ARS | Publication request: Parasitism by protozoan Ichthyophthirius enhanced invasion of Aeromonas hydrophila in channel catfish

Submitted to: Aquaculture America Conference
Publication Type: Abstract Only
Publication Acceptance Date: November 10, 2011
Publication Date: February 29, 2012
Citation: Xu, D., Wei Pridgeon, Y., Klesius, P.H., Shoemaker, C.A. 2012. Parasitism by protozoan Ichthyophthirius enhanced invasion of Aeromonas hydrophila in channel catfish. Aquaculture America 2012. p. 538.

Technical Abstract: In aquaculture production mortality resulting from a single pathogen is rare. More likely, multiple disease agents are present and responsible for disease losses. The ciliated protozoan Ichthyophthirius multifiliis is a common parasite of freshwater fish and frequently causes mass kills of cultured fish leading to high economic losses. Aeromonas hydrophila affects a variety of fish species and causes outbreaks in fish farms with high mortality. Currently there is no information available for the effect of coinfection by Ich and A. hydrophila on bacterial load and survival in channel catfish. The objectives of this study were to 1) determine whether A. hydrophila increased fish mortality in Ich-parasitized channel catfish; and 2) compare the bacterial quantity in different tissues between non-parasitized and Ich-parasitized catfish by real-time polymerase chain reaction (qPCR). Fish were distributed into 20 tanks with 10 fish/tank and received the following treatments: 1) infected by Ich and challenged with A. hydrophila; 2) infected by Ich only; 3) challenged only with A hydrophila; and 4) tryptic soy broth immersion only. For fish challenged with A. hydrophila, fish were exposed to A. hydrophila with aeration for one hour. After challenge, the fish were moved to a 57-l aquarium with flowing water at 0.5 l/min with aeration. The mortality of fish was recorded and dead fish were examined for parasite and A. hydrophila infection twice daily for 2 wk. Two tanks of fish in treatment 1 and treatment 3 were sampled to quantify A. hydrophila in different tissues with qPCR. The skin, gill, liver, kidney, and spleen from two fish in each tank were sampled using aseptic technique at hour 5, day 1, day 2, and day 7 post challenges with A. hydrophila after fish were euthanized with 300 mg/l MS-222. The results demonstrated that the Ich-parasitized catfish showed significantly higher mortality (80%) when exposed to A. hydrophila by immersion than non-parasitized fish (22%). Low mortality was observed in catfish exposed to Ich alone (35%) or A. hydrophila alone (22%). A. hydrophila in fish tissues were quantified by qPCR using a pair of gene-specific primers and reported as genome equivalents per mg of tissue (GEs/mg). Skin, gill, kidney, liver and spleen in Ich-parasitized fish showed significantly higher load of A. hydrophila (9,400 – 188,300 GEs/mg) than non-parasitized fish (4,700 – 42,100 GEs/mg) after exposure to A. hydrophila. This study provides evidence that Ich infection enhances A. hydrophila invasion and results in high fish mortality. The study results highlight the importance of co-infections to the aquaculture industry and will help fish farmers and health managers better define the impact of parasites on fish bacterial diseases.