Program Overview: Parasite-Host Interaction (PHin)Laboratory

Sascha L. Hallett

Department of Microbiology, 226 Nash Hall, Oregon State University, Corvallis, Oregon 97331

The Salmonid Oncorhynchus mykiss and Bryozoan Fredericella indica Serve as Active Hosts for the North American Strain of Tetracapsuloides bryosalmonae, the Cause of Proliferative Kidney Disease

Nilanjana Das, Stephen D. Atkinson, Sascha L. Hallett

Parasite-Host Interactions Laboratory, Department of Microbiology, 226 Nash Hall, Oregon State University, Corvallis, Oregon 97331

The myxozoan parasite responsible for Proliferative Kidney Disease (PKD), Tetracapsuloides bryosalmonae (T. bryo), alternates between salmonid fishes and freshwater bryozoans to complete its life cycle. However, no active hosts (species that shed viable, infectious spores) for the North American (NA) strain have been confirmed. We used fish-to-bryozoan and bryozoan-to-fish transmission experiments to assess the active host status of steelhead trout (Oncorhynchus mykiss) and the bryozoan Fredericella indica. Specific-pathogen-free F. indica colonies were first exposed to effluent from naturally infected O. mykiss and monitored weekly using stereomicroscopy for overt T. bryo infections. Malacospores from infected colonies were then introduced to naïve O. mykiss, which were held at 16˚C and sampled for PCR and histology 2 and 8 wks post-exposure. Additionally, statoblasts and hatched zooids from overtly infected F. indica colonies were screened with stereomicroscopy and qPCR to investigate transmission of T. bryo from zooids to statoblasts. Our results demonstrate that subclinically infected O. mykiss shed viable fish malacospores of the NA strain of T. bryo, as evidenced by the development of overt infections in exposed F. indica. Malacospores shed from the bryozoan subsequently induced clinical PKD in naive O. mykiss, which exhibited kidney swelling upon necropsy, extrasporogonic stages of the parasite in histology, and PCR detections of T. bryo DNA. We found no evidence of vertical transmission within F. indica. Given the widespread rearing and stocking of O. mykiss across North America, these findings highlight the importance of pre-release surveillance in susceptible stocks to prevent dissemination of T. bryo in habitats where infected, active host populations may overlap.

Whole genome sequencing and characterization of virulence and antimicrobial resistance genes in Vibrio harveyi from a marine environment

Ivana Giovanna Zupičić1, Dražen Oraić1, Krešimir Križanović2, Snježana Zrnčić1

1Laboratory for aquatic animal diseases, Croatian veterinary institute, Croatia; 2 Department of Electronic Systems and Information Processing, Faculty of Electrical Engineering and Computing, University of Zagreb, Croatia

Vibrio harveyi is a naturally occurring marine bacterium that has become a significant pathogen of both wild and cultured marine fish and invertebrates. Although vibriosis caused by V. harveyi has been reported for several decades, the bacterium is now globally distributed and has caused mortalities in economically valuable fish species, including in the USA. Over the past fifteen years, outbreaks of vibriosis in Mediterranean aquaculture, particularly affecting European seabass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata), have emerged as a serious economic concern. In this study, sixteen V. harveyi strains were isolated from diseased seabass and sea bream collected from aquaculture facilities in six Mediterranean countries: Croatia, France, Italy, Spain, Tunisia, and Turkey. Whole genome sequencing was performed using ONT MinION and Illumina HiSeq platforms, followed by genome assembly and analysis with established bioinformatics tools. Virulence factors were identified using the VFDB database, while antimicrobial resistance genes (ARGs) and mechanisms were determined using the RGI tool. The genomic findings were compared with results from in vitro susceptibility testing. Out of 171 identified virulence genes, each strain harbored on average 150. Notably, several strains carried sitA (4/16), sitB (3/16), sitC (4/16), sitD (3/16), and the ast gene, which is absent from other Vibrio species in the referenced database, suggesting their potential role as atypical virulence markers. Additionally, five antimicrobial resistance genes were identified across the genomes: CRP, adeF, E. coli parE, APH(3'')Ib, and tet(D). Phylogenetic analysis revealed that the strains clustered into a distinct and closely related group. The presence of virulence and resistance genes did not correlate with the geographic origin of the isolates, possibly due to the semi-enclosed nature of the Mediterranean basin and frequent translocation of seabass and sea bream of different age groups across regions. These findings highlight the potential for region-wide dissemination of genetically similar pathogenic strains and support the need for a unified regional strategy for disease prevention and control.

Seasonal dynamics and spatial distribution of Tetracapsuloides bryosalmonae in the Great Lakes basin

Fusianto C.K.1, Lamichhance S.1, Ahmad Ali S.1 Tattiyapong P.1,2, Eisch E.3, Gorgoglione B.1

1 Fish Pathobiology and Immunology Laboratory, Dept. Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, Michigan State University, East Lansing, MI, USA; 2 Department of Veterinary Microbiology and Immunology, Faculty of Veterinary Medicine, Kasetsart University, Bangkok, Thailand; 3 Fisheries Division, Michigan Department of Natural Resources

Salmonids are restocked in the Great Lakes watershed to maintain a consistent recreational fishing industry and for balanced ecosystem dynamics. The myxozoan parasite Tetracapsuloides bryosalmonae (Malacosporea) may cause Proliferative Kidney Disease (PKD), a severe chronic lymphoid immunopathology, threatening susceptible salmonids in Europe and Northwest America. We are investigating the spatial and seasonal prevalence of T. bryosalmonae in Michigan, opportunistically sampling migratory Chinook and Coho Salmon from DNR-operated fish weirs during seasonal spawning migrations. In October 2023 and 2024 up to 60 fish per stock were sampled from fish weirs installed on both Lake Michigan and Lake Huron sides of Michigan. DNA was extracted from individual posterior kidneys and tested by qPCR specifically targeting T. bryosalmonae 18S rDNA. Positive samples were processed by conventional PCR to retrieve larger genomic sequences from the malacosporean SSU rDNA and CO1 for sequence confirmatory analysis. Our examination revealed a variable infection prevalence across sampled stocks and locations, although most of the specimens were asymptomatic (no clinical signs of PKD) and with a low parasite burden (10^3/10^5/g kidney). Sequence analysis confirmed a sensitive detection of T. bryosalmonae DNA from these salmonid species and revealed some strain diversity. These findings revealed that T. bryosalmonae may be widespread in the Great Lakes watershed, where host availability and environmental conditions are becoming more suitable for completing its two-host lifecycle. We provide important information for the management of natural population restoking in the Great Lakes watershed.