In this field study, two private Oreochromis niloticus fish farms in El Fayoum and El Sharkia governorates showed signs of septicaemia, with a mortality of 20%–25% between autumn 2018 and summer 2019. Postmortem examination of affected fish revealed loose scales, ulceration in the dorsal part of the head, petechial haemorrhages on the body surfaces, unilateral exophthalmia, and corneal opacity together with congested blood vessels of nearly all fleshy portions of the body fins. Internal examination of the viscera showed congestion of the blood vessels in the kidneys and brain, a dark enlarged spleen, and a liver with a marbled appearance. Fish samples (representing 10% of the fish showing clinical signs) were collected and examined for histopathological and bacteriological examination. Aerococcus viridans was isolated from El Fayoum and El Sharkia governorates at rates of 18.18% and 28.89%, respectively, throughout the year, whereas Enterococcus faecalis was isolated at rates of 81.82% and 71.11%, respectively; seasonal prevalence rate of infection were recorded. Antibiotic sensitivity testing showed that the two microorganisms were resistant to ampicillin, oxolinic acid, nalidixic acid, amikacin, and gentamicin, but they were sensitive to amoxicillin and nitrofurantoin. E. faecalis exhibited resistance to erythromycin and oxytetracycline, whereas A. viridans was sensitive to these agents. To study the pathogenicity of such microorganisms in this fish species, experimental infection and reisolation of both bacterial isolates in O. niloticus was performed to confirm the field isolation. The pathological lesions in different organs of the infected fish were fully recorded.

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