Morphological and molecular characterization of Stomachicola muraenesocis Yamaguti, 1934 (Digenea: Hemiuridae) from the daggertooth pike conger Muraenesox cinereus (Forsskål).

Hemiurid digeneans conspecific with Stomachicola muraenesocis Yamaguti, 1934 (the type species of the genus Stomachicola Yamaguti, 1934) were collected from the stomach of the daggertooth pike conger Muraenesox cinereus (Forsskål) off the Persian Gulf of Iran. This study aimed to provide a detailed characterization of Stom. muraenesocis, including measurements, illustrations and scanning electron microscopy (s.e.m.) representations. Comparisons with the original and previous descriptions revealed morphological and metrical variations in several features (i.e. body size and shape, arrangement of reproductive organs, soma to ecsoma length ratio, position of genital opening, number of vitelline tubules and extension of uterine coils) between Stom. muraenesocis from different hosts and localities. This study presents the first molecular sequence data associated with the small (18S) and large (28S) subunit nuclear ribosomal RNA genes (rDNA) for Stom. muraenesocis. Phylogenetic analyses of the 18S dataset placed Stom. muraenesocis as sister lineage to a clade formed of a group of species of Lecithaster Lühe, 1901 (Lecithasteridae Odhner, 1905). In contrast, phylogenetic analyses based on the 28S consistently recovered a sister relationship between Stom. muraenesocis and representatives of the Hemiuridae Looss, 1899. Further comprehensive phylogenetically based classification in light of morphology and taxonomic history of the Hemiuridae and Lecithasteridae is required to infer phylogenetic affinities and historical biogeography of Stomachicola. A comprehensive list of previously reported species of Stomachicola together with their associated hosts, localities and morphometric data is provided.

Differentially expressed transcripts of Tetracapsuloides bryosalmonae (Cnidaria) between carrier and dead-end hosts involved in key biological processes: novel insights from a coupled approach of FACS and RNA sequencing.

Tetracapsuloides bryosalmonae is a malacosporean endoparasite that infects a wide range of salmonids and causes proliferative kidney disease (PKD). Brown trout serves as a carrier host whereas rainbow trout represents a dead-end host. We thus asked if the parasite adapts to the different hosts by changing molecular mechanisms. We used fluorescent activated cell sorting (FACS) to isolate parasites from the kidney of brown trout and rainbow trout following experimental infection with T. bryosalmonae. The sorted parasite cells were then subjected to RNA sequencing. By this approach, we identified 1120 parasite transcripts that were expressed differentially in parasites derived from brown trout and rainbow trout. We found elevated levels of transcripts related to cytoskeleton organisation, cell polarity, peptidyl-serine phosphorylation in parasites sorted from brown trout. In contrast, transcripts related to translation, ribonucleoprotein complex biogenesis and subunit organisation, non-membrane bounded organelle assembly, regulation of protein catabolic process and protein refolding were upregulated in rainbow trout-derived parasites. These findings show distinct molecular adaptations of parasites, which may underlie their distinct outcomes in the two hosts. Moreover, the identification of these differentially expressed transcripts may enable the identification of novel drug targets that may be exploited as treatment against T. bryosalmonae. We here also describe for the first time how FACS based isolation of T. bryosalmonae cells from infected kidney of fish fosters research and allows to define differentially expressed parasite transcripts in carrier and dead-end fish hosts.

Silencing of heat shock protein 90 (hsp90): Effect on development and infectivity of Ichthyophthirius multifiliis.

BACKGROUND: Recently, an increasing number of ichthyophthiriasis outbreaks has been reported, leading to high economic losses in fisheries and aquaculture. Although several strategies, including chemotherapeutics and immunoprophylaxis, have been implemented to control the parasite, no effective method is available. Hence, it is crucial to discover novel drug targets and vaccine candidates against Ichthyophthirius multifiliis. For this reason, understanding the parasite stage biology, host-pathogen interactions, molecular factors, regulation of major aspects during the invasion, and signaling pathways of the parasite can promote further prospects for disease management. Unfortunately, functional studies have been hampered in this ciliate due to the lack of robust methods for efficient nucleic acid delivery and genetic manipulation. In the current study, we used antisense technology to investigate the effects of targeted gene knockdown on the development and infectivity of I. multifiliis. Antisense oligonucleotides (ASOs) and their gold nanoconjugates were used to silence the heat shock protein 90 (hsp90) of I. multifiliis. Parasite stages were monitored for motility and development. In addition, the ability of the treated parasites to infect fish and cause disease was evaluated. RESULTS: We demonstrated that ASOs were rapidly internalized by I. multifiliis and distributed diffusely throughout the cytosol. Knocking down of I. multifiliis hsp90 dramatically limited the growth and development of the parasite. In vivo exposure of common carp (Cyprinus carpio) showed reduced infectivity of ASO-treated theronts compared with the control group. No mortalities were recorded in the fish groups exposed to theronts pre-treated with ASOs compared with the 100% mortality observed in the non-treated control fish. CONCLUSION: This study presents a gene regulation approach for investigating gene function in I. multifiliis in vitro. In addition, we provide genetic evidence for the crucial role of hsp90 in the growth and development of the parasite, suggesting hsp90 as a novel therapeutic target for successful disease management. Further, this study introduces a useful tool and provides a significant contribution to the assessing and understanding of gene function in I. multifiliis.

The Immune Response to the Myxozoan Parasite Myxobolus cerebralis in Salmonids: A Review on Whirling Disease.

Salmonids are affected by the economically significant whirling disease (WD) caused by the myxozoan parasite Myxobolus cerebralis. In the past, it was endemic to Eurasia, but it has now spread to different regions of North America, Europe, New Zealand, and South Africa. Among salmonids, rainbow trout is considered the most highly susceptible host. Upon entering to the host's body, the parasite invades the spine and cranium, resulting in whirling behaviour, a blackened tail, and destruction of cartilage. The disease is characterized by the infiltration of numerous inflammatory cells, primarily lymphocytes and macrophages, with the onset of fibrous tissue infiltration. Several efforts have been undertaken to investigate the role of various immune modulatory molecules and immune regulatory genes using advanced molecular methods including flow cytometry and transcriptional techniques. Investigation of the molecular and cellular responses, the role of STAT3 in Th17 cell differentiation, and the inhibitory actions of suppressors of cytokine signaling (SOCS) on interferons and interleukins, as well as the role of natural resistance-associated macrophage proteins (Nramp) in WD have significantly contributed to our understanding of the immune regulation mechanism in salmonids against M. cerebralis. This review thoroughly highlights previous research and discusses potential future directions for understanding the molecular immune response of salmonids and the possible development of prophylactic approaches against WD.

Genome-wide alternative splicing profile in the posterior kidney of brown trout (Salmo trutta) during proliferative kidney disease.

BACKGROUND: The cnidarian myxozoan parasite Tetracapsuloides bryosalmonae causes chronic proliferative kidney disease (PKD) in salmonids. This parasite is a serious threat to wild and cultured salmonids. T. bryosalmonae undergoes intra-luminal sporogonic development in the kidney of brown trout (Salmo trutta) and the viable spores are released via urine. We investigated the alternative splicing pattern in the posterior kidney of brown trout during PKD. RESULTS: RNA-seq data were generated from the posterior kidney of brown trout collected at 12 weeks post-exposure to T. bryosalmonae. Subsequently, this data was mapped to the brown trout genome. About 153 significant differently expressed alternatively spliced (DEAS) genes, (delta PSI = 5%, FDR P-value < 0.05) were identified from 19,722 alternatively spliced events. Among the DEAS genes, the least and most abundant alternative splicing types were alternative 5' splice site (5.23%) and exon skipping (70.59%), respectively. The DEAS genes were significantly enriched for sodium-potassium transporter activity and ion homeostasis (ahcyl1, atp1a3a, atp1a1a.1, and atp1a1a.5). The protein-protein interaction network analysis enriched two local network clusters namely cation transporting ATPase C-terminus and Sodium/potassium ATPase beta chain cluster, and mixed inclusion of Ion homeostasis and EF-hand domain cluster. Furthermore, the human disease-related salmonella infection pathway was significantly enriched in the protein-protein interaction network. CONCLUSION: This study provides the first baseline information about alternative splicing in brown trout during PKD. The generated data lay a foundation for further functional molecular studies in PKD - brown trout infection model. The information generated from the present study can help to develop therapeutic strategies for PKD in the future.

Therapeutic Intervention with Dietary Chitosan Nanoparticles Alleviates Fish Pathological and Molecular Systemic Inflammatory Responses against Infections.

Marine bio-sourced chitosan nanoparticles (CSNP) are antimicrobial and immunomodulatory agents beneficial for fish medicine. Herein, dietary CSNP was investigated for the amelioration of the systemic inflammatory responses of an induced fish model. One hundred and forty-four rainbow trout were assigned to one pathogen-free and non-supplemented group (negative control), and three challenged groups: non-supplemented (positive control), CSNP-preventive, and CSNP-therapeutic. After a feeding experiment extended for 21 days, the organosomatic indices (OSI) and molecular aspects were assessed. After a challenge experiment extended for further 28 days, CSNP-therapeutic intervention was assessed on fish survival and systemic inflammatory responses on pathology, histo-morphology, and molecular aspects. With CSNP administration, OSI nonsignificantly decreased and the relative expression of targeted inflammatory-mediator genes was significantly increased. The CSNP-therapeutic fish showed an RPS of 80% as compared to the positive control group, and CSNP-therapeutic administration retained the highest gene expression augmentation up to 28 days after the challenge. Notably, the splenic reticulin fibers framework of the CSNP-therapeutic group retained the highest integrity among the groups during the infection. After recovery, reticulin fibers density in the CSNP-therapeutic samples was significantly higher than in the negative control group, which indicates high innate immunity. Thus, CSNP showed promising biotherapeutic features enhancing fish resistance against infections.

Delivering the pain: an overview of the type III secretion system with special consideration for aquatic pathogens.

Gram-negative bacteria are known to subvert eukaryotic cell physiological mechanisms using a wide array of virulence factors, among which the type three-secretion system (T3SS) is often one of the most important. The T3SS constitutes a needle-like apparatus that the bacterium uses to inject a diverse set of effector proteins directly into the cytoplasm of the host cells where they can hamper the host cellular machinery for a variety of purposes. While the structure of the T3SS is somewhat conserved and well described, effector proteins are much more diverse and specific for each pathogen. The T3SS can remodel the cytoskeleton integrity to promote intracellular invasion, as well as silence specific eukaryotic cell signals, notably to hinder or elude the immune response and cause apoptosis. This is also the case in aquatic bacterial pathogens where the T3SS can often play a central role in the establishment of disease, although it remains understudied in several species of important fish pathogens, notably in Yersinia ruckeri. In the present review, we summarise what is known of the T3SS, with a special focus on aquatic pathogens and suggest some possible avenues for research including the potential to target the T3SS for the development of new anti-virulence drugs.

First description of freshwater mite Unionicola sauerensis sp. nov. infesting thick-shelled river mussel Unio crassus.

A sample of 30 thick-shelled river mussels Unio crassus Philipsson (Unionida: Unionidae) was collected from the River Sauer in Luxembourg to acquire data on parasitic infestations of the mussels. Among other parasites, different development stages of freshwater mites were collected from the gills and the mantle of the mussels and were documented with bright-field, stereo, and confocal laser scanning microscopy and microscopic X-ray computed tomography. The retrieved data allowed a morphological description of larvae and female adults of the mites and assigning them to the genus Unionicola Haldeman (Trombidiformes: Unionicolidae) and the subgenus Pentatax Thor. Additionally, adult stages and larvae were barcoded by sequencing a section of the mitochondrial COI and 18S rRNA genes. This resulted in 4 new, similar Unionicola lineages from the adult stages, which differ in at least 14.7% (uncorrected p distance) from those already published. Barcoding of larval DNA was not successful. The comparison with known European species of the genus Unionicola and analysis of the barcoding results allowed the proposal of a new species of the genus Unionicola. The species was named Unionicola sauerensis sp. nov. after the River Sauer in Luxembourg, where the infested mussels were collected.

Synergistic Effect of Biosynthesized Silver Nanoparticles and Natural Phenolic Compounds against Drug-Resistant Fish Pathogens and Their Cytotoxicity: An In Vitro Study.

Fish pathogens causing disease outbreaks represent a major threat to aquaculture industry and food security. The aim of the presented study is to develop safe and effective bioactive agents against two bacterial isolates: Aeromonas hydrophila and Pseudomonas fluorescens. We employed a broth microdilution method to investigate the antibacterial effect of biosynthesized silver nanoparticles (AgNPs); rutin, a natural flavonoid extracted from Ruta graveneoles; and heliomycin, a secondary metabolite produced by marine actinomycetes AB5, as monotherapeutic agents. Moreover, AgNPs in combination with rutin (AgNP + R) and heliomycin (AgNPs + H) were examined for their synergistic effect. The cytotoxic effect of individual bioactive compounds and in combination with AgNPs was investigated on epithelioma papulosum cyprini (EPC) fish cell lines. Individual treatment of AgNPs, rutin, and heliomycin exhibited a dose-dependent antimicrobial activity against A. hydrophila and P. fluorescens. Rutin minimum inhibitory concentration (MIC) showed the lowest cytotoxicity when tested on EPC cell lines, while heliomycin MIC was highly cytotoxic. Combined subtherapeutic doses of AgNPs + R and AgNPs + H displayed additive and synergistic effects against A. hydrophila and P. fluorescens, respectively, with improved results and relative safety profile. The study findings demonstrate that a combination of AgNPs and natural bioactive compounds may represent novel therapeutics fighting fish pathogens potentially affecting the fish farming industry.

Dietary Chitosan Nanoparticles: Potential Role in Modulation of Rainbow Trout (Oncorhynchus mykiss) Antibacterial Defense and Intestinal Immunity against Enteric Redmouth Disease.

Bio-nanotechnology employing bio-sourced nanomaterial is an emerging avenue serving the field of fish medicine. Marine-sourced chitosan nanoparticles (CSNPs) is a well-known antimicrobial and immunomodulatory reagent with low or no harm side effects on fish or their human consumers. In this study, in vitro skin mucus and serum antibacterial activity assays along with intestinal histology, histochemical, and gene expression analyses were performed to evaluate the impact of dietary CSNPs (5 g kg-1 dry feed) on rainbow trout resistance against 'enteric redmouth' disease. Two treatment conditions were included; short-term prophylactic-regimen for 21 days before the bacterial challenge, and long-term therapeutic-regimen for 21 days before the challenge and extended for 28 days after the challenge. Our results revealed higher antibacterial defense ability and positive intestinal histochemical and molecular traits of rainbow trout after dietary CSNPs. The prophylactic-regimen improved trout health while the therapeutic regimen improved their disease resistance and lowered their morbidity. Therefore, it is anticipated that CSNPs is an effective antibacterial and immunomodulatory fish feed supplement against the infectious threats. However, the CSNPs seem to be more effective in the therapeutic application rather than being used for short-term prophylactic applications.

Effect of water temperature on the morbidity of Tetracapsuloides bryosalmonae (Myxozoa) to brown trout (Salmo trutta) under laboratory conditions.

Proliferative kidney disease (PKD) is a disease found in salmonid fish that is widespread in Europe and North America. The dependency of the clinical signs on the water temperature is extensively reported in rainbow trout, but detailed information on brown trout is lacking. In this study, juvenile brown trout were exposed to the spores of Tetracapsuloides bryosalmonae and then kept at different ambient water temperatures (16°C, 19°C and 22°C) for 10 weeks along with recording of morbidity throughout the experiment. At 6, 8 and 10 weeks post-exposure, fish from each temperature group were sampled and underwent pathoanatomical examination to survey disease progression. At 16°C, brown trout showed a significantly higher survival probability compared to those kept in 19°C and 22°C water. Additionally, the parasitic burden (MSQ) was higher and the clinical signs were more pronounced in the brown trout kept at 19°C and 22°C compared with the ones kept at 16°C. This study highlights the correlation of PKD outbreaks and water temperature increases related to global climate change, which will impact the future distribution of brown trout in natural waters.

Kinetics of CD4-1+ lymphocytes in brown trout after exposure to viral haemorrhagic septicaemia virus.

T-helper cells express CD4 as a co-receptor that binds to major histocompatibility complex class II to synchronize the immune response against upcoming threats via mediating several cytokines. We have previously reported the presence of CD4 homologues in brown trout. The study of cellular immune responses in brown trout is limited by the availability of specific antibodies. We here describe the generation of a polyclonal antibody against CD4-1 that allows for the investigation of CD4+ cells. We used this novel tool to study CD4+ cells in different tissues during viral haemorrhagic septicaemia infection (VHSV) using flow cytometric technique. Flow cytometric analyses revealed an enhanced level of surface CD4-1 expression in the infected group in major lymphoid organs and in the intestine. These results suggest an important role for the T-helper cells within the immune response against viruses, comparable to the immune response in higher vertebrates.

Cyprinid herpesvirus 3 (CyHV-3) transmission and outbreaks in Iran: Detection and characterization in farmed common carp.

This study shows the presence of Cyprinid Herpesvirus-3 (CyHV-3) in common carp (Cyprinus carpio) from Iranian carp farms with cumulative mortality up to 80% during 2015-2016. Pathological signs of disease such as gill necrosis, sunken eyes, and increased slime secretion on the skin and fins were observed in affected fish. The extensive fusion of secondary lamellae with necrotic cells, margination of chromatin, and formation of intranuclear inclusion bodies in gill tissues were also observed by histopathological examination. Most tubular epithelial cells and some hematopoietic cells showed intranuclear inclusion bodies in the kidney. The Iranian CyHV-3 isolates showed identity with Asian strains, and displayed the I++ II+ allele of the Asian lineage, as revealed by sequence analysis of the TK gene, Marker I, and Marker II. The detected isolates were also similar to those detected from koi in the same region of Iran, suggesting the probable transmission of CyHV-3 from ornamental to farmed cyprinids. This represents the first report of CyHV-3 from Iranian farmed common carp to the best of our knowledge.

Effects of siRNA silencing on the susceptibility of the fish cell line CHSE-214 to Yersinia ruckeri.

Yersinia ruckeri is a facultative intracellular enterobacterium mostly known as the causative agent of enteric redmouth disease in salmonid fish. In the present study, we applied RNA inhibition to silence twenty pre-selected genes on the genome of a fish cell line (CHSE-214) followed by a gentamicin assay to quantify the effect of silencing on the cells' susceptibility to infection and found that silencing of 18 out of 20 genes significantly reduced the number of Y. ruckeri recovered. These findings improve our understanding of the infection process by Y. ruckeri and of the interactions between this bacterial pathogen and host cells.

Detection of Carp pox virus (CyHV-1) from koi (Cyprinus carpio L.) in Iran; clinico-pathological and molecular characterization.

Cyprinid herpesvirus 1 (CyHV-1) is the causative agent of carp pox characterized by epidermal papillomas in common carp and other cyprinids. In this study, we identified CyHV-1 in koi (Cyprinus carpio) from Iran in 2017 and 2019, showing clinical signs of the carp pox disease. Histopathology showed severe epidermal hyperplasia and the absence of club and goblet cells. Degenerative changes, including spongiosis and single-cell necrosis, were also observed. Keratinocyte dysplasia and a moderate lymphocytic infiltration were observed within the epidermis. PCR of the extracted DNA from skin lesions of affected koi from both outbreaks showed CyHV-1 specific TK amplicons, with high sequence identity (above 99%) among themselves and with other CyHV-1 isolates belong to Cluster I, as well as show 97% similarity to Cluster II isolates. To the best of our knowledge, this is the first report of Carp pox disease (CyHV-1) of koi in Iran and the Middle East.

Modulation of local and systemic immune responses in brown trout (Salmo trutta) following exposure to Myxobolus cerebralis.

Myxobolus cerebralis, the etiological agent of Whirling Disease (WD), is a freshwater myxozoan parasite with considerable economic and ecological relevance for salmonids. There are differences in disease susceptibility between species and strains of salmonids. Recently, we have reported that the suppressor of cytokine signaling SOCS1 and SOCS3 are key in modulating rainbow trout (Oncorhynchus mykiss) immune responses and that resistant fish apparently exhibit effective Th17 cell response after exposure to M. cerebralis. It is unclear whether such molecules and pathways are also involved in the immune response of M. cerebralis infected brown trout (Salmo trutta). Hence, this study aimed to explore their role during immune modulation in infected brown trout, which is considered resistant to this parasite. Fish were exposed to the triactinomyxon (TAM) stages of M. cerebralis and quantitative real-time PCR (RT-qPCR) was carried out to examine local (caudal fin) and systemic (head kidney, spleen) immune transcriptional changes associated with WD over time in infected and control fish. All of the immune genes in the three tissues studied were differentially expressed in infected fish at multiple time points. Brown trout reduced the parasite load and demonstrated effective immune responses, likely by keeping pro-inflammatory and anti-inflammatory cytokines in balance whilst stimulating efficient Th17-mediated immunity. This study increases knowledge on the brown trout immune response to M. cerebralis and helps us to understand the underlying mechanisms of WD resistance.

Aeromonas spp. suggested as the causative agents of red spot disease in northern Vietnamese grass carp Ctenopharyngodon idella.

In northern Vietnam, a disease called 'red spot disease' has been causing high morbidity and mortality in populations of farmed grass carp Ctenopharyngodon idella for about 2 decades. The name 'red spot disease' refers to a condition characterised by haemorrhagic lesions, reddening and ulceration of the skin. Eight different bacterial isolates, namely Aeromonas hydrophila, A. sobria, Staphylococcus epidermidis, Vibrio alginolyticus, Pseudomonas fluorescens, P. luteola, Citrobacter freundii and P. putida, were isolated from diseased grass carp and used for experimental infection of the same species. Fish were challenged with the different bacterial isolates both by immersion and intramuscular injection. Different concentrations of bacteria were tested to evaluate their pathogenicity. Injection with 1 × 105 CFU of A. hydrophila and A. sobria resulted in clinical signs identical to those of red spot-diseased grass carp in Vietnam. None of the other bacterial isolates tested caused any morbidity or mortality in fish challenged either intramuscularly (1 × 106 CFU) or by bath immersion (1 × 106 or 1 × 108 CFU ml-1).

Efficacy of silver nanoparticles to control flavobacteriosis caused by Flavobacterium johnsoniae in common carp Cyprinus carpio.

Flavobacterial infections are among the causes of fish losses in farms with the emergence of antibiotic-resistant isolates. Silver nanoparticles (AgNPs) are known for their potent antimicrobial activity against different types of bacteria. In this study, we evaluated the antibacterial properties of AgNPs (diameter: 23 nm) against Flavobacterium johnsoniae infection in common carp Cyprinus carpio. The assays included both in vitro and in vivo antibacterial tests in addition to evaluation of cell toxicity effects on the fish cell lines. The in vitro results revealed potent inhibitory effects of AgNPs on the growth of F. johnsoniae with a minimum inhibitory concentration of 34 µg ml-1. Fish cell (epithelioma papulosum cyprini and koi carp fin) viability was 95-100% after exposure to 500 ng ml-1 (and lower concentrations) of AgNPs. In the exposure experiment, mortality rates decreased from 45% in the infected non-treated group to 30 and 15% in the intraperitoneal injection and immersion-treated groups, respectively. Neither of the treated groups showed any clinical signs or histopathological lesions. The single-dose treatment with AgNPs during early infection with F. johnsoniae aided in minimizing fish losses.

Acanthocephalan parasites collected from Austrian fishes: molecular barcoding and pathological observations.

Acanthocephalan parasites were collected from the intestinal tracts of 137 predominantly wild fish (1 barbel Barbus barbus, 3 European chub Squalius cephalus, 13 rainbow trout Oncorhynchus mykiss and 120 brown trout Salmo trutta) from 12 localities. The condition factor, intensity of acanthocephalan infection and pathological lesions, if applicable, were documented. Routine bacteriology and virology were performed, and the brown trout were additionally tested for the presence of the myxozoan parasite Tetracapsolioides bryosalmonae by PCR. In total, 113 acanthocephalans were barcoded by sequencing a section of the mitochondrial cytochrome oxidase subunit I (COI) gene. Barcoding of the acanthocephalan tissues resulted in 77 sequences, of which 56 were assigned to Echinorhynchus truttae (3 genotypes), 11 to Pomphorhynchus tereticollis (9 genotypes), 9 to Acanthocephalus sp. (5 genotypes) and 1 to Neoechinorhynchida. Most of these genotypes were detected for the first time. Statistically, the acanthocephalan infection did not have an impact on the condition factor of the brown trout. Infection with P. tereticollis caused more severe pathological changes in the digestive tract than E. truttae. The present study provides new data regarding the distribution of acanthocephalan species in Austria and their impact on individual fish. In addition, new barcoding data from acanthocephalan parasites are presented, and the occurrence of P. tereticollis in European chub in Austria and in brown and rainbow trout in general was confirmed for the first time.

The nature and consequences of co-infections in tilapia: A review.

Co-infections commonly arise when two or multiple different pathogens infect the same host, either as simultaneous or as secondary concurrent infection. This potentiates their pathogenic effects and leads to serious negative consequences on the exposed host. Numerous studies on the occurrence of the bacterial, parasitic, fungal and viral co-infections were conducted in various tilapia species. Co-infections have been associated with serious negative impacts on susceptible fish because they increase the fish susceptibility to diseases and the likelihood of outbreaks in the affected fish. Co-infections can alter the disease course and increase the severity of disease through synergistic and, more rarely, antagonistic interactions. In this review, reports on the synergistic co-infections and their impacts on the affected tilapia species are highlighted. Additionally, their pathogenic mechanisms are briefly discussed. Tilapia producers should be aware of the possible occurrence of co-infections and their effects on the affected tilapia species and in particular of the clinical signs and course of the disease. To date, there is still limited information regarding the pathogenicity mechanisms and pathogen interactions during these co-infections. This is generally due to low awareness regarding co-infections, and in many cases, a dominant pathogen is perceived to be of vital importance and hence becomes the target of treatment while the treatment of the co-infectious agents is neglected. This review article aimed at raising awareness regarding co-infections and helping researchers and fish health specialists pay greater attention to these natural cases, leading to increased research and more consistent diagnosis of co-infectious outbreaks in order to improve control strategies to protect tilapia when infected with multiple pathogens.