Flavobacterium branchiophilum and F. succinicans associated with bacterial gill disease in rainbow trout Oncorhynchus mykiss (Walbaum) in water recirculation aquaculture systems

Bacterial gill disease (BGD) is a common and occasionally devastating disease that affects numerous cultured fish species throughout the world (Starliper & Schill 2012). Outbreaks of BGD tend to occur when environmental conditions deteriorate, and opportunistic pathogens can more readily cause overt disease (Bullock 1972; Schachte 1983). The putative causative agent, Flavobacterium branchiophilum, has been shown to induce BGD under laboratory conditions (e.g. Lumsden et al. 1995; Ostland et al. 1995); however, diagnosis of BGD in the field is generally carried out through light microscopy and/or observation of clinical signs. Therefore, the identity of the bacterial specie(s) present on the gills of BGD-affected fish in culture settings is most often based on visual assessment alone. The observed bacteria using microscopy are presumed to be F. branchiophilum based on organism morphology (numerous long, thin rods; Fig. ​Fig.1.),1.), with this presumption supported by previous BGD research (e.g. Byrne et al. 1995; Ostland et al. 1997; Derksen, Ostland & Ferguson 1998) and various F. branchiophilum-specific diagnostic approaches that have been developed. The latter include ELISA (MacPhee et al. 1995), IFAT (Heo, Kasai & Wakabayashi 1990), and PCR (Toyama, Kita & Wakabayashi 1996). A comprehensive examination of bacterial species present on the gills of naturally infected fish, however, has not been carried out and is desirable to enhance our understanding of BGD and to inform further research (e.g. potential vaccine development). We therefore sought to induce BGD through environmental manipulation (as opposed to pathogen challenge) and to identify the bacterial species involved in typical BGD outbreaks. Our findings are presented in this short communication.