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Antimicrobial resistance poses a growing risk in aquaculture and fisheries due to antimicrobial overuse, contaminated feed, climate change, and antibiotic-laden water discharge, thereby development of resistant bacteria and resistance genes in aquatic ecosystems. This review outlines the main factors, resistance mechanisms, and consequences of antimicrobial resistance (AMR) in aquaculture, focusing on significant fish infections, related physiological changes, and implications for zoonosis and food safety. The bacterial pathogens most associated with antimicrobial resistance in fish and shellfish are Aeromonas spp., Edwardsiella spp., Yersinia ruckeri, Pseudomonas spp., Photobacterium damselae, Vibrio spp., Flavobacterium spp., and zoonotic indicators such as Escherichia coli. Antimicrobial resistance in aquatic systems has serious repercussions, including reduced treatment efficacy, efflux pumps, target alteration, enzymatic inactivation, and horizontal gene transfer. In addition to treatment failure, antibiotic overuse leads to microbial imbalance, immunological and metabolic disruptions in fish, environmental spread of resistance genes, and potential public health hazards. Mitigation strategies such as probiotics, phage therapy, phytobiotics, immunostimulants, immunization, and similar approaches may help reduce reliance on antibiotics in aquaculture. Efficient antimicrobial resistance control in aquaculture requires a One Health approach that integrates surveillance, regulation, and sustainable disease prevention. These findings highlight that AMR in aquaculture is a significant One Health concern requiring effective mitigation, which depends on prudent antimicrobial use, integrated surveillance, and the implementation of safe and sustainable alternatives to antibiotics.

Poor water quality is considered the most critical predisposing factor for disease eruptions in aquaculture facilities. Abrupt decrease or fluctuation in water temperature is a detrimental factor in the occurrence of some low temperature dependent microbes. The ubiquitous pathogen Pseudomonas fluorescens and the saprotrophic water mold Saprolegnia parasitica are ideal examples of this disease pathway. In the current study, the two ubiquitous microbes were isolated from Nile tilapias at an acre-sized earthen pond that was overstocked with more than 25,000 fingerlings during an episode of mass mortalities at mid-February 2023. The peak of mass mortalities occurred after heavy rains and wind storms that hit the country during this time of the year. Most of the dead and moribund fishes were suffering from generalized skin ulcers, fin rot, gill congestion, and cottony wool-like hyphae intimately attached to skin ulcers and fins. Internally, almost all internal organs including the kidney, spleen, and liver were congested. Histopathological examination of affected fish showed hyalinization and necrosis of fin tissues together with inflammatory cell infiltration. Moreover, fungal hyphae were detected within the muscle bundles. The final identity of the retrieved Pseudomonas fluorescens was confirmed using 16S rRNA gene sequence analysis, while Saprolegnia parasitica isolates were confirmed using ITS rDNA gene sequence analysis. All supportive accession numbers for retrieved isolates were published through the Genbank official website.

Aquatic environmental pollution could be a direct trigger of infection through cercarial invasion to skin / gills or indirectly as a predisposing factor that damage the physical barriers of targeted fish resulting in high intensities of EMC infections in all fish vital organs. In the current study, a total of 150 African catfish (Clarias gariepinus) were randomly collected from Mariotteya drain all the way through the Egyptian township of Shabramant located at the historical heart of Giza. Catfish samples were collected in mid-summer during the period from June to July 2024. A well-documented surge in unionized ammonia, water alkalinity, marked decline in dissolved oxygen together with decreased transparency were signaling a chronic case of agricultural and municipal pollution in which all physical mucosal immunological barriers were harshly suppressed. This environmental disruption has resulted in cellular, biological and pathological alterations in which the invasion of digenean cercariae was favored as documented in the histopathological sections made from affected tissues. Parasitological examination revealed the presence of two distinct EMC belonging to Prohemistomum vivax and Cyanodiplostomum spp. which were presumptively identified using the regular morpho-taxonomical methods and confirmed utilizing the sequencing of the ITS2 rDNA gene which is considered good marker in the differentiation between the digenean species and the phylogeny analysis. Histopathological examination of sampled fish tissues has revealed that EMC infection was not only limited to fish musculatures, yet it extends to internal organs including spleen and kidney to present a unique form of systemic EMC infection. EMC were documented within the splenic & renal tissues with remarkable activation of melanomacrophage centers (MMC) denoting the pathophysiological response of hematopoietic tissues to the EMC invasion. The current study sheds light on the growing danger of aquatic pollution and its direct trigger of zoonotic metacercarial spread through the consumption of infected fish flesh.

Cymothoid isopods are among the most devastating parasitic crustaceans in mariculture worldwide. A private marine fish farm in Ismailia province, Egypt suffered mass mortalities in cultured Chelon ramada during the late summer of 2022. The present study aims to reveal the role of the isopod, Nerocila bivittate (N. bivitata) in spread of the evolving prolonged spread of the devastating bacterial disease, Photobacterium damselae subsp piscicida (P. dmselae subsp piscicida) to Chelon ramada. This study was carried out on one hundred and fifty C. ramada collected during different phases of mortalities. Moribund fish showed signs of respiratory distress, erratic movements and poor appetite. Infested fish gills were pale anemic with excessive mucus secretion. Nerocila bivittata was isolated from the gill chamber, buccal cavity, caudal, and dorsal fins of infested fish. The infestation percentage was 26.5 % where gills of infested fish were the most affected tissue. The bipolar bodies of the P. damselae subsp piscicida were remarkably detected within the isopod hemolymph using transmission electron microscope. Further, P. damselae subsp piscida was isolated from both N. bivitata. Both P. damelae subsp piscida and N. bivitata were molecularly confirmed utilizing the 16S ribosomal RNA. In conclusion, N. bivittata infestation is responsible for high economic losses due to fish mortalities, reduced growth rates and increased susceptibility to devastating prolonged bacterial infections.

Parasitic crustaceans affecting cultured marine fish in earthen-pond aquaculture facilities are nasty parasites that trigger cascades of microbial infections with consequent economic losses. In the current study, the role of some parasitic crustaceans in spread and transmission of bacterial infections among cultured marine fishes was investigated utilizing taxonomical, molecular, epidemiological, and pathological assays. A total of 400 marine fish involving Pomadasys stridens, Dicentrarchus labra, Sparus aurata, and Tilapia zilli have been randomly collected from some private earthen pond-based aquaculture facilities. Collected fishes were clinically examined for possible presence of disease symptoms, lesions and/or parasites. Three parasitic crustaceans were identified from gills, skin, and fins of examined marine fish species. The isopod Livoneca redmanii, Caligus, and Lernanthropus species were fully identified. Vibrio alginolyticus was concurrently isolated from both fish and isopod species with 99.76 % similarity between fish and isopod species upon molecular screening utilizing 16SrRNA with sequence alignment. Interestingly, 73.3 % of the isolated bacteria exhibited multi-drug resistance (MDR) with multiple antibiotic resistance (MAR index), values ≥0.2. Further, 56.7 % (17/30) of recovered isolates were resistant to four and five antimicrobials with MAR indices of 0.50 and 0.40. Histopathological examination has revealed remarkable damage of gill filaments and fins of affected fish.

Implementing competent biosecurity strategies is a key factor for maintaining a disease-free aquaculture. Bacterial infections are an eminent threat to all live stages occurring in the marine hatchery. Live foods, including algae, rotifers, and artemia, are potential sources of microbial infections in hatcheries with ineffectual biosecurity plans. In the present study, epidemiological, bacteriological, molecular, and histopathological investigations were carried out on all critical live components of the marine hatchery, including batches of algal cultures, rotifers (Brachionus plicatilis), artemia (Artemia salina), eggs, cultured gilthead seabream (Sparus aurata) larvae, and other inanimate components such as water supply. Clinically, fish larvae had pale skin and exhibited sluggish and frequent erratic swimming patterns. Two Vibrio isolates were presumptively identified by using the API 20E system. After molecular screening and phylogenetic analysis, these isolates were confirmed to be V. alginolyticus and V. vulnificus. Remarkably, V. alginolyticus was the most commonly recovered species from seawater, eggs, algae, rotifers, and artemia cultures. Both of these Vibrio isolates exhibited diverse sensitivities to different antibiotics used in the antibiogram, with maximal susceptibility to florfenicol. Therapeutic and preventive programs were imposed at different rearing stages after tracing the potential sources of infection.

Hard corals are precious marine creatures that comprise a complex form of symbiosis between symbiont algae and coral holobiont. For decades, corals have been challenged by disastrous events of climatic and anthropogenic etiologies. Such complex interactions have resulted in devastating disease episodes among coral populations worldwide. There is a scarcity of information about diseases of hard corals in the Gulf of Aqaba, Red Sea, Egypt. Therefore, the current study aims to investigate various diseases of hard corals in this pristine habitat within the Red Sea. Whitening and dark green dots were the most noticeable morphopathology among hard coral samples collected from Gulf of Aqaba. Some human-based pathogens, such as Rothia kristinae, Cupriavidus pauculus, and Delftia acidovorans, were isolated from some of the examined coral tissues, while the Burkholderia cepacia group was isolated from the nearby sediment. The final identities of the above-mentioned bacterial isolates have been molecularly confirmed using 16S RNA sequence analysis. Pathologically, diseased corals have been observed with changes such as some forms of tissue losses, degenerative changes, and eosinophilic granular amoebocytes/agranular cells infiltration. The frequent detection of some microbial pathogens of human origin could suggest deleterious forms of environmental pollution of anthropogenic origin. Ultimately, the entire existence of hard coral populations is mostly threatened by swiftly erupting climatic changes as well as environmental aquatic pollution. Thus, the current study concludes the real need for extensive ecological, biological, pathological, and immunological studies to determine the eminent threats and propose possible control means for better/sustainable hard coral populations.

Streptococcosis is one of the septicaemic bacterial diseases of public health concern due to its zoonotic potential. Its luxurious existence in wild marine fish from an open water body strongly suggests presence of massive sewage pollution. The aim of the current study was to identify and characterize most common streptococci affecting common pandora (Pagellus erythrinus) inhabiting the Mediterranean coast of Tripoli. A total number of 270 common pandora were clinically examined for possible streptococcal infection and non-streptococcal infection. The fishes were collected from the area extending from Tripoli to Tajoura (east to Tripoli) during the three seasons, summer, autumn, and spring. No fish samples were available during winter due to the bad climate / storms along the entire western Libyan Mediterranean coasts. Streptococcus iniae, Streptococcus dysgalactiae, Streptococcus phocae, Enterococcus faecalis, and other non-streptococcal species, such as Aeromonas hydrophila, Pseudomonas anguilliseptica, and Photobacterium damselae sub species damselae were biochemically identified. Regardless of the season, S. iniae and E. faecalis were the most prevalent streptococcal species (13% and 8.88%, respectively). In contrast, the most retrieved non-streptococcal species were A. hydrophila, followed by Pseudomonas anguilliseptica, with a prevalence of 10%, and 14.44%, respectively. The highest infections were recorded during autumn, followed by summer, then spring with percentages of 53.34%, 50%, and 43.4%, respectively. The majority of the isolates were sensitive to erythromycin, florfenicol, and sulfamethoxazole-trimethoprim. The molecular screening using the sequences of 16S rRNA genes has confirmed the phylogenetic relationship to S. dysgalactiae, E. faecalis, S. iniae, and S. phocae isolates with similarity percentages exceeding 99.6%. The sequences were deposited in the GenBank with accession numbers (OK033868, OK033869, OK033870, and OK033871). To sum up, the obtained results highly suggests that the common pandora fish from Tripoli coast of the Mediterranean is biologically contaminated with various zoonotic streptococci which could be considered as a biological indicator for municipal sewage pollution.

Climate change is now considered one of the greatest challenges and is expected to have a drastic impact on mariculture. The present study aimed to evaluate the impact of climatic changes on the emergence of bacterial pathogens among cultured marine fish in northern Egyptian provinces. 135 samples of three marine fish species, represented as 45 of Dicentrarchus labrax (700 ±25 g), 45 of Sparus aurata (350 ±25 g), and 45 Argyrosomus regius (1 kg ±50 g) were collected from private marine fish farms located in Deeba Triangle, Shataa Damietta(Damietta governorate) and Ismailia province, Egypt. Moribund fishes exhibited erratic swimming behaviour, haemorrhage, erosion and ulcers on the skin. Necropsy findings of infected fish revealed congested liver or pale with engorged gall bladder, congested kidney and spleen. With the presence of serous to hemorrhagic ascetic fluid. Vibrio alginolyticus, Vibrio parahaemolyticus, and Photobacterium damselae subspecies piscicida were the most retrievable bacterial strains from moribund fish. V. alginolyticus was the most prevalent isolated bacterial strain and represented 50%, 50% and 40% of the total isolates recovered from Sparus aurata, Dicentrarchus labrax and Argyrosomus regius, respectively. Retrieved isolates were morphologically and biochemically identified using the API 20E system, followed by further confirmation by sequencing of 16S rRNA genes. The histopathological examination revealed severe inflammatory reactions together with melanomacrophage center alterations within the examined splenic, hepatic, and renal tissues. Data analysis has shown that poor water quality and severe climatic change, especially during the summer, were implicated in the emergence of bacterial infections among cultured marine fish.

Mass fish kills are abrupt events in which a significant number of fish of different ages and species perish in a specific aquatic region. Dramatic waves of mass kills have involved a large variety of economic fish species in both Egyptian fisheries and the aquaculture sector. Several episodes of mass mortalities of various infectious and noninfectious aetiologies have targeted a wide spectrum of freshwater and marine fish in both open-water and captive environments. Kafr El-Sheikh, Sharkia, Dakahlia, Fayoum, Alexandria and Giza were the most affected Egyptian provinces. The Mediterranean basin, Lake Manzala, Lake Mariott, Lake Burullus, River Nile, and some provincial water streams and municipal water drains were all reported to have had one or more episodes of mass kills over the past two decades. These mass kills were mainly attributed to different types / magnitudes of aquatic environmental pollution, which was explicitly aggravated by poor aquaculture / fishing practices. Further, the frequent absence of veterinary guidance has had a great negative impact on providing possible solutions for this disaster in the near future. Ultimately, adopting Good Aquaculture Practice (GAP), competent biosecurity strategies, and regular veterinary supervision will be the most practical solutions that ensure the reliable growth and sustainability of both Egyptian fisheries and aquaculture sectors.

Nile tilapia is the most farmed freshwater fish species across vast areas of the African continent. Although Egypt is considered the largest African producer of Nile tilapia, this fish has been faced with frequent episodes of mortality at both hatchery and farm levels. In the present study, we conducted field screenings to identify the major bacterial pathogens responsible for these mass kills among broodstocks and seeds in Egyptian hatcheries. Aeromonas hydrophila and Streptococcus agalactiae were determined to be the major bacterial threats to hatchery-reared Nile tilapia. The bacterial isolates were presumptively identified using conventional biochemical tests and the API 20 NE miniaturized test. The final identities of the retrieved bacterial isolates were molecularly confirmed using PCR and sequencing of 16S rRNA genes. A. hydrophila and S. agalactiae isolates were found to be sensitive to florfenicol, while the two isolates exhibited resistance to novobiocin and ampicillin. The A. hydrophila isolates were confirmed to be sensitive to oxytetracycline, whereas the S. agalactiae isolates were sensitive to erythromycin. Histopathological examination of the livers of infected fish revealed vacuolar degeneration and necrosis of hepatocytes. Remarkably, infiltrations of Gram-positive cocci were documented within hepatic parenchyma and brain tissues. Further, the infected fish exhibited edematous brains with inflammatory cell infiltration through the meninges. Severe retinal pathology, including collagen fibre disorganization, oedema, and inflammatory cell infiltration was also detected. The current study emphasizes the deleterious impacts of some ubiquitous bacterial pathogens on the health status of Nile tilapia broodstocks and their seeds. Ultimately, we affirm that regular monitoring of water quality, feed quality, proper handling of broodstocks, and accurate diagnosis is a crucial asset in preventing disease spread and mass fish kills in Egyptian hatcheries.

Recently, nanoparticles have attracted attention as a preventive tool for certain infectious diseases affecting fish in aquaculture. Furthermore, freshwater fishes are frequently vulnerable to summer mass morality caused by Aeromonas bacteria. In this regard, we focused on the evaluation of the in vitro and in vivo antimicrobial activity of chitosan (CNPs) and silver (AgNPs) nanoparticles against Aeromonas hydrophila subsp. hydrophila. CNPs and AgNPs were prepared at a mean particle size of 9.03 and 12.8 nm and a charge equalled+36.4 and -19.3 mV for CNPs and AgNPs, respectively. A. hydrophila subsp. hydrophila, Aeromonas caviae, and Aeromonas punctata were retrieved and identified by traditional and molecular techniques. The sensitivity of the obtained bacteria to eight different antibiotic discs was also tested. The antibiotic sensitivity studies revealed the presence of multidrug-resistant (MDR) Aeromonas species (spp.). The bacterium that showed the highest multidrug resistance against the tested antibiotic discs was Aeromonas hydrophila subsp. hydrophila. Therefore, CNPs and AgNPs were in vitro tested against the isolated bacterium and exhibited inhibition zones of 15 and 25 mm, respectively. TEM images also showed that CNPs and AgNPs had an antagonistic action against the same bacterium causing loss of architecture and bacterial death.

The aim of this study is to identify the hidden etiologies of the black chromo-shifting transient phenomenon affecting Nile tilapia, Oreochromis niloticus , farmed in RAS-controlled ponds together with the assessment of the immunological reaction against the chronic irritating effects of the invasive agent. A total of 100 Nile Tilapia were collected from a private farm at Kafrelsheikh Province, located on the northern side of the Egyptian Nile Delta. The clinical history of the affected fish farm showed an unknown chromo-shifting phenomenon where tilapias were exhibiting severe black skin coloration, which gradually disappeared after removal from the tank’s water. A comprehensive gross examination of the collected fish; including parasitological examination of skin and gill scraps, was performed. Blood biochemical testing was performed on the infested blackish O. niloticus and control non-infested fish. The current study showed that the monogenean parasite, Gyrodactylus cichlidarum , was the abundant parasite detected in the infested fish leading to abnormal black discoloration of skin and disruption of the immune system represented by significant increase of cortisol levels, lysozyme activity and different liver enzymes compared to the control. Treatment trials have been applied with moderate degrees of success, where the monogenean count was sharply decreased, and the normal skin color was remarkably restored, at the end of day 14 post-treatment. A triple treatment plan was initiated through 7 days’ application of 0.7 g/m 3 copper sulfates pentahydrate preceded by 1.5 ml/m 3 hydrogen peroxide 40% solution for the same period. One day after the end of the initial treatment, a maintenance dosage of 0.095 ml/m 3 of glutaraldehyde (15%)/quaternary ammonium compounds (QACs) mixture was administered for 3 days. As a supportive/immune-stimulant regimen, a weekly dosage of vitamin C (0.45 g g/m 3 ) and Saccharomyces cerevisiae (0.45 g/m 3 ) was added into the tank’s water to improve the general fish health.

Parameters belonging to the physical status and gonadal size of certain fish provide crucial information for assessing both the productivity and fecundity of declining fish populations. These parameters are vulnerable to the negative impacts of disease agents such as internal or systemic parasites. Although parasitic diseases might influence these parameters, the literature investigating these pathophysiological alterations is scanty. Therefore, the current study represents one of the scarcest studies that document the possible link between parasitism, gonadal health, and the growth of European hake (Merluccius merluccius). Screening of imported European halves indicated a relatively high prevalence of Anisakis pegreffi Larvae 3 infestation, with an 80% prevalence rate, a mean intensity of 24.4, and a mean abundance of 19.5. However, the prevalence of Anisakid larvae infection in native fish was 36% with a mean intensity of 7.36 and mean abundance of 2.65.The current research revealed remarkable ovarian pathology that involved several forms of degenerative changes in ovarian tissues. Such gonadal pathologies were attributed to the damaging effect of the retrieved Anisakis pegreffi Larvae 3. Gonadosomatic index of both heavily infected imported / native hakes was relatively impacted by the progressive gonadal pathology resulting from Anisakis pegreffi L3 infection. Morphometric measurements of the gonads and body have revealed that, gonadosomatic index of both heavily infected imported / native hakes was relatively impacted by the progressive gonadal pathology resulting from Anisakis pegreffi L3 infection.