Prevalence and Treatment of Streptococcus Species in Oreochromis niloticus

Streptococcus agalactiae is a zoonotic pathogen affecting both humans and animals, resulting in significant global economic losses. This study evaluated the impact of diets supplemented with oregano oil on infected Nile tilapia. One hundred Nile Tilapia fish (Oreochromis niloticus) collected from various fish markets were examined using standard phenotypic methods. Streptococcus agalactiae was identified in 28% of samples and confirmed through PCR, producing a 952 bp product specific to the dlt-s gene. The isolates exhibited significant resistance to commonly utilized antimicrobial agents, with the highest resistance observed for amoxicillin at 25/26 (96.1%), followed by sulfa-methoxazole/trimethoprim at 24/26 (92.3%), doxycycline at 19/26 (73%), gentamicin at 17/26 (65.3%), streptomycin at 15/26 (57.6%), and imipenem at 12/26 (46.1%). Reduced antimicrobial resistance was seen against erythromycin (9/26, 34.6%) and ciprofloxacin (4/26, 15.3%). An experimental infection study was performed on 150 obviously healthy Nile tilapia, which were categorized into five groups: G1 (healthy control), G2 (infected, untreated control), G3 (infected, treated with erythromycin), G4 (infected, treated with origanum oil), and G5 (pretreated with origanum oil, subsequently infected and treated with erythromycin). After 4 and 10 days the untreated infected group (G2) exhibited significant decreases in RBC count, hemoglobin, packed cell volume, and lymphocytes, accompanied by a pronounced increase in total leukocyte count and heterophils. In conjunction with elevated serum ALT, AST, creatinine, and glucose levels, there was a significant reduction in GSH-Px activity. Histological examination revealed vascular congestion, edema, and degenerative alterations in the gills, liver, kidneys, and spleen of infected fish. Treatment with erythromycin (G3), origanum oil (G4), or a combination of pretreatment with origanum oil followed by erythromycin (G5) led to significant improvements in blood parameters, oxidative stress markers, immune responses, and histopathological alterations. Increased expression of antioxidant genes (SOD, CAT) and metabolic gene expression (Tfam) was observed in groups administered oregano oil and antibiotics, accompanied by a reduction in the pro-inflammatory cytokine IL-6. This study suggests that incorporating oregano essential oil into the diet by dose 2g/kg bw enhances intestinal antioxidant capacity, strengthens the immune response, and improves resistance to Streptococcus agalactiae.