Flavobacteria as secondary pathogens in carp suffering from koi sleepy disease.

Koi sleepy disease (KSD) is a disease with increasing importance in global common carp aquaculture. Despite the fact that carp edema virus (CEV) is most likely the causative agent of KSD, the disease often presents itself as multifactorial with several parasites and bacteria species present on gills, skin or in internal organs. Therefore, in this study, we analysed and presented initial results on an interaction of flavobacteria and CEV in the development of clinical KSD in carp suffering from proliferative gill disease. We examined selected field samples from Germany and Hungary and confirmed the presence of CEV and flavobacteria co-infections in subset of the samples. In several infection experiments, we studied the transfer and dynamics of both infections. Furthermore, we analysed which Flavobacterium species could be isolated from KSD-affected fish and concluded that Flavobacterium branchiophilum is a possible copathogen. Antibiotic treatment experiments showed that CEV seems to be the primary pathogen causing an insult to the gills of carp and by these enabling other pathogens, including F. branchiophilum, to establish co-infections. Despite the fact that F. branchiophilum co-infection is not required for the development of clinical KSD, it could contribute to the pathological changes recorded during the outbreaks.