Abstract
This research was carried out to study the bacterial etiology of ulcerative syndrome in Nile tilapia (Oreochromis niloticus) and its associated environmental stressors. A total of 90 live and moribund Nile tilapia showing signs of ulcerative dermatitis were collected from various fish ponds in El-Sharkia Province, Egypt, during the summer seasons from June 2019 to August 2020. Collected fish were subjected to full clinical and postmortem investigation followed by bacteriological and molecular identification of the bacterial agents as well as histopathological examinations. Moreover, water samples from the fish sampling sites were collected and analyzed physiochemically and microbiologically. Furthermore, the pathogenicity of the isolated strains was assessed by challenging apparently healthy Nile tilapia to determine the LD50. Clinically, the examined fish exhibited ulcerative dermatitis as focal or fused large ulcers surrounded by red zones of hemorrhages at different body parts. Bacteriologically, Aeromonas veronii biovar sobria and A. caviae were isolated and identified. Moreover, 16S rRNA gene sequencing and phylogenetic analysis verified the identity of these species. Antibiogram profiling of the recovered Aeromonas sp. isolates showed complete resistance to ampicillin, with high susceptibility to gentamycin, trimethoprim/sulphamethoxazole, and erythromycin. Histopathological examination of the affected fish revealed epidermal necrosis with dermal edema and an influx of inflammatory cells into the dermis and hypodermis. The LD50 of A. veronii biovar sobria and A. caviae was estimated as 1.5 × 107 and 1.5 × 108 CFU/fish; respectively. The water temperature and ammonia concentrations recorded a significant rise (P ≤ 0.05) in July and August compared to June; however, the highest pH levels were recorded during August, followed by July. Also, the total bacterial load significantly increased (P ≤ 0.05). Finally, it was concluded that the improvement of water quality helps sustain the health of farmed fish.
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