Flavobacterium columnare in Finnish fish farming : characterisation and putative disease management strategies

Abstract

Lotta-Riina Suomalainen selvitti väitöskirjatyössään syitä flavobakteeritaudin lisääntymiselle. Flavobacterium columnare -bakteerin aiheuttama kalakuolleisuus on vakava uhka suomalaiselle poikaskalan tuotannolle. Flavobakteeri-infektiot muodostavat suurimman osan kalanviljelyssä diagnostisoiduista tautitapauksista ja niiden haitta elinkeinolle on merkittävä. Flavobacterium columnare -bakteeri aiheuttaa kalanpoikasille vakavia kidus- ja ihotulehduksia. Tappiot kalanviljelijälle ovat mittavat, sillä jopa 100 % sairastuneista kaloista voi kuolla. Tauti esiintyy kesäaikaan yleensä viljellyillä lohikaloilla. Sen esiintyminen on lisääntynyt viime vuosien aikana, mutta syitä siihen ei tiedetä.

The number of Flavobacterium columnare infections at Finnish fish farms increased significantly during the 1990s. The pathogen was studied to increase information about the characteristics of this economically important disease and to help develop more effective management strategies. The diversity of 31 F. columnare isolates was studied using molecular methods including 16S rDNA RFLP, RISA, AFLP and sequencing. Seven genetically different groups were found among Finnish isolates using RISA, and F. columnare type strain NCIMB 2248T represented an eighth genotype. These eight genotypes were further characterised according to their growth features, virulence and virulence-related factors. Strains were highly sensitive to even low (0.1%) NaCl concentrations and pH values under 5, being unable to multiply under such conditions. Differences in virulence were also detected. High virulence was associated with the adhesion capacity, and the production of the connective tissue degrading enzyme chondroitin lyase C was significantly higher at 25 °C than at 20 °C. PCR was a more sensitive method than agar plate cultivation for detecting F. columnare in both healthy and infected fish. Fish which had a previous contact with F. columnare were shown to remain as carriers of the disease. Such carrier fish do not, however, form a risk to the fish farming industry, since columnaris disease was not re-induced by concomitant parasitic infection or rearing-related stress. The effects of rearing density and water temperature were also studied. High water temperature was shown to be the key factor affecting the severity of columnaris outbreaks. Transmission of columnaris disease was slower at reduced rearing density, but significant changes in overall mortality between high and low density were not detected.Putative disease management strategies were studied experimentally using rainbow trout (Oncorhynchus mykiss) fingerlings. Manipulation of the water quality to prevent infection was performed by bathing infected fish with 2% and 4% salt or changing the water pH to 4.5. The microbial community of fish skin and gills was manipulated by bathing fish with Pseudomonas sp. MT5, found to be antagonistic to flavobacteria in agar assays. None of these methods was effective against columnaris disease, although 4% salt baths decreased mortality rate of fish at the beginning of the infection.