"In 2012 we found CMS in 89 plants," says Camilla Fritsvold, a researcher at the National Veterinary Institute.
This is the second consecutive year of an increase in the number of detections, which is twice as many CMS cases than in 2010.
"The greatest increase in the number of detections was in the north, while the biggest losses from CMS were reported from central Norway. The reason for this trend is unclear," says Ms Fritsvold.
Cardiomyopathy syndrome (CMS) is a serious heart condition that affects salmon in the sea.
"In 2010, research at the National Veterinary Institute and at the Norwegian School of Veterinary Science in collaboration with Pharmaq, found a new virus, piscine myokarditt virus (PMCV), to cause CMS.
"So far, it appears that there is a clear link between the virus and the disease, and the amount of virus and the extent of damage to the heart."
All the Norwegian PMCV isolates that were examined are very similar and appear to belong to a single genogroup. Virus discovery is an important contribution to the knowledge of CMS, to diagnose, manage and combat the disease better, continued Ms Fritsvold.
"Still we need more research to clarify the relationship between viruses and outbreaks. For example, it remains unclear how the virus takes hold, but waterborne infection from fish to fish appears to be the main source of infection.
"We also know little about how the virus behaves in the diseased fish, when and how long the fish excrete the virus, how long the virus survives and is infectious in the environment outside the fish and why it usually only causes problems on large fish.
"Clearly there is a lot of unanswered questions that we want answers to. If they can be answered, perhaps we can find out how the disease can be prevented," concluded Ms Fritsvold.