WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2025 57 C, the prevalence of the cold-adapted Flavobacterium psychrophilum is favored; etymology: Greek prefix Psychro meaning cold, Greek suffix philos meaning friendly. Additionally, even though the immune system of young fish is generally underdeveloped, whatever little innate protection may exist at that stage is further suppressed due to these low temperatures: the physiological responses of salmonids have been found to be highly temperaturedependent and host-size specific (Buchmann et al. 2024), with 4o C having been previously established as an important immunological threshold. Because of these adverse conditions hatchery morbidity and mortality rates for rainbow trout broodstock and fry could reach up to 70 percent, potentially even eradicating entire stocks if left untreated. Adult fish are also affected by this pathogen, although to a lesser extent, in the form of Bacterial Cold Water Disease (BCWD) and their specific symptoms include loss of appetite, necrosis of the caudal fin and skin lesions. Shared clinical signs manifested on young and developed fish include erosion of tissues, particularly of the caudal fin, darkening of skin, lower jaw skin ulcerations, pale or necrotic gills, and systemic infections affecting the heart, spleen and kidneys, while the young survivors are prone to developing spinal abnormalities (Macchia et al. 2022, Nematolahi et al. 2003). Given the intensification of global aquaculture practices where high-density fish stocking settings are common, rearing environments are thus considered hotspots for disease emergence and spread, further facilitated by the compromised welfare of the farmed organisms, and the inherent variability of aquatic environments. In fact, infectious diseases, primarily caused by opportunistic pathogenic bacteria, have accounted for up to half of production losses in aquaculture (Assefa and Abunna 2018). Recently, in Norwegian salmon aquaculture, syndromic diseases, which are often linked to opportunistic pathogens, have driven mortality rates as high as 25.5 percent in certain zones (Samsing and Barnes 2024). What is an opportunistic pathogen though? In contrast to a ‘primary’ or ‘obligate’ pathogen, opportunistic pathogens have been defined in the scientific literature as “microorganisms associated with disease only in host individuals experiencing atypical environmental stressors or having impaired immune function” (Derome et al. 2016). Both biotic and abiotic surfaces found in an aquaculture rearing environment, such as zooplankton, dry feed, microalgae, tank walls and the surrounding water, normally contain commensal microbial populations. Especially the water microbiota (the assemblage of microorganisms occupying the water body) are widely affected by several environmental parameters. This natural sensitivity and complexity of the aquatic rearing ecosystem is often exemplified by the disruption of the relative balance of microbial communities usually present in water, which can in turn influence the microbial balance on the fish tissues themselves, related to a constantly dynamic interface (Sehnal et al. 2021). When the microbial balance is disrupted, either in the surrounding water or on the fish tissues, it can cause normally harmless bacteria to grow and behave differently within this ecosystem. Such changes, frequently caused by the fluctuations of environmental parameters, such as temperature, salinity, dissolved oxygen concentration, nutrient load, presence of pollutants or pH, could exert stress on the fish themselves, but also selective pressure on certain virulence traits of the surrounding microbes, ultimately turning them pathogenic. The confluence of such factors can have a direct or indirect ‘overload’ effect on the stress response and immune capacities of the host fish, setting the ground for pathologies, and frequently allowing for their mucosal surface defense barriers, such as the skin and the gills, to be circumvented more easily. Such seems to be the case with F. psychrophilum, which has been frequently characterized as an opportunistic pathogen. It can remain in the water for several months (Vatsos et al. 2003), and it has been found not only in fish showing clear signs of disease but also occasionally in wild salmonids, whether they displayed symptoms or not (Van Vliet et al. 2016). Additionally, it has been recently detected in non-salmonid fish, such as sturgeons (Chinchilla et al. 2023) and ayu (Takeuchi et al. 2024), as well as in water samples, sediments, biofilms from aquaculture facilities, and from rivers downstream of fish farms with infected fish (Madetoja et al. 2002, Starliper 2011). Considering the global distribution of this pathogen, the economic ramifications of Flavobacterium psychrophilum infections have been profound and perhaps even underestimated, encompassing direct losses from fish mortality and increased costs for treatment and management strategies (Duchaud et al. 2018). Both BCWD and RTFS have often been treated with antibiotics prophylactically and therapeutically, leading to the development of bacterial strains with resistance to oxolinic acid, oxytetracycline, florfenicol and amoxicillin (Miranda et al. 2016, Saticioglu et al. 2019). Not surprisingly, temporal and geographical genomic studies have also shown that there is a connection between the existence of AMR traits and enhanced pathogenicity in Flavobacterium strains isolated from fish farms, compared to strains isolated from wild environments, exactly due to the selective pressures exercised in aquaculture settings (Duchaud et al. 2018). On the other hand, developing effective vaccines for adult fish has also proven quite challenging since different F. psychrophilum strains show significant genetic differences triggering different immune responses among them, making it challenging for a single vaccine to offer a broad protection. FIGURE 2. Petri dish with agar in which Flavobacterium psychrophilum was embedded and inhibition zones are visible as ‘halos’ around the bacterial culture spots of two positive control strains that are known antibacterial producers and an isolate from an aquaculture unit (B2) being a probiotic candidate strain. (CONTINUED ON PAGE 58)
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