Background: Parasitic and bacterial co-infections have been associated with increasing fish mortalities and severe economic losses in aquaculture through the past three decades. The aim of this study was to evaluate the oxidative stress, histopathology, and immune gene expression profile of gilthead sea bream (Sparus aurata) co-infected with Ergasilus sieboldi and Vibrio alginolyticus. Results: Vibrio alginolyticus and Ergasilus sieboldi were identified using 16 S rRNA and 28 S rRNA sequencing, respectively. The collagenase virulence gene was found in all Vibrio alginolyticus isolates, and the multiple antimicrobial resistance index ranged from 0.286 to 0.857. Oxidant-antioxidant parameters in the gills, skin, and muscles of naturally infected fish revealed increased lipid peroxidation levels and a decrease in catalase and glutathione antioxidant activities. Moreover, naturally co-infected gilthead sea bream exhibited substantial up-regulation of il-1β, tnf-α, and cyp1a1. Ergasilus sieboldi encircled gill lamellae with its second antennae, exhibited severe gill architectural deformation with extensive eosinophilic granular cell infiltration. Vibrio alginolyticus infection caused skin and muscle necrosis in gilthead sea bream. Conclusion: This study described some details about the gill, skin and muscle tissue defense mechanisms of gilthead sea bream against Ergasilus sieboldi and Vibrio alginolyticus co-infections. The prevalence of co-infections was 100%, and no resistant fish were detected. These co-infections imbalance the health status of the fish by hampering the oxidant-antioxidant mechanisms and proinflammatory/inflammatory immune genes to a more detrimental side. Our results suggest that simultaneous screening for bacterial and parasitic pathogens should be considered.

This study explores the use of silver nanoparticles (Ag NPs) and zinc oxide nanoparticles (ZnO NPs), either singly or in combination, for the nanoremediation of aquaculture wastewater. Aquaculture wastewater was treated with varying doses of Ag NPs and ZnO NPs across the following six groups: Group 1 (0.05 mg Ag NPs/L), Group 2 (1 mg ZnO NPs/L), Group 3 (0.05 mg Ag NPs/L + 1 mg ZnO NPs/L), Group 4 (0.025 Ag NPs/L + 0.5 mg ZnO NPs/L), Group 5 (0.1 mg Ag NPs/L + 2 mg ZnO NPs/L), and a control group. Water quality, microbial loads and nanomaterial concentrations were assessed over ten days. Transmission electron microscopy (TEM) showed average particle sizes of 102.5 nm for Ag NPs and 110.27 nm for ZnO NPs. The removal efficiencies of NH₄-N were over 98% across treatment groups. In addition, COD removal efficiencies were 33.33%, 68.82%, 49.59%, 61.49%, and 37.65%. The log-reductions in aerobic plate counts for the nanoparticle-treated wastewater were 1.191, 1.947, 1.133, 1.071, and 0.087, compared to a reduction of 0.911 in untreated wastewater. Silver concentrations ranged from 0.0079 to 0.0192 mg/L, while zinc concentrations ranged from 0.3040 to 0.9740 mg/L, indicating that ZnO-NPs represent a sustainable treatment method for aquaculture wastewater.

In farmed saltwater fish, vaccination is considered the first barrier against pathogenic agents. In Egypt, the vaccine application can diminish the dilemma of summer mass kills of farmed marine fish. The efficacy of booster vaccination with a bivalent vaccine against vibriosis was studied in European seabass. Fish were vaccinated by direct immersion (DI) in water containing bivalent Vibrio alginolyticus and Vibrio parahemolyticus whole cells killed vaccine. Immersion vaccination is more applicable under semi‐intensive fish culture, but less potent than injection. Accordingly, to increase antigen uptake and conferred protection, vaccinated fish randomly received booster vaccination via direct immersion (DI imm.) or intraperitoneal injection (i.p.) at 21 days post‐prime vaccination (21 dpv). Vaccinated and unvaccinated fish were monitored for 3 months and challenged with V. alginolyticus and/or V. parahemolyticus at 42 dpv. Specific and non‐specific immune responses were evaluated in blood samples. At 52 dpv (10 days post‐challenge), the relative immunoglobulin T (IgT) expression was assessed in the gills, hindgut, head kidney and spleen. Notably, imm. + imm. and imm. + i.p. vaccination yielded significantly higher specific antibody titre at 70 dpv, whereas the former was associated with significantly higher IgT expression in the gills and hindgut (p < .05) than the latter. High IgT levels in mucosal organs (gills and hindgut) might be the first indicator of the involvement of IgT in mucosal immune responses. Relative per cent survival among booster‐vaccinated fish ranged from 70% to 90%. The results support the use of this vaccination protocol as a highly effective tool to protect seabass from vibriosis and to improve fish health and environment. Booster doses are strongly recommended under farming environments.

Background: Staphylococcus epidermidis represents an emerging zoonotic threat impacting aquatic ecosystems, livestock, and human health. This study investigated the causative agent behind summer mortality episodes affecting cultured Oreochromis spp. (180 ± 20 g) in Egyptian fish farms, where diseased specimens exhibiting characteristic ulcerative dermatopathy were collected from Ismailia governorate. Results: Comprehensive diagnostics excluded TiLV and NNV, while bacteriological analysis identified Gram-positive cocci producing distinctive white hemolysis-negative colonies on blood agar and red colonies on mannitol salt agar. Polyphasic characterization confirmed the isolates as S. epidermidis (16 S rRNA GenBank MN153038), marking the first genomic record of this pathogen in tilapia. Antimicrobial profiling revealed alarming multidrug resistance (54.5% of tested agents), including resistance to β-lactams (oxacillin, ampicillin, and cefoxitin) that suggests possible methicillin-resistant (MRSE) phenotypes, despite retained vancomycin susceptibility (MIC = 4 µg/mL). Controlled challenge trials demonstrated dose- and route-dependent virulence, with scale removal during immersion exposure precipitated 95% mortality in fingerlings (2.3 ± 0.75 g) versus 55% in intact fish. In contrast, intraperitoneal injection caused 40–50% mortality in adults/juveniles. Notably, silver nanoparticles (AgNPs) exhibited size-dependent antimicrobial activity: 10-nm AgNPs showed superior efficacy (MIC = 1.25 µg/mL; MBC = 2.5 µg/mL) compared to 100-nm AgNPs (MIC = 10 µg/mL) and zinc oxide nanoparticles (MIC = 125 µg/mL). The enhanced activity of smaller AgNPs is attributed to their greater surface area and improved biofilm penetration. Conclusion: These results highlight S. epidermidis as an emerging threat in tilapia aquaculture, particularly given its multidrug resistance. The demonstrated efficacy of AgNPs, especially at smaller particle sizes, offers a promising alternative for controlling such resistant infections in aquaculture settings.

Aeromonas and Shewanella species are opportunistic pathogens and often cause serious diseases in farmed fish. In the present study, persistent natural mortality was observed in El Manzala intensive marine fish farm stocked with European seabass (Dicentrarchus labrax), Damietta, Egypt during the course of winterkills from early December to mid-January. The temperature of the well and fish ponds during episodic die-offs ranged from 13 to 17°C. The diseased European seabass exhibited ulcer, fin hemorrhage, tail fin rot, red and swollen inflamed vents, abdominal distension and hemorrhage, with approximately 30% cumulative mortality. Parasitological examination of diseased fish revealed only mild infestation with Trichodina species in the gills. Furthermore, bacteriological examination of diseased fish resulted in the identification of Shewanella putrefaciens and Aeromonas veronii isolates. These clinical isolates were presumptively identified on the basis of phenotypic characteristics and API 20NE biochemical profiles, followed by 16S rRNA gene sequencing and BLAST alignments. Additionally, the analysis of the neighbor-joining phylogenetic tree supported the above-mentioned identification. RNA polymerase B subunit (rpoB) was considered a proper tight taxonomic tool to differentiate between Aeromonas species. Therefore, Aeromonas isolates were confirmed as Aeromonas veronii based on rpoB housekeeping gene. In addition, antimicrobial susceptibility testing revealed that Shewanella putrefaciens isolates were resistant to oxytetracycline, doxycycline, trimethoprim/sulfamethoxazole and ampicillin, while Aeromonas veronii were susceptible to oxytetracycline, doxycycline and trimethoprim/ sulfamethoxazole. Both isolates were susceptible to florfenicol which was chosen to control these bacterial co-infections in farmed European seabass. This is the first report of Shewanella putrefaciens and Aeromonas veronii natural co-infections in intensively stocked European seabass.