Photobacteriosis in some wild and cultured freshwater fishes in Egypt

Two hundred and seventy wild and cultured fishes (Mugil cephalus, Mugil capito and Nile tilapia) were collected from Alexandria, and investigated for the isolation of Photobacterium. The recovered bacteria were studied for the virulence, pathogenicity and antimicrobial sensitivity. The pathogenesis of the most virulent isolate was done experimentally through pathological investigations. Monitoring of the water quality was also carried out. Three isolates (ph1, ph2 and ph3) of Photobacterium damselae subsp. damselae were obtained from Mugil cephalus, Mugil capito and Nile tilapia, respectively. The first two recovered isolates caused 20% mortality among Oreochromis niloticus and Cyprinus carpio, while the third one (ph3) caused 40% and 30% mortalities among the two fish species, respectively. The infection by these bacteria was accompanied by increase in the unionized ammonia (0.08 mg/L), decrease in the dissolved oxygen (1.8 mg/L) and high pH value (9.8). The experimentally infected fishes showed skin darkening and hemorrhaging of the caudal fin and operculum. Internally, whitish pin-sized nodules were seen in the liver, spleen and kidneys. Histopathologically, the Photobacterium, during the acute phase of the disease, induced degenerative changes in the parenchymatous organs of the infected fishes with marked brain lesions and hematopoietic involvement, while in the chronic stage, granuloma formation and focal necrosis in the internal organs was evident. The three recorded isolates of the Photobacterium were sensitive to erythromycin, streptomycin and ampicillin, but totally resistant to chloramphenicol, oxytetracycline, oxalinic acid, kanamycin and ciprofloxacin. On the other hand, Ph1 isolate was also sensitive to furazolidone while Ph3 was sensitive to neomycin. We were able to isolate three isolates of Photobacterium damselae subspecies damselae, which varied in virulence and antimicrobial sensitivity, from the wild and cultured freshwater fishes at Alexandria, Egypt as a new recorded pathogen in this environment.

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