Monogenean infestation results in mortality and economic losses in farmed tambaqui.
Treatment with vegetal extract is an alternative to control fish diseases.
Annatto extract can be used to treat monogenean gill parasite in aquatic organisms.
100% efficacy of annatto extract against Anacanthorus spathulatus was found.
Hemoglobin, hematocrit, RBC, and WBC counts were higher in fish treated with annatto extract.