Plesiomonas shigelloides as an Emerging Pathogen in Catfish Aquaculture: A Case from a South Texas Commercial Farm

During the summer of 2023, a spontaneous disease outbreak occurred in intensively stocked hybrid catfish (♀ channel catfish, <i>Ictalurus punctatus</i> × ♂ blue catfish, <i>I. furcatus</i>) in earthen ponds on a commercial aquaculture farm in South Texas. The farmer reported 50 to 80 dead fish per pond daily for a month. The fish were market size (1.0 ± 0.3 kg on average), resulting in substantial economic losses. Fifteen moribund fish were submitted for laboratory examination. Grossly, the fish showed distended abdomens, erythematous fins, and inflamed vents. Autopsy demonstrated visceral congestion, distended gastrointestinal tracts, and serosanguineous peritoneal effusion. Bacterial cultures from the internal organs revealed homogeneous bacterial growth after incubation. Presumptive biochemical characterization of the isolated bacteria identified <i>Plesiomonas shigelloides</i>. Further molecular confirmation was achieved by species-specific PCR amplification and 16S-rRNA sequencing. Juvenile catfish were experimentally challenged with the recovered isolates to fulfill Koch's postulates. Moreover, an antibiogram was performed to evaluate the susceptibility of the isolates to a panel of FDA-approved antimicrobials. <i>P. shigelloides</i> isolates were pathogenic to channel catfish and alarmingly multidrug-resistant. We report here, for the first time, <i>P. shigelloides</i> infection in Texas commercial catfish aquaculture, emphasizing its significance as an emerging enteric pathogen that is difficult to treat with FDA-approved antimicrobials.