Search

A disease outbreak in farmed Atlantic cod caused by Yersinia ruckeri is reported. Mortality started following vaccination of cod reared in two tanks (A and B). The accumulated mortality reached 1.9% in A and 4.8% in B in the following 30 days when treatment with oxytetracycline was applied. Biochemical and molecular analysis of Y. ruckeri isolates from the cod and other fish species from fresh and marine waters in Iceland revealed a high salinity-tolerant subgroup of Y. ruckeri serotype O1. Infected fish showed clinical signs comparable with those of Y. ruckeri -infected salmonids, with the exception of granuloma formations in infected cod tissues, which is a known response of cod to bacterial infections. Immunohistological examination showed Y. ruckeri antigens in the core of granulomas and the involvement of immune parameters that indicates a strong association between complement and lysozyme killing of bacteria. Experimental infection of cod with a cod isolate induced disease, and the calculated LD50 was 1.7 × 10(4) CFU per fish. The results suggest that yersiniosis can be spread between populations of freshwater and marine fish. Treatment of infected cod with antibiotic did not eliminate the infection, which can be explained by the immune response of cod producing prolonged granulomatous infection., (© 2013 John Wiley & Sons Ltd.)

Intra-muscular injection of turpentine oil was used to induce acute phase response (APR) in Atlantic cod (Gadus morhua L.). The effects on the serum cortisol, total protein, IgM and pentraxin concentration were examined as well as the effects on natural antibody, anti-trypsin and leukocyte respiratory burst activity. The turpentine injection resulted in a 26 fold increase in the cortisol level after 72 h. Slightly reduced serum protein level in both groups was attributed to the restricted feeding during the experimental period. The IgM serum concentration was significantly reduced after 168 h in the turpentine treated fish while the natural antibody activity was not affected. The anti-trypsin activity was initially suppressed but recovered to normal levels at the end of the experiment. The turpentine injection had little effect on the serum level of the pentraxins, CRP-PI and CRP-PII. The respiratory burst activity was significantly suppressed after 72 h. It is concluded that 1) cod shows a relatively slow humoral and cellular response to APR induction, 2) the increase in serum cortisol level may be the key modulator of the mainly suppressive effects on the immune parameters and 3) pentraxins are not typical acute phase proteins in cod., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Aims: To study the effect of ova disinfection, antibiotic and microbial treatments on the dominant cultivable cod rearing microbiota at pre- and posthatch stages, determining some virulence-related phenotypic traits among bacterial isolates and their relation to larval survival., Methods and Results: Sampling of rearing systems (rearing water, ova, larvae, feeds and supplement) for analysis of cultivable microbiota took place at early stages in 2004 and 2005. Cultivation, phenotypic and genotypic (16S rRNA gene) analyses were performed. The production of putative virulence factors (PVFs), including haemolysin, siderophores and quorum-sensing signals, by bacterial isolates was investigated and related to larval survival. The study was performed during two spawning seasons, evaluating current hatchery practices (ova disinfection and antibiotic treatment of unhealthy larvae) and specific putative probiotics applied to ova and larvae or rotifers. A diversified microbiota (75 operational taxonomic units, OTUs) was observed in cod rearing systems influenced by the feeds and treatments, with prevailing γ-Proteobacteria prior to hatching towards a multiphyla microbiota posthatch. Phenotypic tests demonstrated the heterogeneity within some OTUs. Multivariate analysis of survival data in larval silos and the corresponding larval microbiota was used to divide the genotypic groups into beneficial/harmless and detrimental/opportunistic clusters. PVFs were common among the proposed detrimental/opportunistic OTUs., Conclusions: The results clearly demonstrate the influence of exogeneous feeding and treatments on larval gastrointestinal microbiota and the role of bacteria in larval survival., Significance and Impact of the Study: Increased understanding of the microbiota in rearing systems may contribute to successful implementation of microbial management in cod aquaculture., (© 2010 The Authors. Journal of Applied Microbiology © 2010 The Society for Applied Microbiology.)

Aims: To assess the effects of bacterial treatment at the earliest stages of cod rearing on the microbial load, larval development and performance, testing three bacterial strains (Carnobacterium divergens V41, Arthrobacter sp. and Enterococcus sp.) in vivo that were previously shown to have inhibitory potential towards fish pathogens in vitro., Methods and Results: A bacterial mixture was added eight times to the rearing water from the prehatch to the mid-larval stage (a 38-day period). Microbiological analysis of ova, larvae and rearing water was performed regularly. Larval performance and development were evaluated by survival rate, hypersalinity tolerance and physiological measurements. Different larval survival rates were observed within and between treatments, and possibly explained by variations in larval microflora and established probionts. Larvae from one silo, which had been bathed in the bacterial suspension, showed the highest survival rate (42.1%), lowest Vibrio levels, and were significantly heavier (19.3%) and more stress tolerant than control larvae (P < 0.01). This coincided with the intestinal establishment of two of the tested bacteria., Conclusions: Arthrobacter and Enterococcus strains added regularly to the rearing water from the postfertilized egg stage can become established in larval gastrointestinal tract. The Enterococcus strain was associated with increased larval growth, performance and microflora control, indicating its probiotic nature., Significance and Impact of the Study: Regular application of autochthonous probionts may promote larval welfare, development and stress tolerance at early stages, hence increasing production yield in intensive cod larviculture.

Natural antibodies are present in the serum of vertebrates regardless of antigenic stimulation. Characteristic activity is commonly detected against haptenated proteins, single stranded DNA and thyroglobulin. Natural antibodies are believed to provide an instant protection against pathogens of a broad specificity and to participate in homeostasis. Cod is a poor antibody responder but shows a relatively high level of natural antibodies against haptenated proteins. In this project the specificity, activity and affinity of natural antibodies was studied in different groups of cod and the effects of age/size, environmental temperature, immunisation and infection examined. Antigen driven selection of natural antibodies was also studied in one group of cod. The results demonstrated a broad and yet characteristic specificity, primarily directed against haptenated proteins and possible food antigens. The antibody activity increased with increasing age and at higher temperature whereas immunostimulation by immunisation or infection resulted in variable response. The affinity index of natural antibodies of cod generally did not correlate with changes in the antibody activity but it was in the same range as the affinity index of acquired cod antibodies and that of some mammalian monoclonal acquired antibodies. Analysis of antigen driven antibody selection showed that the natural antibody repertoire of individual cod was heterogeneous with respect to its affinity for haptenated protein.

Pentraxins are important molecules in innate defence and play a role in the acute phase response of both mammals and fish. Isolation of cod pentraxins by affinity chromatography using phosphorylcholine agarose revealed two pentraxin-like proteins, referred to as PI and PII proteins. These varied in their overall charge, pentameric and subunit molecular size, glycosylation and N-terminal amino acid sequences. The PI protein was homologous with the CRP-like pentraxin previously described in cod whereas the PII protein was a new CRP homologue, which was characterized by substantial individual heterogeneity with regard to subunit size and relative density. The results indicate considerable genetic variations in the cod pentraxins.

Survival problems are encountered at early stages of intensive fish rearing and antibiotics are widely used to remedy the situation. Probiotics may provide a potential alternative method to protect larvae from opportunistic and pathogenic bacteria and promote a balanced environment. This study was designed to search for new probiotics to target this critical period in cod rearing. Potential probionts were selected from the natural microbiota of cod aquacultural environment. The selection was based on several criteria: pathogen inhibition potential, growth characteristics, strain identification, metabolite production and adhesion to fish cell lines. Our study demonstrated that 14% of screened bacteria (n=188) had antagonistic properties towards fish pathogens. The majority of these isolates were Gram-positive (81%), belonging to Firmicutes (69.2%) and Actinobacteria (11.5%) phyla based on 16S rRNA gene sequencing. Only 6 (3.2%) of 188 isolates could inhibit all three pathogens tested: Vibrio anguillarum, Aeromonas salmonicida subsp. achromogenes and Vibrio salmonicida. Differences observed in activity intensity and spectrum among inhibitory isolates emphasise the need to develop probiotic mixtures for efficient prophylactic methods. Comparison of growth behaviour of inhibitory isolates and pathogens at cod rearing temperatures, metabolite production and adhesion capacity were considered for final probiont selection. Four promising isolates that could be used as a mixed supplement to rearing water were identified as putative probiotic bacteria. This study emphasises the importance and potential of lactic acid bacteria in aquaculture.

Cod larval culture is currently hampered by high mortalities in the first 2-3 weeks after hatching, often due to infectious diseases. The immune system of cod is not fully competent until 2-3 months after hatching. Conventional vaccination is, therefore, not of value before this time, and the larvae are wholly reliant on non-specific parameters for their defence against infection. A range of substances, generally derived from bacterial, fungal or plant origin, can activate these non-specific parameters. During three hatching seasons, 2001-2003, at the Marine Institute's Experimental Station, Stadur, Grindavik, Iceland, the effects of several immunostimulants on survival and disease resistance of cod larvae and juveniles were examined. Both bathing treatments and administration in the feed were used. One of these substances, lipopolysaccharide (LPS), isolated from the bacterium Aeromonas salmonicida (ssp. salmonicida or achromogenes), appeared in some instances to improve survival and have a beneficial effect on disease resistance. Other substances tested had limited effects. The results emphasize the need for further work in this field.