Anthelmintic efficacy and sublethal effects of trichlorfon in pirarucu (Arapaima gigas) naturally infected with Dawestrema cycloancistrium

Trichlorfon is an organophosphate commonly used to control fish ectoparasites due to its cholinesterase-inhibiting action. Due to recurring issues with monogeneans in pirarucu, this active ingredient has been frequently used in this aquaculture system. The present study aimed to simultaneously evaluate the antiparasitic efficacy and physiological effects of trichlorfon in A. gigas juvenile naturally infected with monogenean Dawestrema cycloancistrium. In the in vitro assay, parasitized gill arches were exposed to trichlorfon (0.1 mg/L - 10 g/L) under controlled conditions, with parasite motility analyzed every 30 min. A 100 % mortality rate was observed after 30 min at 10 g/L and after 3 h at 10 mg/L; concentrations ≤ 1 mg/L caused only a reduction in parasite motility. In the in vivo trial, 96 juveniles were separated into four groups: control, 10 g/L for 30 min, 10 mg/L for 3 h and 10 mg/L for 96 h. The 10 mg/L for 96 h treatment showed the highest efficacy (96.9 %), significantly surpassing the others (49.7 % and 42.3 %; p < 0.05). However, this prolonged exposure induced hypochromic anemia, leukopenia, reduction in ALT and AST levels, and hyperglycemia. It is concluded that trichlorfon short-bath protocols, such as 10 g/L for 30 min or 10 mg/L for 3 h did not cause significant physiological alterations but resulted in only partial efficacy (∼50 %). Prolonged exposures at 10 mg/L for 96 h should not be recommended for A. gigas due to their hematotoxic and hepatotoxic effects.