in B.C. and worldwide. But the only agent that we found statistically correlated with the severity and occurrence of the condition was PRV.”
Di Cicco said the same association between the virus and the disease has been demonstrated in Europe.
There have been more than 50 studies of PRV and HSMI, he said, but this is the first over the fish’s life cycle in B.C. and only the second conducted in the field. The first was in 2006 in Norway.
Independent fish biologist Alexandra Morton said that while farmed fish can recover from HSMI fairly easily, the virus would put wild salmon at risk. The disease makes fish lethargic, she said, making them vulnerable to predators.
“It’s the behaviour it causes that would be lethal, not necessarily the disease itself,” she said.
“You can’t lie around with the flu as a wild fish — there’s nowhere to hide.”
HSMI is also believed to be triggered by stress, which wild salmon regularly experience as they face predators and swim up streams, Morton said.
Last summer, Morton visited aquaculture farms along the coast to test for PRV. Her research is ongoing.
Jeremy Dunn, executive director of the B.C. Salmon Farmers Association, called the new study important.
He noted that while HSMI is associated with PRV, the specific cause has not been established and the population of the fish studied entered the marine environment without the virus.
“The single farm identified in the study had an overall healthy population of salmon, with a low mortality, which showed normal behaviour and growth rates,” he said in a statement. “There is further research ongoing to better understand how HSMI develops and its root causes.”
In 2015, a federal court judge ordered DFO to tighten its rules on the transfer of fish into open-net pens after smolts bred by Marine Harvest Canada tested positive for PRV.
The judge pointed to the perceived link between PRV and HSMI in his ruling.
About PRV and HSMI
• A durable virus concentrated in red blood cells that may move into the heart and skeletal muscle
• Shown to infect B.C. chinook, sockeye and Atlantic salmon
• Highly contagious, but low virulence or disease-causing ability
• Can be present for months without causing disease or death
• Samples show presence in Pacific coast salmon as early as 1988
• On the farm that was studied, 20 per cent tested positive for PRV in May 2013, rising to 100 per cent by October
• Found in 80 per cent of farmed Atlantic salmon tested by the B.C. Ministry of Agriculture
HEART AND SKELETAL MUSCLE INFLAMMATION DISEASE
• Heart and skeletal muscle inflammation is a disease that causes lesions on the heart
• Makes fish lethargic
• Believed to be triggered by stress
• First observed in 1999 Atlantic salmon farmed in Norway
• Not on the Canadian Food Inspection Agency’s list of reportable diseases, which are considered of significant importance to human or animal health
• Mortality on farms tends to be low, however it may kill up to 20 per cent of fish on an affected farm. Farm-wide mortality was not shown to increase with HSMI in this study
• HSMI has never been reproduced without the presence of PRV
• PRV has never been successfully cultured, so a cause and effect relationship between PRV and HSMI in Atlantic salmon cannot be definitively established