Serotypes and Virulence Gene Profiles of Streptococcus agalactiae Isolates From Non-Tilapia Freshwater Fish

Streptococcus agalactiae is a multi-host pathogen and a major threat to aquaculture, particularly tilapia production. This study characterised the serotype distribution and virulence gene profiles of 97 S. agalactiae isolates obtained from non-tilapia freshwater species in Vietnam, including carp species not previously recognised as hosts, namely black carp, grass carp and common carp. Three serotypes were identified among non-tilapia isolates, Ia (44.3%), III (39.2%) and Ib (16.5%). Virulence gene distributions varied substantially within and between serotypes; however, all isolates consistently carried the fbsA and cfb genes. Distinct serotype-associated gene patterns were observed. Serotype Ia isolates possessed fbsA, fbsB, bca, hylB and cfb but lacked lmb and spb1-F, whereas serotype III isolates harboured pavA, cylE, fbsA, lmb, scpB, cfb and cspA but lacked bac. Serotype Ib isolates carried pavA, cylE, fbsA, hylB, cfb and cspA, while spb1-F and bac were absent. Overall, this study identifies additional non-tilapia hosts of S. agalactiae in freshwater aquaculture provides the first evidence of infection in black carp, grass carp and common carp. These findings demonstrate the diversity of circulating serotypes and virulence gene profiles. These findings indicate increasing epidemiological complexity and highlight the need for integrated prevention and disease management strategies targeting this pathogen.