Emergence, identification, and characterization of a novel Streptococcus agalactiae CPS type Ia ST7 strain (Ia-2021) causing large scale mortalities in tilapia aquaculture

In 2021 streptococcosis caused by Streptococcus agalactiae capsular polysaccharide (CPS) type Ia ST7 (referred to as Ia-2021) emerged in Mexico and spread to other regions in Central and South America devastating the industries in the affected countries. To address this emergent issue, the present research aimed to: (1) document case reports of the disease, (2) reproduce clinical signs under laboratory conditions, (3) use comparative genomics to investigate the evolutionary history of the emergent strain, and (4) develop an assay to discriminate from other S. agalactiae. Experimental injection challenges recapitulated clinical signs and gross lesions of disease observed in the field including erratic swimming, external hemorrhages, pale livers, hemorrhaging and necrosis of the intestine, and pustules. Whole genome sequencing, comparative bacterial genomics, and phylogenetic analyses yielded a DNA target suitable for a discriminatory PCR assay, which reliably differentiated the CPS type Ia-2021 isolates from other CPS type Ia isolates. These new primers were included in a previously established multiplex PCR (mPCR) for S. agalactiae CPS typing. The validated mPCR provides a "one-stop shop" for S. agalactiae CPS typing and can reliably assign unknown isolates to one of the ten CPS types, as well as discriminate between the Ia-2021 strain from other Ia isolates. This assay fills a critical need in epidemiological surveillance in tilapia aquaculture and provides an important tool to ensure appropriate CPS types are incorporated into vaccines. The work presented here is a critical first step to accelerate research to address this emergent disease and develop practical solutions for tilapia farmers to prevent streptococcosis caused by S. agalactiae CPS type Ia-2021.