Experimental transmission of segmented filamentous bacteria (SFB) in rainbow trout Oncorhynchus mykiss

Rainbow trout gastroenteritis (RTGE) has been the cause of acute mortality in farmed rainbow trout in Europe since 1992. Epidemiological analysis has indicated a strong association with high production levels and suggested an infectious aetiology. The condition is characterised by the presence of large numbers of segmented filamentous bacteria (SFB) in the intestine, but the role of these in the disease has not been confirmed, in part because the organisms cannot be cultured. Therefore, other approaches need to be developed to investigate the role of SFB in RTGE. Faecal material from clinically affected RTGE trout, either untreated or heat-inactivated, was administered to fish from a susceptible stock, to determine whether the SFB could be transferred artificially and survive in or colonise the new host. Using histology and nested PCR, SFB were detected in the pyloric caeca of fish 23 to 30 d after challenge with untreated faeces. Histological changes in the intestine and the presence of an unidentified Gram-negative coccus were also significantly associated with exposure to untreated faeces. Upregulation of IFN-γ, IL-17A/F and IL-22 gene expression in proximal intestine suggested a low-level immune response to the challenge.