Harmful Aeromonas spp. associated with the pathogenic tilapia parasite Gyrodactylus cichlidarum – Double trouble?

Co-infections involving monogenean parasites and pathogenic bacteria frequently result in increased fish mortality. Among the most problematic pathogens affecting tilapia aquaculture are monogenean parasites, such as the cosmopolitan Gyrodactylus cichlidarum , as well as opportunistic bacteria like Aeromonas sp. Bacterial infections are assumed to be opportunistic, facilitated by the parasite's breaching the fish host's first line of defense (mucus-covered skin) with its attachment hooks and through feeding. Here, using scanning electron microscopy, metagenomics, microbiology and matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) we characterized bacterial communities associated with farmed tilapia and G. cichlidarum . We demonstrate that bacterial communities associated with G. cichlidarum were significantly different and independent from those collected from fish or the environment and enriched in members of the genus Aeromonas ; in particular, the pathogenic species Aeromonas veronii and A. caviae . The strong and specific association between Aeromonas and this monogenean suggests that G. cichlidarum might in effect be both a pathogen itself and a vector for a secondary pathogen – a hypothesis yet to be tested. • Electron microscopy shows bacteria associated with Gyrodactylus cichlidarum. • Metagenomics show that Aeromonas spp. are associated with this parasite. • MALDI-TOF MS identified bacterial species known to be fish pathogens. • This parasite-bacterial association may explain frequent co-infections in aquaculture. • Gyrodactylus cichlidarum may itself be a pathogen and a vector for bacterial pathogens.