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Dr. Gary Marty
Dr. Gary Marty

Scientists claim to have potentially found a case of HSMI in farmed salmon in BC – however, a careful look at the evidence suggests they are making a big deal about a disease that has already been reported.

The condition known as Heart and Skeletal Muscle Inflammation (HSMI) was diagnosed in Norway for the first time in the 1990s and is associated with low mortality in farmed Atlantic salmon (0-20%).  The primary concern for farmers is that affected populations have poor growth rates and feed conversion.

Extensive surveillance in BC has shown the Norwegian form of HSMI is not present in either wild Pacific or farmed Atlantic salmon.

The cause of HSMI remains enigmatic 

Some researchers have suggested a link between the condition and the salmonid virus Piscine Reovirus (PRV); however, repeated attempts to show a causal relationship between PRV and HSMI in BC fish have failed. The most recent study by Garver et al. (2016), published in the journal PLoS One, showed that transmission of PRV from infected to uninfected fish did not result in HSMI. In many parts of the world, including Norway, PRV is common and abundant in fish that never develop HSMI.

Thus, according to available information, the condition of HSMI cannot be diagnosed by genetic technology.

However, according to a press release May 21st from Fisheries and Oceans Canada (DFO), scientists have detected HSMI using “integrated technologies”.

“Applying newly introduced and integrated technologies, a team of international researchers, led by Fisheries and Oceans Canada’s (DFO’s) Dr Kristi Miller, has diagnosed a potential Heart and Skeletal Muscle Inflammation (HSMI) in farmed Atlantic salmon samples collected from a BC aquaculture facility in 2013-2014”, stated the press release.

But this disease is not new to BC, and according to an expert in salmonid pathology, the diagnosis of HSMI might be misleading.

“From the information available to me, Dr Miller's team is not reporting a new disease in BC; instead, they are reporting a different name (HSMI) for a disease of farmed Atlantic salmon in BC that has been reported publicly since at least 2011,” Dr Gary Marty, Senior Fish Pathologist for the Ministry of Agriculture in BC, told Fish Farming Expert.

Subsequent stories by the media have been quick to draw (incorrect) conclusions about the condition saying such things as “A feared viral disease proven deadly in Norwegian fish farms has been confirmed for the first time by federal scientists studying farmed salmon in BC (cbc.ca)”.

A more thorough understanding of the condition quickly provides clarity into the situation.

The hallmark indicators of HSMI

“Three things are required,” Marty said, as he explained the requirements for veterinarians to diagnose the disease syndrome HSMI in BC. “Characteristic inflammation in the heart (based on histopathology), characteristic inflammation in the muscle (based on histopathology), and characteristic clinical signs as assessed by the attending veterinarian”.

The third requirement – clinical signs of HSMI (lethargy, poor eating and slower than normal growth) – is very important as a hallmark indicator for the condition HSMI as described in Norway.

According to Dr Marty, all of the BC veterinarians that deal with farm fish health (private company veterinarians, BC government veterinarians, and DFO veterinarians) use the criteria (above) for assigning a diagnosis of HSMI (histopathological AND clinical signs).

A team of researchers, led by DFO scientist Dr Kristi Miller, has claimed to diagnose a potential case of HSMI in farmed Atlantic salmon samples collected from a BC aquaculture facility in 2013.

What the press release does not mention, is how the same fish that Miller is now reporting to have HSMI is the same group of fish that Marty publicly reported on in 2013 as having "inflammation of the heart and skeletal muscle, which are two features of HSMI".

However, the population of fish in 2013 was otherwise healthy, which by definition, is not consistent with HSMI. “The few fish that died had severe inflammation and other changes in the heart, but the attending veterinarian told me that the rest of the fish were eating and growing well, with low mortality,” said Dr Marty.

Furthermore, the diagnosis of HSMI by Dr Miller's group seems to be based on a different case definition than that used by BC veterinarians.

As Dr Marty explained, all scientists working with this outbreak seem to agree that the histopathology of the fish is similar to HSMI.  The difference in diagnoses is whether clinical signs are included in the diagnosis. This was explained in a document published 2015 by the Canadian Science Advisory Secretariat “Although some pathologists have summarized lesions present in samples from the audit program as ‘HSMI-like’ or ‘consistent with HSMI’, these diagnoses have not been consistent with a clinical pattern that matches HSMI”.

No clinical signs of disease

As part of the collaboration between DFO, the Pacific Salmon Foundation and Genome BC (known as the Strategic Salmon Health Initiative, or SSHI), the BC Salmon Farmers Association provided the samples that were tested. In response to the announcement by DFO, a press release was issued.

“We believe that more investigation into the health of wild and farm-raised salmon is imperative”, the BCSFA said.

“The findings announced by the SSHI investigators regarding a potential diagnosis of HSMI in fish from one Atlantic salmon farm in BC are important… However, there is no consensus amongst the scientific community about the finding as the fish sampled in this farm showed no clinical signs of disease. Government and industry should expedite the science, provide necessary funding, and work collaboratively for the sake of the aquaculture industry and for wild salmon.”

Dr Marty explained that as part of the DFO/BC Fish Health Auditing and Surveillance Program, since 2006 he has occasionally diagnosed “unexplained heart lesions” as the cause of death. But these lesions have never been accompanied by clinical symptoms of Norwegian HSMI, making it inappropriate to diagnose HSMI.

“The single farm identified in 2013 as having fish with a potential diagnosis of HSMI had an overall healthy population of salmon, which showed normal behavior and growth rates, which are not consistent with HSMI,” continued the press release. “In addition, government fish health experts identified liver and gill lesions in the same fish that are not typical of HSMI.”

What caused those lesions from 2013 is unclear - it may be as simple as environmental stressors that caused these lesions, as Dr Marty explained that “transient adverse environmental conditions”, such as exposure to algal toxins, may have been the cause of the unexplained lesions.

However, what is clear, is that this sudden diagnosis of HSMI isn’t based on a current outbreak – the disease was described by Dr Marty in 2013 in a report that is now publicly available.

And Dr Marty did not diagnose those fish as having HSMI, “[because] clinical signs of the BC disease do not match Norwegian HSMI, I do not recommend calling the BC disease HSMI”.

To date, there has never been a confirmed report of HSMI in either wild or farmed Pacific salmon species anywhere in the world.