Strep by Strep: Characterizing Novel Agents of Piscine Streptococcosis in the Americas

1Taylor I. Heckman, 1Diem Thu Nguyen, 1Eric Littman, 2Eileen E. Henderson, 3Adrián López‐Porras, 4Alejandro Perretta, 1Alexander Chow, 5Hasan C. Tekedar, 6Benjamin R. LaFrentz, 7John P. Hawke, 8Dave Marancik, 9Hugh Mitchell, 6Craig Shoemaker, 10Ruben Avendaño-Herrera, 11Cesar Ortega, 12Ulisses Pereira, 13Bill Keleher, 14Guillermo Risatti, 15Michael W. Hyatt, 14Salvatore Frasca, Jr., 16Kaveramma Mukkatira, 16Mark Adkison, 17Timothy J. Welch, 18Geoffrey C. Waldbieser, 5Matt J. Griffin, 1Esteban Soto

1Department of Medicine & Epidemiology, School of Veterinary Medicine, University of California, Davis, CA, USA; 2California Animal Health and Food Safety Lab, School of Veterinary Medicine, University of California, Davis, CA, USA; 3 Department of Biosciences, UiO, Norway; 4Instituto de Investigaciones Pesqueras, Universidad de la República, Uruguay; 5College of Veterinary Medicine, Mississippi State University, MS, USA; 6USDA-ARS, Aquatic Animal Health Research Unit, AL, USA; 7Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, LA, USA; 8Department of Pathobiology, School of Veterinary Medicine, St. George’s University, Grenada; 9AquaTactics Fish Health, WA, USA; 10Facultad de Ciencias Biológicas and Interdisciplinary Center for Aquaculture Research (INCAR), Universidad Andrés Bello, Chile; 11Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma del Estado de México, México; 12Departamento de Medicina Veterinária Preventiva, Universidade Estadual de Londrina, Brazil; 13Kennebec River Biosciences, ME, USA;  14Connecticut Veterinary Medical Diagnostic Laboratory, Department of Pathobiology and Veterinary Science, University of Connecticut, CT, USA; 15Wildlife Conservation Society, New York Aquarium, NY, USA; 16California Department of Fish and Wildlife, CA, USA; 17USDA-ARS, National Center for Cool and Coldwater Aquaculture, WV, USA; 18USDA-ARS, Warmwater Aquaculture Research Unit, MS, USA

ABSTRACT

Piscine streptococcosis was first described in 1957 in Japanese cultured rainbow trout (Oncorhynchus mykiss) and has since been established as a devastating disease of wild and farmed fish worldwide. Outbreaks are estimated to cost the global aquaculture industry billions of dollars annually, and development of sustainable control strategies is complicated by the diverse nature of the responsible pathogens. Streptococcus iniae and S. agalactiae are the primary agents of piscine streptococcosis, but S. dysgalactiae and Lactococcus garvieae have also emerged as pathogens of concern in American fisheries and aquaculture. Effective management of streptococcal outbreaks requires definitive diagnosis of the causal agent, but biochemical tests and single gene sequencing techniques can lead to erroneous identifications in closely related species, and often lack resolution to the strain level. A multilocus sequence analysis (MLSA) scheme was developed to better discriminate between species and strains of piscine streptococcal pathogens in a reproducible and disseminable manner. This MLSA scheme was applicable in the genotyping of clinical isolates of S. iniae, S. agalactiae, and S. dysgalactiae from recent outbreaks in fish species across North and South America. MLSA revealed novel clades of fish-associated S. dysgalactiae that diverged from the established, terrestrial, subspecies of S. dysgalactiae – S. dysgalactiae subsp. dysgalactiae and S. dysgalactiae subsp. equisimilis. Representative isolates from these two fish-associated clades revealed associations between genetic group and host-specific virulence in intramuscular and intracoelomic challenges of white sturgeon (Acipenser transmontanus) and Nile tilapia (Oreochromis niloticus). The MLSA scheme was further adapted to type L. garvieae. A pared down version successfully differentiated isolates from healthy and diseased fish from North America, with differences in clade virulence observed between representative L. garvieae isolates in an intracoelomic challenge of rainbow trout (O. mykiss). Continuing research to elucidate the genetic basis for such strain heterogeneity will be crucial for reducing the impact of these pathogens as they continue to expand in host and geographic range.