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HAVBRUK2-Stort program for havbruksforskning

Role of nutritional status for PD infection and recovery

Alternative title: Kva rolle spelar ernæringsstatus under eit PD utbrot

Awarded: NOK 2.0 mill.

Salmon farmers in Norway experience around 16% mortality during the seawater phase, and one of the major causes are viral diseases such as pancreas disease (PD). PD is caused by the salmonid alphavirus (SAV), and the infection causes loss of pancreatic tissue and damages the heart and muscle. This results in problems with nutrient digestion and poor feed utilization, and further to reduced slaughter quality, poor growth and increased mortality, and represents an important welfare challenge for the industry. In this project, the role of single nutrients and nutritional status at the time of infection on modulation of the immune response, how the fish handles the infection and how long it takes before the fish recovers has been investigated. This has been done through different approaches using challenge trials where the fish is infected with a virus, and cell models to study how immune cells respond to virus and whether different rearing temperatures influence the immune response. The nutritional status of the fish can be influenced by both nutrients in the feed, environment, diseases and genetics, and the combination of these factors determine how strong the fish is when infected. When fish get PD, the ability to digest nutrients is reduced because the pancreatic tissue is damaged. The pancreas produces digestive enzymes needed to break down nutrients like proteins and lipids to smaller pieces (molecules) that the fish can utilize. This means that the fish will develop a poor nutrient status and become slimmer, because it is dependent on utilizing the stored reserves. Nutrients can influence the immune response in different ways, for instance because they are used to form inflammation signals, or directly stimulate or inhibit the immune response. This also means that the nutritional status at the time of infection is important for the outcome of the disease. Due to the increasing need for sustainable feed and limited access to fish oils, the composition of the salmon feed has changed to contain more plant lipids. A lower content of fish oils and higher content of plant oils reduces the content of the long chain marine omega-3 fatty acids EPA and DHA, and also changes the content of omega-6, saturated and unsaturated fatty acids. In general, the fatty acid composition in the feed is reflected in most tissues, which means that fish that have eaten more plant lipids will have less omega-3 fatty acids in the cell membranes. The composition of the cells influences how the fish responds to an infection, for instance through modulation of the inflammatory response. An optimal response is important during an infection to quickly mobilize the immune system and fight the infection, but it is also important that the inflammation does not persist. Results from this project shows that salmon that have eaten a high level of omega-6 fatty acids develop a stronger inflammation reaction after an artificial induced PD infection, and that the inflammation lasts longer. This fish also used longer time to repair tissues and recover. In comparison, positive effects of having a diet with a low level of omega-6 compared to omega-3 fatty acids were shown, and that the level of saturated lipids may influence the outcome. Environmental climate change implies that farmed fish may be exposed to more frequent periods of sub-optimal water temperatures, which results in several physiological and fitness consequences. Triploid salmon are considered for commercial aquaculture due to environmental and production purposes. Triploids are functionally sterile, which eliminate genetic interactions between escaped farmed and wild fish and mitigate pre-harvest maturation in the production. However, triploids have been shown to be more sensitive to sub-optimal environmental conditions and the requirement for some nutrients differ. Fish exposed to temperatures above the optimum for growth alter the metabolism, resulting in changes in nutritional status. Results from the present project shows that fish kept at high temperature had lower concentrations of antioxidants in the liver, while they produced more endogenous antioxidants to protect the muscle. In cell models it was shown that the immune response is influenced by the rearing temperature and ploidy, and the response was modulated by functional amino acids. At high temperature several of the measured responses were lower, but supplementation with arginine could restore the immune response to similar levels as seen in fish reared at optimal temperature. The practical meaning of this will have to be followed up in future studies. In conclusion, it was shown that nutritional status and single nutrients influence the immune response both in challenge trials (in vivo) and in models (in vitro). The results from this project contributes to increasing the knowledge on how nutrients modulate fish health and are useful to reduce production losses and increase welfare in fish farming.

The salmon aquaculture industry experiences around 20% fish mortality during the seawater phase, and one of the major causes is viral diseases such as pancreas disease (PD). The combined effects of PD on mortality, growth, feed conversion and quality results in large economic consequences for the fish farmers, and represent a major welfare challenge. Growth in teleosts involves the GH-IGF system, and this may be altered during a PD outbreak. Thus it is interesting to investigate whether the GH-IGF system is altered during a PD outbreak, and whether it plays a role for the growth regulation and regeneration of tissues when the fish is in recovery. The nutritional status of the fish at the time of infection may modulate the immune response and hence the disease progression. The essential amino acid histidine and histidine derivatives has anti-inflammatory properties and modify the immune response, however the potential immunomodulatory role of histidine to viral diseases such as PD is not known. Vegetable oils used in aquafeeds are devoid of essential marine omega-3 fatty acids, and since the tissue fatty acid composition reflects the feed composition, the use of vegetable oils in the feed may have consequences for the immune response and inflammation process. Triploid Atlantic salmon is being considered for aquaculture, however the underlying causes for physiological differences between diploid and triploid, including potential differences in the immune response to viral diseases such as PD has not been studied in detail. The results from the present project will increase the basic knowledge on how nutrients influence fish health and disease susceptibility, and will serve as a basis for the development of best practice protocols on how to reduce production losses and increase welfare in commercial salmon farming.

Funding scheme:

HAVBRUK2-Stort program for havbruksforskning