Efficacy of hydrogen peroxide to control parasitic infestations on hatchery-reared fish

The efficacy of hydrogen peroxide to control external parasitic infestation s on juvenile (10-33-g) rainbow trout Oncorhynchus mykiss was evaluated in three clinical field trials. Fish were exposed to hydrogen peroxide concent rations ranging from 0 to 560 mg/L for 30 min once every other day for a to tal of three treatments. Pre- and posttreatment skin scrapes and gill wet m ounts of test fish were microscopically examined to identify and enumerate external parasites. Infestation severity was classified as nonexistent (0 o rganisms), low (1-10 organisms), moderate (11-20 organisms), or high (great er than or equal to 21 organisms). In trial 1, pretreatment skin examinatio ns revealed a severe infestation of the protozoan Ambiphrya on all fish exa mined. Posttreatment skin examinations conducted within 24 h of the last tr eatment indicated that all hydrogen peroxide treatments eliminated Ambiphry a, whereas control fish remained severely infested with the protozoan. In t rial 2, pretreatment examinations of skin and gill samples indicated a high infestation of the trematode Gyrodactylus (skin) and the protozoan Trichod ina (gills) on all fish. Posttreatment examinations conducted within 24 h o f the last treatment indicated that Gyrodactylus was eliminated from the sk in of all treated fish; however, the high infestation of Trichodina remaine d on the gills of the test fish. All control fish had high infestation leve ls of bath parasites. A high infestation of Ambiphrya was found on the skin of test fish before treatment (trial 3). Posttreatment examinations conduc ted 14 d after the last treatment revealed that 56% of the fish were parasi te free, whereas the remaining test fish had low infestation levels. Contro l fish remained severely infested with the parasite. Based on the efficacy data, all hydrogen peroxide treatment regimens were efficacious in the cont rol of Ambiphrya and Gyrodactylus.