Eighteen recently imported San Francisco piranha (Pygocentrus piraya) were found to have mobile cutaneous parasites. Fish were anesthetized with 75 ppm tricaine methanesulfonate (MS-222) (FINQUEL, Argent Chemical Laboratories, Inc., Redmond, WA, USA) and two to seven parasites per fish manually removed. To prevent conspecific aggression, fish were maintained at 15 ppm MS-222 in individual enclosures until simultaneously re-introduced. Parasites were identified as branchiuran fish lice, Dolops nana. Gill biopsies and skin scrapes of four randomly selected fish had abundant Dactylogyrus sp. trematodes. One fish had a single Braga sp. cymothoid isopod removed from under the operculum.
Treatment for immature D. nana and Dactylogyrus sp. was performed with 0.5 ppm trichlorfon (DyLox [Masoten], Bayer Environmental Science, Research Triangle Park, NC, USA) weekly for 8 weeks. A 25% water change was performed prior to treatment, and as necessary to maintain water quality. The day after the last treatment most fish developed incoordination and depression. A 50% water change was performed, aeration increased, and carbon filtration added. Affected fish were treated once or twice with atropine (1 mg/kg IM) and once with dexamethasone sodium phosphate (1 mg/kg IM); eight died and the rest improved. The next day fish were normal, a 50% water change was performed, clinical signs recurred, atropine and dexamethasone were repeated, and another fish died. Over the course of 48 hours the two 50% water changes and continuous carbon filtration should have removed the trichlorfon from the system, yet clinical signs recurred after the water change. Tank and replacement water parameters measured (pH, ammonia, nitrites, nitrates, copper, and chlorine) were within expected ranges.
Dead fish were examined grossly and one autolyzed fish was not further evaluated. Histopathology was performed on five and tissues from three were pooled for ancillary diagnostics. All were in good body condition with moderate to abundant coelomic adipose. Gross changes included erythematous and congested gill filaments partially covered by excessive mucoid secretions. No parasites were present. Histologically, there was mild to moderate acute branchitis and mild blunting and swelling of secondary lamellae. There was mild diffuse hepatic accumulation of lipid vacuoles and hydropic degeneration. Hepatic organophosphate levels were undetectable.
It could not be determined if the morbidity and mortality were related to a water parameter abnormality or treatment complication, but surviving fish were normal and parasite free.
The authors thank the Prospect Park Zoo and New York Aquarium staffs for their care and management of these fish, Dr. Tim Georoff for assistance with the procedure, and Nina Palmer of the Queens Zoo for technical support.