Molecular surveillance and pathogenicity assessment of Lactococcus garvieae and Streptococcus iniae in Rainbow trout

Document Type : Original Article

Authors
Ali Shirazinezhad 1
Hossein Khaj 2
Seyyed Mohammad Hossein Hosseini 1
Fatemeh Dabiri 1
Hajar Molaee 1
Yahya Tahamtan 1
Masoumeh Hayati 1
1 Department of Microbiology, Shiraz Branch, Razi Vaccine and Serum Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Shiraz, Iran.
2 Department of Immunology, Shiraz Branch, Razi Vaccine and Serum Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Shiraz, Iran.
10.22034/zmm.2025.736067
Abstract
Background & Objectives: Streptococcosis, primarily caused by Streptococcus iniae and Lactococcus garvieae, causes high mortality and economic losses in rainbow trout (Oncorhynchus mykiss) worldwide. The disease is systemic, associated with septicemia and hemorrhages, and vaccination is the most effective preventive strategy. Since successful vaccination depends on highly virulent strains, this study isolated and characterized streptococcal pathogens from diseased fish to select a potent vaccine candidate.
Materials & methods: In this experimental study, 111 clinical samples were collected from rainbow trout with clinical signs of streptococcosis from commercial fish farms. Identification was conducted via experimental culture methods, biochemical profiling and 16S rRNA gene PCR were used for bacterial characterization. In vivo virulence was assessed by determining the median lethal dose (LD₅₀) in rainbow trout.
Results: Bacteriological analysis of clinical samples revealed that 95% of isolates were gram-positive cocci by staining. Molecular techniques confirmed 37.84% of isolates as Lactococcus garvieae (1100 bp product) and 18.92% as Streptococcus iniae (300 bp product), while 43.24% of isolates yielded no identifiable L. garvieae or S. iniae. Virulence assessment via the Reed and Muench method revealed LD₅₀ values of 10²·⁴¹ for L. garvieae isolate LG001 and 10²·⁸³ for S. iniae isolate SE001, indicating high pathogenicity in both isolates.
Conclusion: This study provides essential data for the selection of a highly virulent streptococcal strain as a vaccine candidate and offers a scientific basis for developing an effective vaccine to control streptococcosis in rainbow trout, thereby reducing economic losses in aquaculture and minimizing zoonotic risks.
Keywords
Lactococcus garvieae
LD₅₀
Rainbow trout
Streptococcus iniae
Zand Molecular Microbiology
Volume 1, Issue 2
Autumn 2025
Pages 88-97