Occurrence of Argulus sp. Infestation with Dual Bacterial Co-infection Caused by Aeromonas hydrophila and Enterococcus faecalis in Oscar Fish (Astronotus ocellatus): A Case Report.

A case of mild Argulus sp. infestation was reported in 30 Oscar fish (Astronotus ocellatus). Infestation was managed with diflubenzuron (0.01 mg/L of water). Following the second treatment, six fish developed panexophthalmia, with three mortalities-two showing ascites. A single fish exhibiting clinical signs of ascites was successfully sampled, from which ascitic fluid, whole blood, and visceral organs were collected for subsequent laboratory analyses. Microbiological analysis of ascitic fluid identified Aeromonas hydrophila and Enterococcus faecalis, where A. hydrophila was sensitive to tetracycline and enrofloxacin and E. faecalis was sensitive to tetracycline but intermediate to enrofloxacin. Haematology revealed microcytic normochromic anemia, leukocytosis dominated by neutrophils/heterophils and monocytes, and lymphopenia. Histopathology showed severe secondary lamellar fusion in gills, granulomatous inflammation in multiple organs, hepatic degeneration, and mild other parasitic co-infection (Monogenean and Trichodina sp.). Elevated un-ionized ammonia (NH₃) was the only abnormal water quality parameter. Surviving symptomatic fish received intramuscular enrofloxacin (10 mg/kg body weight, 7 days) and topical water bandage application, resulting in full recovery in two cases and clinical improvement in the third. This case underscores that even mild Argulus sp. infestations can facilitate opportunistic bacterial infections under environmental stress, leading to systemic pathology. Integrated management combining parasite control, targeted antibiotic therapy, water quality improvement, and topical care can yield favorable outcomes in ornamental fish health.