Abstract
Background: Streptococcosis, as a bacterial disease with broad tropism in fish and one of the causes of septicemia. Enterococcus faecalis is one of the causative agents of streptococcosis that can be isolated in tilapia.
Aim: This study was undertaken to complete the reporting gap on the pathogenicity profile and clinical symptoms of E. faecalis bacterial infection in red tilapia (Oreochromis hybrid). The study is expected to be enriching information regarding recognizable clinical signs in the field that can lead to the diagnosis of streptococcosis caused by E. faecalis, especially in the Indonesian aquaculture environment.
Methods: The method used in this artificial infection study using red tilapia, which were divided into two types of route groups infection, namely intraperitoneal (IP) and peroral (PO) with bacterial concentrations given for each route of infection was 2.1 × 108 CFU ml-1; 2.1 × 107 CFU ml-1; and 2.1 × 106 CFU ml-1. One group was given BHI broth media sterile as a non-infectious control. Clinical symptoms, changes in swimming habits and consuming feed, external and internal organ lesion, and leucocytes profile changes were observed during the observation period along 14 days to evaluate the infectious effect of each treated fish group. The lethal dose (LD50) was estimated with the Spearman-Kärber method. The evaluation of leukocyte profile was performed to find leukocytosis as the clinical sign of infection.
Results: The results showed variations in clinical symptoms inflicted on fish through death or the moribund stage. The highest mortality occurred in the treatment group of 2.1 × 108 CFU ml-1 with the PO route. The bacterial concentration of 2.1 × 107 CFU ml-1 given either PO or IP can cause mild infection symptoms but did not cause mortality. The LD50 of PO and IP route was obtained at 1.99 × 108 and 0.79 × 108 CFU ml-1, respectively. The total leukocytes in the infected fish group increase significantly (p < 0.05) by 2-3 fold compared with the non-infection group. There was found the bacteria’s discovery on the blood smear examination from fresh dead fish or moribund fish in the treatment group of 2.1 × 108 CFU ml-1, both PO and IP.
Conclusion: E. faecalis with low pathogenicity can lead to septicemia which is characterized by a total increase in leukocytes, bacteria’s discovery on the blood smear examination, and variation clinical symptoms systemically in the treated fish.

Key words: Enterococcus faecalis, experimental infection, red tilapia, septicemia, streptococcosis