Heart and skeletal muscle infection (HSMB) and cardiomyopathy syndrome (CMS) are two viral diseases that are causing an increasing problem for Norwegian aquaculture. There are still no vaccines against these diseases. For pancreatic disease (PD) a vaccine is introduced, but with limited effect. The disease is still a major problem for the industry. All these three viral diseases affect all hearts of salmon.
Functional feed directed to disease is usually documented through infection challenges. This documentation is expensive, time-consuming and has therefore been used to a limited extent. Both lab and field trials often provide no or uncertain data. Also, such trials do not always have the necessary sensitivity and reproducibility necessary to reveal the effects nutrition has on health.
In the project, we wanted to develop a biological fish model to test the effects of nutrition on cardiac function and circulation in fish. The model was planned to be used to investigate how feed can be used to strengthen overall cardiac health, reduce tissue damage in the heart after disease, and to ensure faster recovery after a virus infection. Testing and validation of the model showed that it was not precise enough to simulate tissue damage in the heart after infection and to distinguish between different feed based on krill meal. An alternative in vitro model with immune cells was then used to investigate basic mechanisms of feed effects. The results were promising but will require further development for a practical application.
An infection trial with PD was performed to verify the fish model, showing that feed with krill meal gave a significantly better growth 10 weeks after infection. Histopathology and microarray of the heart showed that the diets with krill meal gave less severe PD symptoms and faster rehabilitation of the fish. The positive effect of krill meal in the feed on growth was much more pronounced in infected than in uninfected fish.
The project was ended with a test of two diet concepts based on low and high levels of krill meal in an experiment with Atlantic salmon. The fish was grown from approx. 1 kg to 4.5 kg at BioMar's trial license at Dønna. The experiment aimed to test the dietary effects on heart health, fillet colour and yield and meat quality. An important goal was to test the effect of melanin spots in the fillet that is a large problem in Norwegian salmon farming.
Large-scale field trials are always prone to unexpected events and uncontrolled conditions. Use of only two cages per feed code also limits statistically valid conclusions but is a compromise when doing large-scale trials. Unfortunately, there were too few melanin stains in the fillet to identify any dietary effects. The fish given the diet codes with krill meal had a darker skin colour. The diet with the highest level of krill meal had a slightly better fillet colour than the control group. This diet code also had significantly more triangular hearts and no rounded (malformed) hearts indicating better heart health.
Histopathology, qPCR and microarray showed that the fish was infected with HSMB and CMS. The most obvious infection was from the PMCV virus that causes CMS. PMCV was found in about half of the fish in the two groups fed krill meal compared to the control group. The difference was not significant, but the results are promising and will be followed up in new trials.
A dose-response effect of krill meal was detected on the gene expression (transcriptome) in the gills with many changes in genes associated with virus defence. These findings correspond to gill histology data and are important documentation for a new feed that can help strengthen both cardiac and gill health.
A major part of loss in Atlantic salmon farming is caused by the viral diseases HSMI, CMS and PD, which infect the heart and circulatory system leading to severe cardiomyopathy and anemia. The lack of efficient vaccines substantiate the need of developing alternative methods functional feeds that can be offered to the fish once a disease outbreak has been confirmed in order to reduce damage on the cardiovascular system and to promote efficient repair.
Development of functional fish feeds to improve the cardio-vascular system and such mitigate disease have been studied to a limited extent and has been given low priority despite its large potential when compared to the well documented nutritional effects on humans with cardiac failures and cardiomyopathy. Hitherto, progress has been partly hampered by the lack of a suitable model. The current use of lab and field trials do not have the required sensitivity or reproducibility needed to identify significant effects of feed supplements promoting improved cardiovascular performance and regeneration after injury.
The main innovation of this project will be a cardiovascular challenge model for clinical nutrition studies in fish. This will comprise a unique test system for sensitive and reproducible evaluation of cardiovascular performance and recovery following dietary interventions for mitigation of viral-induced pathologies.
The cardiovascular challenge model will be used to follow up BioMar's pilot studies indicating that functional feeds can reduce histopathology score in heart tissue and mortality after HSMI infection. Key diets identified with the new test system will be further developed in large scale experiments at Biomar's trial concessions investigating effects on both health and flesh quality.
BioMar anticipate that the project results in one or more functional feeds that can strengthen cardiovascular health before stressful episodes and that can mitigate the negative effects of viral diseases and such become a tool for reducing loss for the farmers.