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Title: Two in vitro methods for screening potential parasiticides against Ichthyophthirius multifiliis using Tetrahymena thermophila

Author
item Xu, Dehai
item ZHANG, QI-ZHONG - Guangzhou University
item Zhang, Dunhua

Submitted to: Journal of Fish Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2015
Publication Date: 4/7/2015
Publication URL: http://handle.nal.usda.gov/10113/62185
Citation: Xu, D., Zhang, Q., Zhang, D. 2015. Two in vitro methods for screening potential parasiticides against Ichthyophthirius multifiliis using Tetrahymena thermophila. Journal of Fish Diseases. 39:285-294.

Interpretive Summary: Ichthyophthirius multifiliis, commonly called “Ich”, is one severe ciliate parasite that infects most species of fresh water fish worldwide, leads to high fish mortality and causes heavy economic loss in the aquaculture. Currently parasiticides to control this parasite are limited and no effective parasiticide can be used to treat this parasite. Effective screen methods are urgently needed to find parasiticides that can kill parasite Ich. Ich is an obligated parasite, requires fish as host to survive and cannot be cultured in vitro. Tetrahymena thermophila is a free living protozoa in water similar to Ich and can be easily cultured in large quantities within a short period of time. This study evaluated whether T. thermophila can be used in plate count method and MTS assay to screen potentially effective parasiticides against Ich. Three Tetrahymena species, different concentrations of culture media, and antibiotics penicillin/streptomycin were evaluated to select the most efficacious combination for screening parasiticides against Ich. Then the in-vitro methods were used to compare lethal concentrations of copper sulfate, formalin, and malachite green on T. thermophila and parasite Ich. The tested parasiticides showed similar or higher lethal concentrations to T. thermophila than Ich. Results in this study demonstrated that the parasiticides that killed T. thermophila would kill parasite Ich and these in vitro methods using T. thermophila could be used to screen novel parasiticides against parasite Ich.

Technical Abstract: Ichthyophthirius multifiliis (Ich) is a severe ciliate parasite that infects most species of fresh water fish worldwide. The disease causes high fish mortality and heavy economic loss in aquaculture. Currently parasiticides to control this parasite are limited not only because of environmental safety concerns of their usage but also because of difficulties to culture the parasite under in vitro conditions for toxicity assays. This study used species of Tetrahymena, Ich-related and cultivable ciliate protozoa, to evaluate their responses to tested parasiticides. Trials were conducted to evaluate interactions among three species of Tetrahymena, different concentrations of culture medium and antibiotics to select the most efficacious combination for screening parasiticides. The results demonstrated that T. thermophila grown in 2% culture medium without penicillin/streptomycin were sensitive for screening potential parasiticides against Ich in vitro and this procedure was used throughout the study. Plate counting and MTS assays were used to compare lethal concentrations or median lethal concentrations (LC50) of copper sulfate, formalin, and malachite green between T. thermophila and Ich theronts or between T. thermophila and Ich tomonts. T. thermophila showed similar or slightly higher lethal concentrations or LC50 for tested parasiticides compared to Ich theronts or tomonts. Since the parasiticides that killed T. thermophila would kill Ich theronts or tomonts, these in vitro methods using T. thermophila can be used to screen novel parasiticides against Ich.