Parasite Ich Could Transmit Edwardsiella Ictaluri Into Channel Catfish

There is limited information on whether parasites act as vectors to transmit bacteria in fish.  In this study, we used parasite Ichthyophthirius multifiliis and fluorescent bacterium Edwardsiella ictaluri as a model to study the interaction between parasite, bacterium and fish. The theronts after exposure to E. ictaluri showed significantly higher fluorescing bacteria (23-39 %) than control theronts (~6 %).  Theronts exposed to E. ictaluri at 4 × 10⁷ CFU mL^-1 showed a higher percentage (~60 %) of fluorescent theronts compared to those (42 %) exposed to 4 × 10³ CFU mL^-1 at 4 h. All tomonts (100 %) carried the bacterium after exposure to E. ictaluri.  E. ictaluri survived and replicated during tomont division.  Confocal microscopy demonstrated E. ictaluri was associated with the tomont surface.  Among theronts released from tomonts exposed to E. ictaluri, 31 – 66 % was observed with attached E. ictaluri.  Sixty percent of fish exposed to theronts treated with 5×10⁷ E. ictaluri mL^-1 were positive for E. ictaluri at 4 h as determined by qPCR or fluorescent microscopy.  Fluorescent E. ictaluri were observed on trophonts in skin and gill wet mounts of dead fish.  This study demonstrated that Ich could vector E. ictaluri to channel catfish.  The study results will help fish farmers and health managers better define the impact of parasites on bacterial diseases in fish. The vectoring ability of parasites is particularly important at fish farms because the introduction of parasites either from wild fish or from other farms could concomitantly involve the introduction and/or transmission of bacterial diseases.