Control of Salmincola californiensis (Copepoda: Lernaeapodidae) in rainbow trout, Oncorhynchus mykiss (Walbaum): a clinical and histopathological study.

A stock of rainbow trout, Oncorhynchus mykiss, held at an experimental facility, was found to be heavily infested with the lernmaeapodid copepod Salmincola californiensis. The efficacy and effects of treatment were compared with ivermectin or manual removal of parasites as a means of control of S. californiensis. One group of fish was orally intubated with 0.2 mg ivermectin active ingredient kg-1 fish. A second treatment was administered after a further 14 days. In a second group of fish, parasites were manually removed from the gills using forceps. These fish were sampled for up to 21 days post-first removal of parasites. In the ivermectin-treated fish adult parasites became inactive and changed colour within 18 h of the initial treatment. Copepods began to disappear by day 3 post-treatment and by day 31 almost all embedded female parasites had disappeared. Gills were clinically normal apart from cavitation deformity resulting from parasite attachment. Post-ivermectin treatment, there was an increase in the number of eosinophilic granular cells surrounding the bulla of attached S. californiensis, but from day 31 post-treatment these were replaced by macrophages and epithelioid cells to form a necrotic focus. In manually picked fish there was extensive haemorrhage in the interlamellar spaces as a result of parasite removal. At sites of parasite removal tissue necrosis was minimal and healing was rapid. At the end of the sampling period the structure of the gill was improved. The use of oral dosage with ivermectin is an effective treatment for S. californiensis and could be particularly beneficial for use with endangered salmon broodstocks infested with the parasite.