Antibiotic resistance of Aeromonas salmonicida isolated from Atlantic salmon, Salmo salar L., in Scotland

Abstract. Antibiotic sensitivity patterns of 304 isolates of Aeromonas salmonicida from 229 outbreaks of furunculosis among salmon in Scotland between 1988 and 1990 were investigated. Fifty-five per cent were resistant to oxytetracycline and 37% resistant to oxolinic acid. Multiple resistance was common (52%) and 18 out of 19 antibiograms which were found in the first year recurred in the succeeding year. More than a quarter of the outbreaks were associated with two or more A. salmonicida variants distinguishable by their antibiotic sensitivity patterns. The implications of these findings in the control of furunculosis are considered.     

Plasmid-mediated antibiotic resistance in Aeromonas salmonicida isolated from Atlantic salmon, Salmo salar L., in Scotland

Abstract. Forty oxytetracycline-resistant isolates of Aeromonas salmonicida obtained from outbreaks of furunculosis in Atlantic salmon in Scotland were tested for their susceptibility to 12 antibacterial agents. There were 10 resistance patterns with multiple resistance to two to six antibacterial agents. Transferable R-plasmids encoding oxytetracycline resistance were demonstrated in 11 out of the 40 isolates. The resistance transferred was multiple; to oxytetracycline, streptomycin, sulphamethoxine and trimethoprim, or to oxytetracycline and one or two of these in combination. Oxytetracycline resistance was transferred in a single, large step, >250-fold increase, and the MICs for individual transconjugants from mating populations were the same.     

Nocardiosis in tank-reared Atlantic salmon, Salmo salar L.

Nocardiosis is an infection caused by aerobic Gram-positive, branching, filamentous rods of the genus Nocardia . The organism is resident in both soil and plants ( Austin & Austin 1993 ; Frerichs 1993 ), and is closely related to Mycobacterium spp. Nocardiosis caused by Nocardia asteroides and N. seriolae (previously N. kampachi ) has been reported in several finfish species, both freshwater and marine. The first reported incidence was recorded by Valdez & Conroy (1963) in neon tetras, Hyphessobrycon innesi (Myers), and subsequently in other fish species, some of commercial significance including rainbow trout, Oncorhynchus mykiss (Walbaum) ( Snieszko, Bullock, Dunbar & Pettijohn 1964 ), brook trout, Salvelinus fontinalis (Mitchill), ( Campbell & MacKelvie 1968 ) and yellowtail, Seriola quinqueradiata (Temminck & Schlegel; Kubota, Kariya, Nakamura & Kira 1968 ). Despite two of these incidences occurring in salmonids, infection by Nocardia spp. in Atlantic salmon, Salmo salar L., has not previously been reported.     

Reproductive strategies in Atlantic salmon, Salmo salar L.

Abstract. Life-history strategies are means by which animals solve the problems of successful reproduction in varying environments. Their development patterns are consequences of responses to the opportunities the environment offers them. Understanding them requires an understanding of the way they evolved, their ontogenetic development, their physiological control, and their adaptive value. The present paper views the salmonids as marine fishes, which have radiated into fresh water through using river beds as protected spawning grounds. It also takes the view that the maturation process has priority over somatic growth in fish, and that it has already been initiated by the time of first feeding. Its completion is environmentally dependent, and can be arrested annually. Whether or not it will be arrested depends on the status of the energy stores of the individual at particular critical times of year. This mechanism has adaptive value both for immediate reproductive success — adequate energy to provision the next generation — and for later overwinter survival, ensuring that if energy stores are inadequate for reproduction they are spared. Atlantic salmon show variation in their reproductive patterns, and examples are given from laboratory and aquaculture experiments to demonstrate some environmental controls which result in these variations. A hypothetical model is presented to account for the operation of these controls.     

Humoral immune response in Atlantic salmon, Salmo salar L., to cellular and extracellular antigens of Aeromonas salmonicida

Abstract. The putative virulence factors of Aeromonas salmonicida , the aetiological agent of furunculosis in salmonids, are candidates for protective antigens in effective vaeeines against furunculosis. In this report, the authors have compared the immunogenieily of eell-associated and extracellular antigens of A. salmonicida in Atlantic salmon, Salmo salar L., to that in rabbit. The animals were immunized with formalin-killed whole cells and formalin-inactivated extracellular products (ECP), either separately or in combination. The ability of the antigens to induce antibody production was studied by elisa and Western blotting techniques. These results confirm previous reports that far more structures are immunogenie in rabbit compared to the antibody responses elicited in salmon. However, in both species, some antigens were dominant, including a caseinolytic protease in addition to the A-protein and high and low MW LPS.     

Immunological responses in Atlantic salmon, Salmo salar L., against purified serine protease and haemolysins from Aeromonas salmonicida

Abstract. A method for purification of the 70-KDa extracellular serine protease of Aeromonas salmonicida by hydrophobic chromatography and ion exchange is described. The purified protease, adsorbed onto mineral particles, was used for immunization of salmon. Other groups of salmon were immunized with particles coated with purified glycerophospholipid:cholesterol acyltransferase (GCAT) or with the acyltransferase complexed with LPS (GCAT-LPS). Humoral immune responses assayed after 6 weeks by ELISA, showed relatively good responses against GCAT-LPS, while titres of antisera against the purified protease and GCAT were scarcely above those of control sera. However, the antibody responses to each toxin were shown by Western blotting to be specific and qualitatively similar to responses seen in rabbits. The toxin preparations were also used (in combination with whole bacteria) for vaccination of salmon. On challenge 3 months later, only GCAT-LPS elicited significant immunological protection. However, a more convincing protection was seen when total extracellular product was present in the vaccine.     

Haemorrhagic kidney syndrome of Atlantic salmon, Salmo salar L.

This report describes a new syndrome affecting farmed Atlantic salmon on the Canadian east coast that has resulted in increased morbidity and mortality in affected stocks. The major pathological findings are apparent only microscopically and include renal interstitial haemorrhage and acute tubular necrosis and tubular casting. As a result, the disease has become known as haemorrhagic kidney syndrome (HKS). Affected fish are lethargic and anorectic, and lack external lesions. Clinically, HKS fish are anaemic, hypoproteinaemic and hyperosmolalic, with increased serum concentrations of sodium and chloride. At necropsy, internal changes ranged from apparently normal to include one or several of the following: swelling and/or patchy reddening of the kidney, pale gills, exophthalmos, serosanguinous ascites, darkening of the posterior intestine and splenomegaly. Ultrastructurally, viral inclusions were found in the cytoplasm of erythrocytes of HKS fish, and there were unusual electron‐dense inclusions within the tips of renal tubular microvilli of HKS fish. The significance and relevance of the ultrastructural findings to HKS are unknown. Virus isolation was attempted using CHSE, RTG‐2, FH‐10, BB and EPC cell lines; no virus was isolated. Bacteriological analysis failed to reveal significant pathogens. Analysis of tissues for heavy metals and pesticides was negative. Assays for clostridial toxins, lipopolysaccharide and verotoxins were negative. The aetiology of HKS remains unresolved.     

Modified extracellular product antigens of Aeromonas salmonicida as potential vaccines for the control of furunculosis in Atlantic salmon, Salmo salar L.

Abstract. Extracellular product (ECP) antigens of Aeromonas salmonicida were modified in an attempt were tested on an antigen-induced proliferation assay, for the ability to induce antibodies as measured by dot blot dot assay and as vaccines in vaccination/challenge trials. Modifications tested included particularization on to polystyrene beads, coating on to sheep red blood cells, mixing with BCG vaccine as adjuvant, and attachment to the T-independent carrier Fieoll. The only modification that resulted in increased protection levels was the particularization on to polystyrene beads.     

Tissue localization of Aeromonas salmonicida in Atlantic salmon, Salmo salar L., following experimental challenge

Three groups of Atlantic salmon, Salmo salar L., were exposed to live, colony-forming, radiolabelled Aeromonas salmonicida bacteria in a bath challenge: (1) fish with artificial wounds; (2) fish with a reduced epidermal mucus layer caused by removal of the mucus layer on two occasions by a swabbing procedure; and (3) a control group of untreated fish. Fish were killed 2, 6 and 24 h after challenge, and radioactivity (cpm g^–1) was measured in the blood, mucus, skin, wound area, gills, anterior kidney, posterior kidney, spleen, midgut and hindgut. The highest levels of radioactivity were measured in the wound areas and in the gills. There was a significant positive correlation between the levels of radioactivity in the gills and blood, and between the mucus and skin at 2 h post-challenge. Two hours after the bath challenge, live A. salmonicida bacteria were found in the blood of fish in the 'swabbed' and 'artificial wound' groups, and not in the control group. Twenty-four hours after the bath challenge, the kidney of fish from all groups contained viable bacteria, whereas the blood was negative.     

Infectious pancreatic necrosis in Atlantic salmon, Salmo salar L

A clinical and histopathological review was carried out of 21 outbreaks of acute infectious pancreatic necrosis (IPN) in Scottish Atlantic salmon, Salmo salar L., farms (13 marine and eight fresh water) during 1991-2004. A distinctive syndrome was evident in both post-smolts in sea water and fry in fresh water, where liver lesions, which had not previously been associated with IPN, became a consistent finding in addition to the more typical pancreatic and intestinal changes. Initial cases were described in post-smolts in Shetland, but by the end of the period of investigation this type of pathology had extended down the West coast of Scotland and into Ireland. Limited viral strain analysis suggested that similar strains were involved in both fresh water and sea water and that these differed from earlier isolates from rainbow trout, Oncorhynchus mykiss (Walbaum). In fresh water, recovered fish frequently developed a greatly distended intestine associated with accumulation of undigested food. In sea water, after the initial, often significant (50% or more), losses, there were many fish which failed to grow and became chronically emaciated and prone to sea louse infection. Although use of transfer diets containing immune enhancers and the selection of IPN resistant broodstock has reduced losses the disease remains a serious cause of economic loss.     

Exophiala salmonis infection in Atlantic salmon Salmo salar L.

Abstract. Infection of Atlantic salmon, Salmo salar L., in seawater with Exophiala salmonis is described. Histological effects consisted principally of granuloma formation, especially in the posterior kidney and spread appeared to occur both by extension and by the liaematogenous route. The nature of the outbreak suggested that the original infection occurred via contaminated food.
A comparison is made between this condition and systemic mycoses in other species.