In Vitro Anthelmintic Effect of Selected Agents Against Microcotyle sebastis and Microcotyle caudata, the Gill Monogenean Parasites of Rockfish, Sebastes spp.

The in vitro efficacy of various anthelminthic agents was investigated against adults and eggs of the Microcotyle sebastis and M. caudata. Praziquantel (PZQ), ivermectin (IVM), trichlorfon (TCF), fenbendazole (FNBZ), febantel (FBT), flubendazole (FLBZ), thiabendazole (TBZ), formalin, hydrogen peroxide and sodium hypochlorite (NaOCl) were selected as test agents. The hatching rate of M. sebastis and M. caudata eggs was significantly reduced by formalin (200, 300 mg/L), hydrogen peroxide (50-200 mg/L) and NaOCl (75-200 mg/L) treatment. However, seven anthelminthic agents, including PZQ, did not effectively inhibit the hatching of eggs. 50-200 mg/L NaOCl treatment significantly killed M. sebastis and M. caudata adults. In particular, IVM 200 mg/L for 30 min treatment showed 100% killing activity against M. sebastis adults and 80% killing activity against M. caudata adults. However, PZQ, TCF, FNBZ, FBT, FLBZ and TBZ treatments showed mortalities of less than 33%. When anthelmintics were directly applied to adult worms attached to the gills in vitro, PZQ 200 mg/L for 30 min treatment induced detachment of M. sebastis (99.2%) and M. caudata (100%) from the gills. However, most of the worms recovered after the treatment. IVM 200 mg/L induced detachment of more than 83% of M. sebastis and M. caudata on the gills and, consequently, death of the detached worms. All the other agents induced negligible effects on the adult worms attached to the gills. This result suggests that NaOCl and IVM bath treatments effectively control M. sebastis and M. caudata in sebastid species aquaculture, and formalin, hydrogen peroxide and NaOCl are effective against eggs. An in vivo study will be necessary to evaluate their anthelmintic effects on aquaculture farms.