Background and Aim: Streptococcosis is a common bacterial disease in red tilapia, in which Enterococcus faecalis infection has not been widely reported. This study aimed to evaluate the efficacy of pellets that contain chicken E. faecalis-induced immunoglobulin Y (IgY) to treat and prevent streptococcosis in red tilapia.

Materials and Methods: We conducted a 28-day study for immunoprophylaxis and immunotherapy, each using four groups with two replications: Healthy control fish (KS), non-IgY pellets (PA and TA), pellets with 25% egg yolk containing E. faecalis-induced IgY (PB and TB), and pellets with 50% egg yolk containing E. faecalis-induced IgY(PC and TC). Indirect enzyme-linked immunosorbent assay was performed on prototype pellets produced with an IgY suspension at 1.63 mg/mL as the standard optical density curve. For the immunoprophylaxis study, pellets of 3% of the average body weight of the experimental fish (0.50 g per fish per day) were given daily until day 14 before the challenge test with E. faecalis (2.1 × 10⁹ Colony-forming unit/mL peroral) on day 15. The data from the observation period on days 15–28 were analyzed. For the immunotherapy study, pellets of 3% of the average body weight (0.50 g per fish per day) were given daily for 21 days (days 8–28) 7 day spost-infection. The data from the immunotherapy study were collected during the observation period on days 8–28. Statistical analysis was performed on non-specific immune variables: Total leukocytes, monocytes, lymphocytes, neutrophils, phagocytic activity, and macrophage capacity; and the semi-quantitative distribution of melanomacrophage centers (MMCs) in the lymphoid organs, such as spleen and liver. Photomacrographic data were analyzed descriptively and qualitatively by comparing the healing process and clinical signs found between experiments in the immunotherapy study.

Results: The pellet with 50% egg yolk with an IgY at 2.43 mg/g pellet, 3% of body weight once daily, was the best formula on experimental fish. The administration of this formulation can also increase non-specific immunity and the distribution of MMCs in the spleen and liver with a survival rate of 55% for 14 days of challenge period in the immunoprophylaxis study and 70% for 21 days of therapy period in the immunotherapy study.

Conclusion: Immunoglobulin Y can be a prophylactic and therapeutic agent against streptococcal infections caused E. faecalis in red tilapia with an optimum dosage of 2.43 mg/g pellet.

Keywords: Enterococcus faecalis, immunoglobulin Y, immunoprophylaxis, immunotherapy, red tilapia, streptococcal infection.