Disease outbreaks in farmed Amazon catfish (Leiarius marmoratus x Pseudoplatystoma corruscans) caused by Streptococcus agalactiae, S. iniae, and S. dysgalactiae

The Amazon catfish is one of the main farm-raised native fish in Brazil. In spite of its importance, the main health-associated issues have been poorly characterized in these fish. In this study, we describe outbreaks of S. agalactiae, S. iniae, and S. dysgalactiae in Amazon catfish farms in detail. The genetic diversity and antibiotic resistance patterns of different isolates were also evaluated. Thirty-five diseased fish were sampled from four commercial farms, and were subjected to bacteriological examinations. The isolates were genotyped by REP-PCR, and antimicrobial susceptibility of six antibiotics was evaluated using the disc diffusion assay. Fifteen Streptococcus spp. isolates were identified (S. agalactiae = 10 strains; S. dysgalactiae = 1; S. iniae = 4), which were further confirmed by MALDI-TOF. S. agalactiae and S. dysgalactiae infections were successfully achieved under experimental conditions, which confirmed Koch's postulates. Three distinct genetic patterns were verified by REP-PCR for S. agalactiae; S. iniae isolates were indistinguishable by this technique. In antimicrobial susceptibility tests, all S. agalactiae isolates were classified as wild-types to erythromycin (ERY); S. agalactiae strains, non-wild-types to amoxicillin (n = 2 isolates), florfenicol (n = 4), norfloxacin (n = 2), oxytetracycline (n=3) (OXY), and trimethoprim-sulfamethoxazole (n = 6) (SXT); S. dysgalactiae strain, non-wild-type to OXY and ERY; and S. iniae isolates, non-wild-types to both SXT (n = 1) and OXY (n = 3). This study represents the first description of Streptococcus iniae, S. dysgalactiae, and S. agalactiae serotype II infections in farm-raised Amazon catfish. In addition, it is the first study that describes OXY resistant S. agalactiae isolates from farmed native fish in Brazil.