Effects of clioquinol on the scuticociliatosis-causing protozoan Miamiensis avidus in olive flounder Paralichthys olivaceus

Scuticociliatosis is a devastating and intractable protozoal disease in olive flounder, leading to a significant loss throughout the year. This study aimed to investigate a systemically effective antiscuticociliatosis agent for olive flounder for better absorption into the infected internal organs. The in vitro and in vivo antiscuticociliatosis effects of clioquinol (CQ) were examined after screening 30 biocidal agents against the highly pathogenic scuticociliate Miamiensis avidus. CQ was the most potent in vitro drug of those tested against cultured M. avidus. CQ was the least toxic in healthy olive flounder among the drugs that exhibit high potencies. In olive flounder, a single intramuscular injection of 40 mg/kg CQ significantly reduced mortality caused by artificial infection with M. avidus, and 10–20 mg/kg CQ increased fish survival times. CQ was also effective in naturally infected scuticociliatosis. Ciliate cell numbers were lower when CQ was injected in most organs, including the brain. CQ was well absorbed by the internal organs after intramuscular injection. This study suggests that CQ can be considered as a potential antiscuticociliatosis agent for systemic administration in olive flounder.