Ichthyophthirius multifiliis as a potential vector of Edwardsiella ictaluri in channel catfish

There is limited information on whether parasites act as vectors to transmit bacteria in fish. In this trial, we used Ichthyophthirius multifiliis and fluorescent Edwardsiella ictaluri as a model to study the interaction between parasite, bacterium, and fish. The percentage (23-39%) of theronts fluorescing after exposure to E.?ictaluri was significantly higher than control theronts (~?6%) using flow cytometry. Theronts exposed to E.?ictaluri at 4?×?10(7) CFU?mL(-1) showed a higher percentage (~?60%) of fluorescent theronts compared to those (42%) exposed to 4?×?10(3) CFU?mL(-1) at 4?h. All tomonts (100%) carried the bacterium after exposure to E.?ictaluri. Edwardsiella ictaluri survived and replicated during tomont division. Confocal microscopy demonstrated that E.?ictaluri was associated with the tomont surface. Among theronts released from tomonts exposed to E.?ictaluri, 31-66% were observed with attached E.?ictaluri. Sixty percent of fish exposed to theronts treated with 5?×?10(7) 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.