Record of the occurrence of Trichodina spp. and monogeneans associated with Serratia spp. bacteriosis in tambaqui (Colossoma macropomum)

This study reports an outbreak of circular lesions on the dorsal region of fattening tambaqui (Colossoma macropomum) in June 2025 in Alvorada D'Oeste, Rondônia. Approximately 180 fish were affected, showing a history of reduced feed consumption two weeks prior to the event. Water analysis revealed mostly adequate physicochemical parameters (neutral pH and low ammonia), however, alkalinity and hardness were below ideal in some tanks. This low buffering capacity caused stress in the fish, increasing their susceptibility to disease. Physical examination did not detect parasites macroscopically, with the dorsal lesion being the only evident clinical sign. Laboratory investigations identified a multifactorial etiology for the outbreak. The bacterium Serratia spp. was detected in the lesions themselves. In addition, microscopy of skin mucus revealed an extremely high load of the protozoan Trichodina spp., as well as the presence of the cyanobacterium Oscillatoria spp. (indicative of excess organic matter). In the gills, Trichodina spp. and the parasite Monogenea were found, in smaller numbers. It was concluded that the suboptimal water parameters stressed the fish. This stress, combined with the excess organic matter, triggered a population explosion of Trichodina spp. and monogeneans. These parasites damaged the skin and gills, creating entry points for the bacterium Serratia spp., which established the bacterial infection responsible for the observed dorsal lesions.