In recent years, bacterial disease outbreaks in sea-farmed Atlantic salmon in Norway have become increasingly frequent. This concerns both well-known diseases such as furunculosis and yersiniosis, but also bacteria never or seldom associated with disease previously. The background for this situation remains unclear, although a steep rise in the use of physical treatment methods for removing salmon lice has occurred concurrently, and such treatments are often reported shortly prior to disease outbreaks.
It is broadly recognised amongst epidemiologists that the development of infectious diseases relies upon an interplay between three factors, i.e. the agent (here bacteria), the host (here salmon) and their shared environment (here sea cages). Whilst there are no indications of neither the involved bacteria nor the salmon having undergone fundamental changes of relevance recently, the salmon are now increasingly exposed to potentially stressful and/or harmful stimuli, e.g. during physical delousing. Repeated and/or long-term stress may cause impairments to an animal’s immune system, but in the case of several disease-associated bacteria from terrestrial animals, host stress hormones have also been shown to directly stimulate their ability to cause disease. This phenomenon however, coined ‘microbial endocrinology’, remains poorly studied amongst bacteria from fish.
Our goal is to study whether (and to what extent) stress hormones from salmon, e.g. excreted during physical delousing, may cause selected fish-associated bacteria to become more ‘aggressive’ and thereby perhaps contribute towards explaining the observed rise in disease outbreaks. An array of complementary methods will thus be employed to map changes to the bacteria's physical traits (phenotypes) and gene regulation (transcriptomes), and particularly such of know association to disease development, while under stress hormone exposure. Specific tools will be developed/refined for measuring stress hormones.
Following years of minor bacterial disease problems in farmed Atlantic salmon in Norway, this has again become a major cause of mortality, poor welfare and economic loss. The expanding bacterial disease problems often relate to the sea-phase of salmon farming, where handling-intensive operations are likely to cause fish stress and stress hormone production. For many bacteria capable of causing disease in mammals, exposure to stress hormones may stimulate their ability to do so. The study of such host-microbe interactions has been coined 'microbial endocrinology', a field largely unexplored for bacterial fish diseases.
Here, we hypothesise:
1. The accumulation of stressors facing today’s farmed salmon has now shifted the overall balance within the epidemiological triad in favour of increased bacterial disease incidence.
2. Disease-associated bacteria in fish react by activating disease mechanisms when sensing a rise in host stress hormones.
3. Severe stress in combination with high fish densities during handling procedures may cause concentrations of waterborne stress hormones to trigger ‘activation’ of the bacteria.
To investigate this we will expose selected fish-associated bacteria to stress hormones in the lab. Effects upon the bacteria’s disease-causing potential will then be studied using a series of complementary techniques, allowing identification of disease-related genes that become activated. Selected bacteria will be similarly studied after intimate exposure, via water, to experimentally stressed salmon. We will also develop tools for sensitive monitoring of stress hormone levels in salmon stocks.
Our overall ambition is to create new knowledge and increased understanding of important bacterial diseases in Norwegian aquaculture through unravelling whether microbial endocrinologcal processes are important facilitators of bacterial diseases in fish. This can hopefully facilitate development of better mitigation strategies within the aquaculture industry.