Streptococcus dysgalactiae subsp. dysgalactiae from farm-raised channel catfish in Mississippi, USA, with notes on fish associated Group C Streptococci

Authors: BALAMI, SUJITA - Mississippi State University; KHOO, LESTER - Mississippi State University; Older, Caitlin; Waldbieser, Geoffrey; CAMUS, ALVIN - Mississippi State University; HECKMAN, TAYLOR - Mississippi State University; WARE, CYNTHIA - Mississippi State University; Richardson, Bradley; HAWKE, JOHN - Louisiana State University; LOPEZ-PORRAS, ADRIAN - University Of Oslo; SOTO, ESTEBAN - University Of California, Davis; Shoemaker, Craig; LAWRENCE, MARK - Mississippi State University; PETERMAN, ANN - Mississippi State University; PETERMAN, MARK - Mississippi State University; HANSON, LARRY - Mississippi State University; GRIFFIN, MATT - Mississippi State University

Submitted to: Journal of Fish Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2025
Publication Date: 5/30/2025
Citation: Balami, S., Khoo, L.H., Older, C.E., Waldbieser, G.C., Camus, A.C., Heckman, T.I., Ware, C.C., Richardson, B.M., Hawke, J.P., Lopez-Porras, A., Soto, E., Shoemaker, C.A., Lawrence, M.L., Peterman, A.E., Peterman, M.A., Hanson, L.A., Griffin, M.J. 2025. Streptococcus dysgalactiae subsp. dysgalactiae from farm-raised channel catfish in Mississippi, USA, with notes on fish associated Group C Streptococci. Journal of Fish Diseases. e00080-25. https://doi.org/10.1111/jfd.14166.
DOI: https://doi.org/10.1111/jfd.14166

Interpretive Summary: Lancefield serological group C Streptococcus dysgalactiae subspecies dysgalactiae (GCSD) is an emerging cause of fish disease. GCSD have primarily been associated with Asian mariculture, but recent cases in wild and cultured fish in the United States have been reported. Researchers at Mississippi State University in collaboration with scientists from the USDA ARS Warmwater Aquaculture Research Unit, Louisiana State University, University of Oslo, University of California, and USDA ARS Aquatic Animal Health Research Unit conducted research into GCSD isolated from wild fish, farmed Nile tilapia, and farmed channel catfish in the United States. Phylogenetic and genomic analyses were conducted to understand the similarity of the isolates obtained from fish affected in the U.S. with those previously reported from fish in other countries, as well as GCSD collected from human and other terrestrial animals. GCSD obtained from catfish grouped with other fish isolates, which were distinct from human- and other animal-associated isolates. Genomic analyses suggested the GCSD obtained from catfish may represent a novel subspecies. Collectively, these results indicate the range of GCSD is expanding, and should be further researched as a potential threat to U.S. aquaculture.

Technical Abstract: Lancefield serological group C Streptococcus dysgalactiae (GCSD) is an emerging cause of fish disease worldwide, largely associated with mariculture in Asia. In the United States (US), GCSD had been a pathogen of minimal concern for fish, but recent cases indicate a putative emergence in wild and cultured freshwater fish populations in the Americas. The current study discusses three novel cases of GCSD-associated streptococcosis in US farm-raised channel catfish (Ictalurus punctatus) occurring in Mississippi, US, in 2022 and 2023. Infected fish presented with pendulous abdomens, red swollen vents, and petechiae on the mouth, fins and ventral lateral abdomen. Clinical isolates were cultured from the kidney and brain and initially confirmed as GCSD by 16S rRNA gene sequencing, followed by phylogenetic analyses using the superoxide dismutase gene (sodA), 16S/23S internal transcribed spacer region and nine genes within a multilocus sequence analysis scheme for piscine pathogenic Streptococcus. Catfish GCSD were highly homologous to fish strains from published reports from Asia and South America, forming a discrete phylogenetic clade, separate from human and other terrestrial animal isolates. Pairwise genomic comparisons suggest this fish-associated group does not belong to any currently recognised subspecies and may represent a unique subtype. The broadening host and geographic range of GCSD, coupled with the evidence of a discrete aquatic lineage and recent reports from freshwater fishes in the southeastern US, suggest GCSD is a potential emergent threat to catfish aquaculture in the US.