CMS is a contagious disease, new study confirms
Cardiomyopathy syndrome (CMS) is a communicable disease that can affect not only large and mature Atlantic salmon, but also pre-smolts, post-smolts and those in early sea phases, research in Norway has concluded.
CMS in Atlantic farmed salmon is an infectious viral disease that causes severe inflammation in the inner layer of the salmon heart, both in the anterior and cardiac chambers.
The disease is widespread along the entire Norwegian coast and most often affects large salmon in the last year they are in the sea. Moderately increased mortality or CMS outbreaks therefore have major financial consequences.
Together with sea lice, CMS is now considered one of the most important problems for the Norwegian aquaculture industry and is a growing problem for farmers in Scotland, Ireland, and the Faroe Islands, according to an article published by the Norwegian University of Life Sciences (NMBU) associated with a forthcoming doctoral dissertation by scientist Camilla Fritsvold.
Infection trials
“The aim of this study was to establish a model for infection trials for CMS and use this to find out if CMS is a contagious disease. In addition, we would follow, describe and compare the development of these heart changes in a long-term infection trial, in a field outbreak of the disease and using several methods in an infection trial of shorter duration,” said Fritsvold in the article.
“Finally, we investigated whether blood samples and mucus samples, which can be taken without killing the fish, can be used to detect piscine myocarditis virus (PMCV), which is the virus that causes CMS, and we compared the results from these tests with standard CMS diagnostics.”
Until 2010/2011, the cause of the disease was unknown, with many different hypotheses.
Some thought it was a physiological condition, others claimed that it was caused by environmental factors, while others believed that it was a contagious disease, probably caused by a virus.
“In the first study, we presented the results from the first successful infection trial which shows that CMS is a communicable disease,” said Fritsvold.
Fritsvold and her colleagues injected unvaccinated Atlantic salmon post-smolts with tissue homogenate made from heart and kidney tissue from large farmed salmon with CMS. The experimental fish then developed heart changes in line with previous descriptions of CMS in large farmed salmon. Despite a very long observation period of 42 weeks, both the incidence and severity of the heart changes remained steadily high, and no signs of healing were seen.
CMS-sick salmon cannot fight PMCV
In the next study, a group of young Atlantic salmon of approximately 1 kg were diagnosed with typical CMS unusually early after launch. Several samplings were examined from this group until slaughter almost 10 months later. The study showed that the infected fish had an unusually high amount of PMCV-specific RNA.
“CMS-sick salmon do not seem to be able to fight the PMCV infection and get rid of the virus, and the heart inflammation does not seem to become less violent, heal or be repaired over time,” said Fritsvold.
Heart samples
In the most recent study, Atlantic salmon pre-smolts were infected with an injection into the abdominal cavity of spleen tissue material from CMS-sick fish. Histopathological and real-time PCR (RT-PCR) examinations of tissue samples were compared with results from real-time RT-PCR examinations of blood samples and of mucus from the surface of the fish, from immunohistochemistry and from a new in situ hybridization method - RNAscope.
The results confirm previous findings and show that when sampling fish with clear CMS changes in the heart, the heart itself is the best organ for the detection of PMCV-specific RNA, and it does not matter which part of the heart the samples are taken from.
“We also detected PMCV-specific RNA in plasma in early sampling,” said Fritsvold. “This can mean that the fish go through a phase of the infection where the virus spreads in the blood (viremia). At the same time, a relatively large amount of viral RNA was also detected in central-kidney samples. This means that plasma from blood samples and tissue samples from the central kidney may be used to detect CMS in the early stages, before the classic heart changes have occurred and can be diagnosed with histopathology.”
Contributes to better and safer diagnoses
Fritsvold found that the RNAscope ISH method for in situ detection of PMCV in histological tissue sections was both more sensitive and easier to interpret than the IHK method. The RNAscope ISH method can thus be a valuable additional method for histopathology in the diagnosis of CMS in the early stages of the disease, and in common diagnostics when there are cases of atypical heart changes or mixed infections.
“Our study provides increased insight into which factors play together and lead to CMS in the fish, and how the disease develops in the fish. In addition, we hope that the new RNAscope-ISH method can contribute to better and more secure CMS diagnoses and disease resolution, by making it easier to distinguish early and atypical cases of CMS from other, similar and important heart diseases.”
