ARS | Publication request: Enhanced susceptibility of channel catfish to the bacterium Edwardsiella ictaluri after parasitism by Ichthyophthirius multifiliis

Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 7, 2012
Publication Date: February 14, 2012
Repository URL: http://hdl.handle.net/10113/55522
Citation: Xu, D., Shoemaker, C.A., Martins, M., Wei Pridgeon, Y., Klesius, P.H. 2012. Enhanced susceptibility of channel catfish to the bacterium Edwardsiella ictaluri after parasitism by Ichthyophthirius multifiliis. Veterinary Microbiology. 158:216-219.

Interpretive Summary: Host–pathogen interactions are rarely one-on-one in aquaculture systems where fish may be concurrently infected by two or more pathogens. Bacterium Edwardsiella ictaluri and parasite Ichthyophthirius multifiliis (Ich) are two common pathogens of catfish which cause major losses to catfish aquaculture. There is no information available on whether Ich infection will affect the susceptibility of catfish to E. ictaluri. The objective of this study was to evaluate the susceptibility of channel catfish to the bacterium E. ictaluri and determine bacterial loads in different fish organs after parasitism by Ich. The results demonstrated that Ich-parasitized catfish showed higher mortality (88.3-91.7%) when exposed to E. ictaluri than non-parasitized fish (10%). The lymphocyte counts were lower in fish co-infected by Ich and E. ictaluri (100-1,100 cells /µl) than non-infected control fish (5,100-14,100 cells /µl). The bacterial loads in fish infected by 5,000 theronts / fish were 40-2000 fold higher than non-parasitized fish. The parasite infection enhanced the susceptibility of channel catfish to bacterial invasion and caused fish blood changes and high mortality. This work suggests that prevention of parasite infection in fish will not only reduce the direct damage caused by the parasite but also reduce fish mortality due to bacterial co-infection. The study results are important to the aquaculture industry and will help fish farmers and health managers better define the impact of parasites on bacterial diseases in fish.

Technical Abstract: Bacterium Edwardsiella ictaluri and parasite Ichthyophthirius multifiliis (Ich) are two common pathogens of cultured fish. There is no information available on whether Ich infection will affect the susceptibility of catfish to E. ictaluri. The objective of this study was to evaluate the susceptibility of channel catfish to the bacterium E. ictaluri and determine bacterial loads in different fish organs after parasitism by Ich. Hematological parameter changes following co-infection by Ich and E. ictaluri were also determined. Fish received the following treatments: 1) infected by I. multifiliis at 5,000 theronts/fish and exposed to E. ictaluri; 2) infected by I. multifiliis at 1,000 theronts/fish and exposed to E. ictaluri; 3) infected by I. multifiliis alone; 4) exposed to E. ictaluri alone; and 5) non-infected control. E. ictaluri in fish organs were quantified by quantitative real-time polymerase chain reaction and reported as genome equivalents per mg of tissue (GEs/mg). The results demonstrated that the Ich-parasitized catfish showed significantly (P<0.05) higher mortality (88.3-91.7%) when exposed to E. ictaluri than non-parasitized fish (10%). The lymphocyte counts were significantly lower in fish co-infected by Ich and E. ictaluri (100-1,100 cells /µl) than non-infected control fish (5,100-14,100 cells /µl). The bacterial loads in fish infected by 5,000 theronts / fish ranged from 6,497 to 163,898 GEs/mg which was between 40-2000 fold higher than non-parasitized fish (49-141 GEs/mg). The parasite infection enhanced the susceptibility of channel catfish to bacteria invasion. The co-infection by Ich and E. ictaluri caused fish hematological changes and high mortality.