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


Correspondence: De-Hai Xu, U.S. Department of Agriculture, Agricultural Research Service, Aquatic Animal Health Research Unit, 990 Wire Road, Auburn, AL 36832, USA. Tel.: +1 334 887 3741; fax: +1 334 887 2983; e-mail: dehai.xu@ars.usda.gov


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 × 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. 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 × 107E. 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.