Aquaculture in Argentina is undergoing growth and development. Rainbow trout (Oncorhynchus mykiss ) is the main farmed species, with a production of 4,080 tons in 2023, concentrated in the Piedra del Águila and Alicurá reservoirs (Neuquén and Río Negro provinces). Although these environments are recognized by SENASA as free from WOAH -notifiable diseases affecting salmonids, certain infectious agents can negatively impact production. Aeromonas hydrophila and Plesiomonas shigelloides are Gram-negative bacteria widely distributed in aquatic environments, recognized as opportunistic pathogens in fish and with zoonotic potential, capable of causing gastrointestinal, cutaneous, or systemic infections in humans. This study aims to report the detection of these bacterial species from farmed rainbow trout in Neuquén province, Argentina.
As part of health monitoring activities conducted by the Centro de Ecología Aplicada del Neuquén (CEAN) in trout farms from the province of Neuquén , fish presenting fin and operculum lesions, localized hemorrhages, and in some cases cutaneous ulcers were sampled and subjected to anatomical-pathological examination. Kidney samples were cultured on TSA and TYES agar and incubated at 15 °C for 7 days. Isolates were examined by Gram staining, oxidase and catalase tests, motility assessment, and biochemical identification using API® 20E. Flavobacterium psychrophilum was confirmed by PCR using primers fpPPIC1F/fpPPIC1R.
No significant mortality was observed in the monitored stocks at the time of sampling. From fish in two farms, cream-colored colonies developed on TSA , consisting of Gram-negative rods, oxidase- and catalase-positive, motile, and identified as A. hydrophila by API® 20E (codes 3047525 and 3047147, the latter presumptive). In one farm, P. shigelloides was detected, forming brown-grayish colonies on TSA, also Gram-negative rods, oxidase- and catalase-positive, motile, identified by API® 20E (code 7144004). In all cases, F. psychrophilum co-occurred, producing orange-yellow colonies on TYES, long Gram-negative rods with gliding motility, and PCR-positive results.
The detection of A. hydrophila and P. shigelloides in farmed rainbow trout in Argentina constitutes a novel finding. In both cases, co-infections with F. psychrophilum, responsible for bacterial cold-water disease (BCWD) and rainbow trout fry syndrome (RTFS) in salmonid aquaculture worldwide , were recorded, suggesting pathogenic interactions that could exacerbate mortality events. The presence of these bacteria, even in the absence of acute disease outbreaks , poses a latent risk to trout health—particularly under stress or unfavorable environmental conditions—and to human health. These findings highlight the need to strengthen health monitoring programs, implement biosecurity measures, and promote the surveillance of zoonotic pathogens under a One Health approach.