CMS now one of biggest farmed salmon disease problems

Since its inception in 2005, the TriNation conference has aimed to contribute to collaboration and knowledge sharing between academia and the farming industry in Scotland, Ireland and Norway.

In the beginning, pancreatic disease (PD) was the main theme, but during the nearly 20 years the conference has been organised, the theme has changed and in 2022, 135 participants from 11 countries gathered to exchange information about salmon heart health in general, and the viral diseases PD, caused by salmonid alphavirus (SAV); cardiomyopathy syndrome (CMS), caused by piscine myocarditis virus (PMCV); and myocarditis (HSMI), caused by piscine reovirus (PRV), in particular.

In the current issue of Fish Farming Expert’s sister magazine, Norsk Fiskeoppdrett (Norwegian Fish Farming - subscription required), Tore Hovland of Aqua Kompetance AS, Chris Mitchell of WellFish Diagnostics and Sonal Patel of the Norwegian Veterinary Institute - who are on the steering committee and reference group for the conference - present a summary of the new knowledge that emerged during the last TriNation conference.

Vaccination impact

They write, among other things, that a review of the status of viral diseases in Scotland, Ireland, and Norway showed that the topic was very current, and CMS is highlighted as clearly the biggest problem in all three countries. The PD situation in both Norway and Scotland has improved, although there has been an increase in PD detections in Ireland.

“From 2021 to 2022, a significant reduction in SAV2 and SAV3 detections in Norway is reported. What is the cause of the decline is unclear, but there are many indications that vaccination has had an effect,” write the experts.

In Norway’s production area 7, the buffer zone against the PD-free area, there have been no detections since 2020. A combination of zonal fallowing, an increased focus on biosecurity followed by a local vaccination initiative is believed to be key to the success.

Scotland on top of PD

“The Scottish farming industry is less concerned about PD than we are in Norway. All fish are vaccinated, and PCR is used for systematic monitoring of infection status. Clinical outbreaks are rarely seen, and when they do occur, mortality is low, while some reduced appetite and loss of growth are more common,” they write.

Heart and skeletal muscle inflammation (HSMI) is not a major concern in any of the three countries. Detections are made along the entire Norwegian coast, a total of 188 in 2021 compared to 161 the previous year. In Ireland, HSMI has not been detected in farmed salmon since 2015, despite the virus associated with HSMI being present in the sea. In Scotland, two tests were carried out in 2020, without the fish appearing to become ill.

“Salmon infected with PMCV and exposed to stress develop heart disease (CMS) more easily than what we see in salmon infected with PRV, which do not develop HSMI to the same extent. It therefore does not appear that stress affects the fish’s resistance to HSMI to the same extent as for CMS.”

Experiments show that stress does not increase the amount of piscine reovirus (PRV) in fish.

“The same experiments investigated whether an ongoing infection with the PRV makes the fish less susceptible to infection with the salmonid alphavirus (SAV). Unfortunately, that does not seem to be the case,” write the three article authors.

CMS

From Norwegian farming, a generally high mortality was once again reported, with CMS as the single cause that contributes the most.

“The viral disease is a problem along the entire coast, with detections in all production areas. Also, from Ireland and Scotland, CMS is highlighted as the most serious problem, with clinical outbreaks at six of the 13 sites that were in operation in Ireland in 2021. Mortality occurs especially after the fish have been handled in connection with delousing which can cause stress.”

During the two days of the conference, several presentations on CMS were held.

“Diagnostic methods are being further developed, research is being done on infection transmission, and new vaccines are being developed”, were some of the things that emerged.

The next TriNation conference is planned to be held in Norway in the spring of 2024.

The upcoming conference is being planned and the location and date will be announced soon.

“For the next conference, we look forward to a stronger participation from business players with professional contributions and to good discussions about the topics that affect these important viruses that affect fish health,” conclude Hovland, Mitchell and Patel.