ARS | Publication request: Ichthyophthirius multifiliis as a potential vector of Edwardsiella ictaluri in channel catfish

Submitted to: FEMS Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 6, 2012
Publication Date: February 23, 2012
Repository URL: http://hdl.handle.net/10113/55521
Citation: Xu, D., Shoemaker, C.A., Klesius, P.H. 2012. Ichthyophthirius multifiliis as a potential vector of Edwardsiella ictaluri in channel catfish. FEMS Microbiology Letters. 329:160-167.

Interpretive Summary: The ability of parasites to enhance mortality caused by bacterial diseases in fish is presently receiving attention in aquaculture research. There is limited information on whether parasites act as vectors to transmit pathogenic bacteria in fish. In this trial, we used a protozoan parasite Ichthyophthirius multifiliis and fluorescent Edwardsiella ictaluri (a gram negative bacterium) as a model to study the interaction between the parasite, the bacteria and the fish host. We tested the hypothesis that Ich can vector E. ictaluri into channel catfish. Almost 60 % of theronts (infective parasite stage) carried E. ictaluri compared to control theronts (less than 6 %) 4 h post exposure to fluorescent E. ictaluri. All treated tomonts (reproductive parasite stage) (100 %) demonstrated fluorescent bacteria 2 – 8 h post exposure to E. ictaluri and no fluorescent bacteria were observed on control tomonts. When fish were exposed to theronts treated with E. ictaluri, 100%, 90% and 60% of fish showed E. ictaluri at 4 h, 1 and 2 d, respectively. This study demonstrated that Ich can vector E. ictaluri into channel catfish. Ich theronts and tomonts carried E. ictaluri after exposure to the bacterium. Tomonts exposed to E. ictaluri could pass E. ictaluri to infective theronts released from the tomonts and the theronts transmitted the bacterium 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.

Technical Abstract: There is limited information on whether parasites act as vectors to transmit pathogenic 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 host. The percentage (23-39 %) of theronts fluorescing after exposure to E. ictaluri was significantly (P < 0.05) higher than control theronts (~6 %) using flow cytometry. 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 to 2 d as determined by qPCR or fluorescent microscopy. Fluorescent E. ictaluri were observed on or near trophonts in skin and gill wet mounts of dead fish. This study demonstrated that Ich could vector E. ictaluri to channel catfish.