A commercial rapid optical immunoassay detects Streptococcus agalactiae from aquatic cultures and clinical specimens

The BioStar STREP B Optical ImmunoAssay (STREP B OIA) (BioStar® OIA® Strep B Assay Kit; BioStar Incorporation, Louisville, CO, USA), commonly used for diagnosis of human maternal group B streptococcus (GBS) colonization, was evaluated for its diagnostic and analytical sensitivity and specificity to aquatic animal GBS isolates, cross-reactivity, and diagnosis and recovery of GBS directly from clinically- infected fish swabs. STREP B OIA identified 25 known fish and dolphin GBS isolates. Thirteen non-GBS negative control isolates from fish and other animals were negative, giving 100% analytical specificity and no cross-reactivity. Three groups of 6 Nile tilapia (Oreochromis niloticus) (mean weight of 40.60 ± 1.70 g) each were inoculated intraperitoneally with either 106 colony-forming units (cfu) GBS/fish, 106 cfu Streptococcus iniae/fish or 100 μL of tryptic soy broth (TSB) and observed for mortality for 7 days. The nare and brain of all fish were swabbed and subjected to the STREP B OIA for detection of GBS antigen immediately after swabbing (0 h) or 24, 48 and 72 h post-swabbing and compared to conventional culture on trypticase soy agar with 5% sheep blood. The STREP B OIA method demonstrated a diagnostic sensitivity of 75.0% and a diagnostic specificity of 69.2% compared to direct TSA. The percent agreement between OIA and culture was 100%. GBS antigen could be retrieved by OIA following 72-h storage of swabs. These results demonstrate the utility of the STREP B OIA to identify GBS from culture and directly from swabs of clinically- infected fish.