الفهرس 
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Abstract
In recent years, fish mortality rates have increased suddenly due to mixed and multiple infections. The following study was designed to examine the co-infections between bacteria and parasites in some fish species from different aquatic environments in Egypt and the probability that parasites can transmit bacteria. The study shedded the light on the interaction of these pathogens and the stress induced by climatic change and deteriorating water quality parameters. The work was carried out first by conducting a survey of concurrent natural parasite-bacteria infections in fish was conducted in 2019 on 379 moribund and recently dead fresh- and marine water fishes of various body weights, collected seasonally from the River Nile at Giza, from Deeba triangle, Manzla lake and from Qaroun Lake in the Fayoum province. Fish were examined for parasites and bacteria and identified to the genus level. Clinostomum and/or Euclinostomum spp. as well as Pseudomonas were found in freshwater fish while Isopod, and Vibrio, Pseudomonas and Aeromonas were isolated in marine water fish. The prevalences of overall infestation and concurrent infections were recorded and discussed in relation to seasonality. Investigation of the phenotypic and genotypic characterization of Vibrio alginolyticus recovered from freshly dead or moribund fish and isopod was carried out to implicate isopod in the transmission of V. alginolyticus. Biochemical, antibiogram, species-specific PCR and phylogenetic analysis of 16S rRNA gene were done. In addition, physicochemical analysis of water and identification of isopod were performed. Results revealed deteriorated water quality, identical antimicrobial susceptibility profiles of V. alginolyticus strains from fish, isopods, and water. They were susceptible to erythromycin, imipenem, and trimethoprim/sulphamethoxazole, but resistant to ampicillin, lincomycin, colistin, and penicillin. In the species-specific PCR using the collagenase gene, all tested isolates showed the characteristic V. alginolyticus amplicon of 737 bp. The phylogenetic analysis of partial 16S rRNA gene sequence revealed that V. alginolyticus strains from fish and isopod showed 100% similarity with each other and 99.23% similarity with water strain, suggesting that the isopod was the likely source of infections. A similar study was attempted to investigate the correlation between Clinostomum infestation and transmission of Ps. Putida in Oreochromis niloticus and Tilapia zilli fish in River Nile at Bahr El-azam, Giza. Isolates from freshly dead or moribund fish, metacercarias and water samples were retrieved during spring and summer seasons. Isolates were subjected to bacteriological examination, biochemical tests, antibiogram, PCR, sequencing and phylogenetic tree analysis. Water samples were analyzed for physicochemical parameters using standard methods. Results indicated that water quality was deteriorated, Ps. Putida strains from fish, clinostomum and water samples showed the exact antimicrobial susceptibility profile where they were found to be highly resistant to ceftriaxone, ampicillin, ceftazidime, cefoxitin and cefazolin and ciprofloxacin. On the other hand, they were highly sensitive to Tobramycin, Amikacin, Imipenem, gentamicin, levofloxacin and norfloxacin. In PCR, targeting the 16s rRNA gene, all tested isolates showed the characteristic amplicon of 380 bp. Sequencing and phylogenetic analysis of Ps. Putida strains confirmed the identity of these isolates, indicating that the clinostomum was probably the source of infection.
In conclusion, the study showed that the magnitude of clinostomum infestations and the high prevalence of isopods were linked to deteriorated water quality combined with high levels of pollution. It is suggested that the parasites investigated in this study could be a potential vector in the transmission of bacteria in fish.