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Fish farming creates conditions where disease transmission is enhanced and antibiotic treatments are commonly used to cure bacterial diseases to prevent severe losses due to infections. Ability to persist in such an environment has been suggested to lead to the evolution of high virulence. Columnaris disease caused by Flavobacterium columnare is a growing problem in freshwater fish farming. Transmission of the disease is poorly known, and survival of F. columnare in the rearing environment has not been studied. This paper addresses both transmission of columnaris disease and survival strategy of F. columnare. Saprophytic activity of F. columnare was studied by infecting rainbow trout fingerlings before and immediately after death and by following bacterial shedding from the fish carcasses. From fish killed immediately after infection, bacteria were shed at high rates for 5 days, and from fish exposed to F. columnare post mortem for 8 days. In another experiment, rainbow trout fingerlings were experimentally infected with F. columnare and monitored for transmission of the bacteria post infection until and after the death of the fish. The transmission of columnaris disease to living rainbow trout was the most efficient from dead fish, from which bacteria were shed into water at higher rates than from living fish. We also found that F. columnare can survive at least for 5 months in both sterilized distilled and lake water. These results show that death of the host causes no cost for F. columnare; it thrives in alive and dead fish, and in water. Saprophytism may have been a transition stage to pathogenicity of this originally harmless water bacterium, and maintained as an effective transmission and survival strategy of F. columnare. Our findings also suggest that F. columnare may be able to persist in the rearing environment during antibiotic treatments of the living fish.

Roach (Rutilus rutilus) were irradiated with a single dose of ultraviolet B (UVB) radiation (0.4 J/cm2) in order to study the effects of UVB on the nonspecific immune defense mechanisms of fish. Neutrophils and macrophages were isolated from the head kidney of fish on days 1-14 postirradiation. Both random and directed migration of neutrophils, studied by migration under agarose assay, were suppressed on day 1 after UVB irradiation. The respiratory burst of phorbol 12-myristate 13-acetate-stimulated neutrophils and macrophages was also suppressed at days 1 and 2 after UVB irradiation. The suppression of migration and respiratory burst were restored or the responses were even enhanced later, but on the other hand spontaneous cytotoxicity of neutrophils toward 51chromium-labeled K562 target cells stayed suppressed throughout the 14 day follow-up. This study indicates that UVB radiation has the potential to suppress the functioning of phagocytes and to compromise the immune system of fish.