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Emerging Infections in Aquatic Animals

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A special issue of Pathogens (ISSN 2076-0817). This special issue belongs to the section "Emerging Pathogens".

Deadline for manuscript submissions: closed (28 February 2023) | Viewed by 19754

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Special Issue Editors

Dr. Spencer Russell

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Guest Editor

Department of Fisheries and Aquaculture, Center for Innovation in Fish Health, Vancouver Island University, Nanaimo, BC V9R 5S5, Canada
Interests: aquatic animal diseases; aquaculture; fish pathology

Prof. Dr. John S. Lumsden

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Guest Editor

Department of Pathobiology, University of Guelph, Guelph, ON N1G 2W1, Canada
Interests: fish pathology; autophagy; aquatic animal health

Special Issue Information

Dear Colleagues,

Healthy aquatic ecosystems are home to a diversity of invertebrates, fish, mammals, and plants. All aquatic animals can be affected by infectious diseases caused by pathogens such as viruses, bacteria, fungi, protozoa, and parasites. Infection and disease are normal phenomena in aquatic animal populations; however, under certain conditions these infections can become severe, causing excessive morbidity and mortality, with negative population impacts. Predisposing factors can include the introduction of new agents to a population or stressful conditions that promote disease such as climate change, habitat alteration, water quality perturbations or the introduction of non-native species. These stressors can directly impact the prevalence and severity of disease in aquatic populations.

This Special Issue of Pathogens will emphasize emerging infectious diseases in aquatic animals. We plan to assemble a series of contributions offering a comprehensive view of emerging aquatic animal diseases caused by pathogenic organisms. We invite submissions on the newest advances in pathogen identification, characterization, and detection, host–pathogen interactions, host immune response to infection, and the epidemiology of disease in aquatic animals.

We look forward to collaborating with you.

Dr. Spencer Russell
Prof. Dr. John S. Lumsden
Guest Editors

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Keywords

aquatic animals
emerging diseases
pathogens

Published Papers (11 papers)

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Research

26 pages, 3394 KiB

Open AccessArticle

A New Sturgeon Herpesvirus from Juvenile Lake Sturgeon Acipenser fulvescens Displaying Epithelial Skin Lesions

by Sharon Clouthier, Marek Tomczyk, Tamara Schroeder, Cheryl Klassen, André Dufresne, Eveline Emmenegger, Thomas Nalpathamkalam, Zhuozhi Wang and Bhooma Thiruvahindrapuram

Pathogens 2023, 12(9), 1115; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12091115 - 31 Aug 2023

Viewed by 1141

Abstract

Herpesvirus infections of sturgeon pose a potential threat to sturgeon culture efforts worldwide. A new epitheliotropic herpesvirus named Acipenser herpesvirus 3 (AciHV-3) was detected in hatchery-reared Lake Sturgeon Acipenser fulvescens displaying skin lesions in central Canada. The growths were discovered in the fall, reached average prevalence levels of 0.2–40% and eventually regressed. No unusual mortality was observed. The cellular changes within the lesions included epithelial hyperplasia and were reminiscent of other herpesvirus infections. The virus was not evident in lesions examined by electron microscopy. Skin tissue homogenates from symptomatic sturgeon produced atypical cytopathic effects on a primary Lake Sturgeon cell line, and next-generation sequence analysis of the DNA samples revealed the presence of an alloherpesvirus. A new genotyping PCR assay targeting the major capsid protein sequence detected AciHV-3 in symptomatic Lake Sturgeon as well as other apparently healthy sturgeon species. Bayesian inference of phylogeny reconstructed with a concatenation of five alloherpesvirus core proteins revealed a new Alloherpesviridae lineage isomorphic with a new genus. The presence of AciHV-3 homologs in cell lines and sturgeon sequence datasets, low sequence divergence among these homologs and branching patterns within the genotyping phylogeny provide preliminary evidence of an endogenous virus lifestyle established in an ancestral sturgeon. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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8 pages, 1882 KiB

Open AccessCommunication

Detection and Quantification of Nocardia crassostreae, an Emerging Pathogen, in Mytilus galloprovincialis in the Mediterranean Sea Using Droplet Digital PCR

by Anna Cutarelli, Francesca Carella, Francesca De Falco, Bianca Cuccaro, Fabio Di Nocera, Donatella Nava, Gionata De Vico and Sante Roperto

Pathogens 2023, 12(8), 994; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12080994 - 28 Jul 2023

Viewed by 1181

Abstract

Nocardia crassostreae is a novel pathogen responsible for infections in oysters (Crassostrea gigas) and mussels (Mytilus galloprovincialis). N. crassostreae is also responsible for nocardiosis both in immunocompetent and immunocompromised patients. We investigated N. crassostreae DNA in mussels grown in marine sites of the Mediterranean Sea in the Campania Region. We examined 185 mussel pooled samples by droplet digital PCR (ddPCR) and real-time quantitative PCR (qPCR), each pool composed of 10 mussels and 149 individual mussels. ddPCR detected N. crassostreae DNA in 48 mussel pooled samples and in 23 individual mussel samples. qPCR detected N. crassostreae DNA in six pooled samples and six individual mussel samples. The two molecular assays for the detection of N. crassostreae DNA showed significant differences both in the pooled and in individual samples. Our study demonstrated that ddPCR outperformed real-time qPCR for N. crassostreae DNA detection, thus confirming that ddPCR technology can identify the pathogens in many infectious diseases with high sensitivity and specificity. Furthermore, in individual mussels showing histological lesions due to N. crassostreae, the lowest copy number/microliter detected by ddPCR of this pathogen was 0.3, which suggests that this dose could be enough to cause infections of N. crassostreae in mussels. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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11 pages, 980 KiB

Open AccessArticle

Secreted Extracellular Products of Flavobacterium covae as Potential Immunogenic Factors for Protection against Columnaris Disease in Channel Catfish (Ictalurus punctatus)

by Mohamed Sayed, Lakshmi Narayanan, Manal Essa, Mark Lawrence, Attila Karsi and Hossam Abdelhamed

Pathogens 2023, 12(6), 808; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12060808 - 07 Jun 2023

Cited by 1 | Viewed by 1178

Abstract

Columnaris disease caused by Flavobacterium covae leads to substantial economic losses in commercially important fish species worldwide. The US channel catfish (Ictalurus punctatus) industry is particularly vulnerable to this disease. Therefore, there is an urgent need to develop a vaccine to reduce the economic losses caused by this disease. Secreted extracellular products (SEPs) are considered to be essential bacterial virulence factors that often provide immunogenicity and protection. The current study sought to identify the main SEPs of F. covae and to evaluate their potential to provide protection in channel catfish against columnaris disease. SDS-PAGE analysis of SEPs revealed five protein bands with molecular weights ranging from 13 to 99 kDa. Mass spectrometry analysis showed that these SEPs were hypothetical protein (AWN65_11950), zinc-dependent metalloprotease (AWN65_10205), DNA/RNA endonuclease G (AWN65_02330), outer membrane protein beta-barrel domain (AWN65_12620), and chondroitin-sulfate-ABC endolyase/exolyase (AWN65_08505). Catfish fingerlings were vaccinated with SEPs, SEPs emulsified with mineral oil adjuvant, or heat-inactivated SEPs, or they were sham-immunized through intraperitoneal (IP) injection. After 21 days, an F. covae challenge showed 58.77% and 46.17% survival in the catfish vaccinated with the SEPs and the SEPs emulsified with adjuvant compared to the sham-vaccinated control (100% mortality within 120 h post-infection). However, the heat-inactivated SEPs failed to provide significant protection (23.15% survival). In conclusion, although SEPs contain potentially important immunogenic proteins, further work is needed to optimize their use for long-lasting protection against columnaris disease in fish. These results are significant given the economic impact of columnaris disease on fish farming worldwide. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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18 pages, 3345 KiB

Open AccessArticle

Retrospective Screening of Anthrax-like Disease Induced by Bacillus tropicus str. JMT from Chinese Soft-Shell Turtles in Taiwan

by Jia-Ming Tsai, Hsin-Wei Kuo and Winton Cheng

Pathogens 2023, 12(5), 693; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12050693 - 10 May 2023

Cited by 1 | Viewed by 2097

Abstract

Bacillus cereus is ubiquitous in the environment and a well-known causative agent of foodborne disease. Surprisingly, more and more emerging strains of atypical B. cereus have been identified and related to severe disease in humans and mammals such as chimpanzees, apes, and bovine. Recently, the atypical B. cereus isolates, which mainly derive from North America and Africa, have drawn great attention due to the potential risk of zoonosis. The cluster of B. cereus carries several anthrax-like virulent genes that are implicated in lethal disease. However, in non-mammals, the distribution of atypical B. cereus is still unknown. In this study, we conducted a retrospective screening of the 32 isolates of Bacillus spp. from diseased Chinese soft-shelled turtles from 2016 to 2020. To recognize the causative agent, we used various methods, such as sequencing analysis using PCR-amplification of the 16S rRNA gene, multiplex PCR for discriminating, and colony morphology by following previous studies. Furthermore, the digital DNA-DNA hybridization (dDDH) and average nucleotide identity (ANI) values were calculated, respectively, below the 70 and 96% cutoff to define species boundaries. According to the summarized results, the pathogen is taxonomically classified as Bacillus tropicus str. JMT (previous atypical Bacillus cereus). Subsequently, analyses such as targeting the unique genes using PCR and visual observation of the bacteria under various staining techniques were implemented in our study. Our findings show that all (32/32, 100%) isolates in this retrospective screening share similar phenotypical properties and carry the protective antigen (PA), edema factor (EF), hyaluronic acid (HA), and exopolysaccharide (Bps) genes on their plasmids. In this study, the results indicate that the geographic distribution and host range of B. tropicus were previously underestimated. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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18 pages, 6006 KiB

Open AccessArticle

Development of Bioluminescent Virulent Aeromonas hydrophila for Understanding Pathogenicity

by Eda Ozdemir, Hossam Abdelhamed, Ozan Ozdemir, Mark Lawrence and Attila Karsi

Pathogens 2023, 12(5), 670; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12050670 - 02 May 2023

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Abstract

Virulent Aeromonas hydrophila (vAh) strains that cause motile Aeromonas septicemia (MAS) in farmed channel catfish (Ictalurus punctatus) have been an important problem for more than a decade. However, the routes of infection of vAh in catfish are not well understood. Therefore, it is critical to study the pathogenicity of vAh in catfish. To this goal, a new bioluminescence expression plasmid (pAKgfplux3) with the chloramphenicol acetyltransferase (cat) gene was constructed and mobilized into vAh strain ML09-119, yielding bioluminescent vAh (BvAh). After determining optimal chloramphenicol concentration, plasmid stability, bacteria number–bioluminescence relationship, and growth kinetics, the catfish were challenged with BvAh, and bioluminescent imaging (BLI) was conducted. Results showed that 5 to 10 µg/mL chloramphenicol was suitable for stable bioluminescence expression in vAh, with some growth reduction. In the absence of chloramphenicol, vAh could not maintain pAKgfplux3 stably, with the half-life being 16 h. Intraperitoneal injection, immersion, and modified immersion (adipose fin clipping) challenges of catfish with BvAh and BLI showed that MAS progressed faster in the injection group, followed by the modified immersion and immersion groups. BvAh was detected around the anterior mouth, barbels, fin bases, fin epithelia, injured skin areas, and gills after experimental challenges. BLI revealed that skin breaks and gills are potential attachment and entry portals for vAh. Once vAh breaches the skin or epithelial surfaces, it can cause a systemic infection rapidly, spreading to all internal organs. To our best knowledge, this is the first study that reports the development of a bioluminescent vAh and provides visual evidence for catfish–vAh interactions. Findings are expected to provide a better understanding of vAh pathogenicity in catfish. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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10 pages, 1144 KiB

Open AccessArticle

Comparative Evaluation of Booster Vaccine Efficacy by Intracoelomic Injection and Immersion with a Whole-Cell Killed Vaccine against Lactococcus petauri Infection in Rainbow Trout (Oncorhynchus mykiss)

by Tryssa de Ruyter, Eric Littman, Zeinab Yazdi, Mark Adkison, Alvin Camus, Susan Yun, Timothy J. Welch, William R. Keleher and Esteban Soto

Pathogens 2023, 12(5), 632; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12050632 - 22 Apr 2023

Cited by 4 | Viewed by 1506

Abstract

Lactococcus petauri is an important emergent bacterial pathogen of salmonids in the USA. The purpose of this study was to evaluate the protection conferred to rainbow trout (Oncorhynchus mykiss) against L. petauri by formalin-killed vaccines in immersion and injectable forms, as well as the enhanced protection afforded by booster vaccination. In the first challenge, fish were immunized via intracoelomic injection (IC) or immersion (Imm) routes alone. Approximately 418 degree days (Temperature in degree Celsius × days post-immunization) (dd) Imm, or 622 dd IC post-vaccination, fish were challenged via IC with wild-type L. petauri. In the second experiment, initial Imm vaccination was followed by booster vaccination via Imm or IC routes 273 dd post-immunization along with appropriate PBS controls. The various vaccination protocol efficacies were evaluated by challenging fish with L. petauri by cohabitation with diseased fish 399 dd post-booster administration. A relative percent survival (RPS) of 89.5% and 28% was recorded in the IC and Imm single immunization treatments, respectively. In the second study, an RPS of 97.5%, 10.2%, 2.6% and −10.1% plus approximately 0%, 50%, 20%, and 30% bacterial persistence was recorded in the Imm immunized + IC boosted, Imm immunized + mock IC boosted, Imm immunized + Imm boosted, and Imm immunized + mock Imm boosted treatments, respectively. Only the Imm immunized + IC injection boosted treatments provided significant protection when compared to unvaccinated and challenged treatments (p < 0.05). In conclusion, although both Imm and IC vaccines appear safe for trout, the inactivated Imm vaccines seem to provide only mild and temporary protection against lactococcosis; whereas IC immunized trout develop a significantly stronger protective response in both challenges. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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21 pages, 3647 KiB

Open AccessArticle

Impact of Infection with Flavobacterium psychrophilum and Antimicrobial Treatment on the Intestinal Microbiota of Rainbow Trout

by David Huyben, Maureen Jarau, Janet MacInnes, Roselynn Stevenson and John Lumsden

Pathogens 2023, 12(3), 454; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12030454 - 14 Mar 2023

Cited by 3 | Viewed by 1970

Abstract

The diversity and composition of intestinal microbiota in rainbow trout have been studied using next-generation sequencing (NGS), although few studies have examined the effects of antimicrobials. We evaluated the effect of antibiotics florfenicol and erythromycin and infection with or without Flavobacterium psychrophilum on the intestinal microbiota in rainbow trout juveniles (30–40 g) using NGS. Prophylactic oral antibiotic treatments were administered for 10 days before groups of fish were injected intraperitoneally with virulent F. psychrophilum. Intestinal content (allochthonous bacteria) was collected at day −11, 0, 12, and 24 p.i., and the v3–v4 region of the 16S rRNA gene was sequenced using Illumina MiSeq. Before prophylactic treatment, Tenericutes and Proteobacteria were the most abundant phyla identified and Mycoplasma was the most abundant genus. Fish infected with F. psychrophilum had decreased alpha diversity and a high abundance of Mycoplasma. Fish administered florfenicol had increased alpha diversity compared to the control at day 24 p.i., although both florfenicol and erythromycin-treated fish had a higher abundance of potential pathogens, specifically Aeromonas, Pseudomonas, and Acinetobacter. Mycoplasma disappeared after treatment but appeared again after day 24. This study demonstrates that prophylactic oral treatment with antibiotics florfenicol and erythromycin as well as F. psychrophilum infection changed the composition of intestinal microbiota in rainbow trout juveniles that did not recover by day 24 p.i. and further long-term effects on the host need to be investigated. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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28 pages, 7699 KiB

Open AccessEditor’s ChoiceArticle

Genomics of Tenacibaculum Species in British Columbia, Canada

by Joseph P. Nowlan, Ashton N. Sies, Scott R. Britney, Andrew D. S. Cameron, Ahmed Siah, John S. Lumsden and Spencer Russell

Pathogens 2023, 12(1), 101; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens12010101 - 06 Jan 2023

Cited by 4 | Viewed by 2389

Abstract

Tenacibaculum is a genus of Gram-negative filamentous bacteria with a cosmopolitan distribution. The research describing Tenacibaculum genomes stems primarily from Norway and Chile due to their impacts on salmon aquaculture. Canadian salmon aquaculture also experiences mortality events related to the presence of Tenacibaculum spp., yet no Canadian Tenacibaculum genomes are publicly available. Ribosomal DNA sequencing of 16S and four species-specific 16S quantitative-PCR assays were used to select isolates cultured from Atlantic salmon with mouthrot in British Columbia (BC), Canada. Ten isolates representing four known and two unknown species of Tenacibaculum were selected for shotgun whole genome sequencing using the Oxford Nanopore’s MinION platform. The genome assemblies achieved closed circular chromosomes for seven isolates and long contigs for the remaining three isolates. Average nucleotide identity analysis identified T. ovolyticum, T. maritimum, T. dicentrarchi, two genomovars of T. finnmarkense, and two proposed novel species T. pacificus sp. nov. type strain 18-2881-A^T and T. retecalamus sp. nov. type strain 18-3228-7B^T. Annotation in most of the isolates predicted putative virulence and antimicrobial resistance genes, most-notably toxins (i.e., hemolysins), type-IX secretion systems, and oxytetracycline resistance. Comparative analysis with the T. maritimum type-strain predicted additional toxins and numerous C-terminal secretion proteins, including an M12B family metalloprotease in the T. maritimum isolates from BC. The genomic prediction of virulence-associated genes provides important targets for studies of mouthrot disease, and the annotation of the antimicrobial resistance genes provides targets for surveillance and diagnosis in veterinary medicine. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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19 pages, 5106 KiB

Open AccessArticle

Phaeohyphomycosis due to Exophiala in Aquarium-Housed Lumpfish (Cyclopterus lumpus): Clinical Diagnosis and Description

by Colin T. McDermott, Charles J. Innis, Akinyi C. Nyaoke, Kathryn A. Tuxbury, Julie M. Cavin, E. Scott Weber, Deana Edmunds, Stéphane Lair, Jill V. Spangenberg, Amy L. Hancock-Ronemus, Catherine A. Hadfield, Leigh A. Clayton, Thomas B. Waltzek, Connie F. Cañete-Gibas, Nathan P. Wiederhold and Salvatore Frasca, Jr.

Pathogens 2022, 11(12), 1401; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens11121401 - 23 Nov 2022

Viewed by 2182

Abstract

Phaeohyphomycosis caused by Exophiala species represents an important disease of concern for farmed and aquarium-housed fish. The objective of this study was to summarize the clinical findings and diagnosis of Exophiala infections in aquarium-housed Cyclopterus lumpus. Clinical records and postmortem pathology reports were reviewed for 15 individuals from 5 public aquaria in the United States and Canada from 2007 to 2015. Fish most commonly presented with cutaneous ulcers and progressive clinical decline despite topical or systemic antifungal therapy. Antemortem fungal culture of cutaneous lesions resulted in colonial growth for 7/12 samples from 8 individuals. Amplification of the internal transcribed spacer region (ITS) of nuclear rDNA identified Exophiala angulospora or Exophiala aquamarina in four samples from three individuals. Postmortem histopathologic findings were consistent with phaeohyphomycosis, with lesions most commonly found in the integument (11/15), gill (9/15), or kidney (9/15) and evidence of fungal angioinvasion and dissemination. DNA extraction and subsequent ITS sequencing from formalin-fixed paraffin-embedded tissues of seven individuals identified E. angulospora, E. aquamarina, or Cyphellophora sp. in four individuals. Lesion description, distribution, and Exophiala spp. identifications were similar to those reported in farmed C. lumpus. Antemortem clinical and diagnostic findings of phaeohyphomycosis attributable to several species of Exophiala provide insight on the progression of Exophiala infections in lumpfish that may contribute to management of the species in public aquaria and under culture conditions. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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18 pages, 3385 KiB

Open AccessArticle

A First Report of Molecular Typing, Virulence Traits, and Phenotypic and Genotypic Resistance Patterns of Newly Emerging XDR and MDR Aeromonas veronii in Mugil seheli

by Abdelazeem M. Algammal, Reham A. Ibrahim, Khyreyah J. Alfifi, Hanaa Ghabban, Saad Alghamdi, Ahmed Kabrah, Ahmed R. Khafagy, Gehan M. Abou-Elela, Nermeen M. Abu-Elala, Matthew Gavino Donadu and Reham M. El-Tarabili

Pathogens 2022, 11(11), 1262; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens11111262 - 29 Oct 2022

Cited by 21 | Viewed by 1966

Abstract

Aeromonas veronii is associated with substantial economic losses in the fish industry and with food-borne illness in humans. This study aimed to determine the prevalence, antibiogram profiles, sequence analysis, virulence and antimicrobial resistance genes, and pathogenicity of A. veronii recovered from Mugil seheli. A total of 80 fish were randomly gathered from various private farms in Suez Province, Egypt. Subsequently, samples were subjected to clinical, post-mortem, and bacteriological examinations. The retrieved isolates were tested for sequence analysis, antibiogram profile, pathogenicity, and PCR detection of virulence and resistance genes. The prevalence of A. veronii in the examined M. seheli was 22.5 % (18/80). The phylogenetic analyses revealed that the tested A. veronii strains shared high genetic similarity with other A. veronii strains from India, UK, and China. Using PCR it was revealed that the retrieved A. veronii isolates harbored the aerA, alt, ser, ompAII, act, ahp, and nuc virulence genes with prevalence of 100%, 82.9%, 61.7%, 55.3%, 44.7%, 36.17%, and 29.8%, respectively. Our findings revealed that 29.8% (14/47) of the retrieved A. veronii strains were XDR to nine antimicrobial classes and carried bla_TEM, bla_CTX-M, bla_SHV,tetA, aadA1, and sul1 resistance genes. Likewise, 19.1% (9/47) of the obtained A. veronii strains were MDR to eight classes and possessed bla_TEM, bla_CTX-M, bla_SHV,tetA, aadA1, and sul1 genes. The pathogenicity testing indicated that the mortality rates positively correlated with the prevalence of virulence-determinant genes. To our knowledge, this is the first report to reveal the occurrence of XDR and MDR A. veronii in M. seheli, an emergence that represents a risk to public health. Emerging XDR and MDR A. veronii in M. seheli frequently harbored aerA, alt, ser, ompAII, and act virulence genes, and bla_TEM, sul1, tetA, bla_CTX-M, bla_SHV, and aadA1 resistance genes. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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22 pages, 7132 KiB

Open AccessArticle

Prospective Longitudinal Study of Putative Agents Involved in Complex Gill Disorder in Atlantic salmon (Salmo salar)

by Ana Herrero, Hamish Rodger, Adam D. Hayward, Chris Cousens, James E. Bron, Mark P. Dagleish and Kim D. Thompson

Pathogens 2022, 11(8), 878; https://0-doi-org.brum.beds.ac.uk/10.3390/pathogens11080878 - 03 Aug 2022

Viewed by 1870

Abstract

Complex gill disorder (CGD) is an important condition in Atlantic salmon aquaculture, but the roles of the putative aetiological agents in the pathogenesis are uncertain. A longitudinal study was undertaken on two salmon farms in Scotland to determine the variations in loads of CGD-associated pathogens (Desmozoon lepeophtherii, Candidatus Branchiomonas cysticola, salmon gill pox virus (SGPV) and Neoparamoeba perurans) estimated by quantitative PCR. In freshwater, Ca. B. cysticola and SGPV were detected in both populations, but all four pathogens were detected on both farms during the marine stage. Candidatus B. cysticola and D. lepeophtherii were detected frequently, with SGPV detected sporadically. In the marine phase, increased N. perurans loads associated significantly (p < 0.05) with increases in semi-quantitative histological gill-score (HGS). Increased Ca. B. cysticola load associated significantly (p < 0.05) with increased HGS when only Farm B was analysed. Higher loads of D. lepeophtherii were associated significantly (p < 0.05) with increased HGS on Farm B despite the absence of D. lepeophtherii-type microvesicles. Variations in SGPV were not associated significantly (p > 0.05) with changes in HSG. This study also showed that water temperature (season) and certain management factors were associated with higher HGS. This increase in histological gill lesions will have a deleterious impact on fish health and welfare, and production performance. Full article

(This article belongs to the Special Issue Emerging Infections in Aquatic Animals)

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