الفهرس 
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Abstract
In the last few years there were episodes of colossal mortalities in tilapia fish farms during summer season. This study was intended to investigate the possible microbial causes of such mortalities. A total of 250 moribund fish samples were collected from Kafr El-Sheikh, Sharqia and El Fayoum governorate. The most apparent clinical signs were hemorrhagic patches and darkness on skin, protruded vent, exophthalmia, and abdominal distension. The postmortem findings were enlarged, friable spleen and liver, abscesses, serosanguinous ascitic fluid and enlarged gallbladder. The molecular identification of Tilapia Lake Virus by PCR confirmed the absences of the virus in all examined samples collected from Kafr El-Sheikh governorate. A total of 48 pathogenic bacterial isolates were retrieved from the infected fish. Namely, Streptococcus agalactiae (14), Enterococcus faecalis (10), Lactococcus garvieae (10), Streptococcus dysgalactiae (9), Aeromonas hydrophila (3), and Vibrio cholera (2). The predominance of Streptococci infections points to its intense involvement in such mortalities. All isolates were confirmed by 16s rRNA gene sequencing and phylogenetic analysis. The molecular screening confirmed the existence of Hyl, cylE, scpB and camp genes in all S. agalactiae strains. E. faecalis isolates possessed Hyl (50%), Asa1 (40%), CylA (70%), GelE (60%), Esp (50%), EF3314 (50%) and Ace (60%). L. garvieae isolates possessed Hly1 (40%), Hly2 (60%), Hly3 (60%), NADH oxidase (70%) and Eno (50%). The isolates of S. dysgalactiae possessed Mig (33%), brpA (55%), Lmb (77%), scpB (33%) and Rip (44%). The histopathological examination revealed that S. agalactiae and L. garvieae cause meningitis in tilapia. Ultimately, isolates showed variable antibiotic resistance profile against the majority of tested antibiotics.