Marco Rozas-Serri

Introduction: Brazil is among the world’s largest producers of Nile tilapia ( Oreochromis niloticus ), a species increasingly challenged by a complex of cocirculating pathogens that threatens production sustainability. Methods: We conducted a structured epidemiological surveillance program across the three main tilapia-producing macro-regions of Brazil (Southeast, Central-West, and Northeast) between 2023 and 2025. A total of 26,057 diagnostic samples, generating 68,514 pathogen-specific tests, were analyzed for Aeromonas spp., Edwardsiella spp., Flavobacterium spp., Francisella orientalis , Infectious Spleen and Kidney Necrosis Virus (ISKNV), Lactococcus spp., and Streptococcus spp. using standardized bacteriological, real-time PCR, and MALDI-TOF MS protocols, and antimicrobial susceptibility (minimum inhibitory concentration, MIC) was determined for four antimicrobials. Generalized linear models (with a farm-clustered mixed-model sensitivity analysis), time-series analysis (ADF stationarity, ARIMA), co-detection network analysis, and Gamma-log MIC modelling were applied. Results: The cumulative overall detection positivity rate (DPR) was 18.1%, with Flavobacterium spp. showing the highest cumulative positivity (21.6%) on the smallest testing base, and Lactococcus spp. (18.4%), ISKNV (23.3%), and Streptococcus spp. (15.2%) representing the dominant, most widely sampled agents; S. agalactiae serotype Ia was not detected by serotype-specific qPCR. Detection was structured principally by macro-region and pathogen-specific temporal dynamics, whereas host developmental stage exerted a heterogeneous, generally weak influence. Five of the seven pathogens showed stationary, endemically compatible dynamics; Streptococcus spp. was non-stationary, consistent with active geographic expansion into the Northeast, whereas non-stationarity in Flavobacterium spp. reflected sampling instability rather than directional change. Quarterly aggregation minimized forecast error among the frequencies evaluated. Co-detection was predominantly independent, with only a weak module linking the three dominant agents. Antimicrobial resistance was structured by pathogen and antibiotic, with significant pathogen-specific interactions, and the fry stage showed higher MICs. Discussion: These findings reframe the historically Streptococcus -centered view of Brazilian tilapia health and, within the constraints of a structured but non-probabilistic design, provide a descriptive, data-driven baseline to inform regionalized early-warning systems and rational antimicrobial stewardship in Brazilian tilapia aquaculture.

Piscirickettsiosis, caused by the facultative intracellular bacterium Piscirickettsia salmonis , remains the primary infectious disease economically limiting salmonid aquaculture in Chile, while its incidence has also increased in farmed salmonids in Ireland, Scotland, Australia, and Canada in recent years. This comprehensive review synthesizes 181 peer‐reviewed publications (2014–2026) to integrate transformative advances in P. salmonis genomics, pathogenesis, epidemiology, host immunity, and control strategies, updating the foundational 2014 review by Rozas and Enríquez. Comparative genomic studies have consistently resolved LF‐89‐like and EM‐90‐like isolates as two distinct phylogenomic clusters, with marked differences in pan‐genome composition, lineage‐specific proteins, ribosomal operon organization, and Dot/Icm‐associated genomic architecture, supporting genogroup‐level divergence that should not be inferred from any single threshold‐based metric such as Average Nucleotide Identity (ANI) alone. Genogroup‐specific disease phenotypes are also well established. LF‐89 produces chronic granulomatous infections, whereas EM‐90 causes acute hemorrhagic septicemia. Mechanistically, P. salmonis evades cell‐mediated immunity by inhibiting phagosome–lysosome fusion and suppressing MHC‐I–mediated antigen presentation, impairing CD8 ⁺ cytotoxic T‐lymphocyte priming and likely explaining the consistently low vaccine efficacy and genogroup‐restricted cross‐protection observed under field conditions. Treatment failures persist despite absent widespread acquired resistance, attributable to intracellular persistence, biofilm formation, and pharmacokinetic–pharmacodynamic mismatches. Despite exponential growth in mechanistic knowledge, a substantial translational gap persists between research findings and operational implementation. This review identifies critical priorities requiring urgent investment: genogroup‐specific bivalent vaccine development, climate‐adaptive biosecurity frameworks, molecular diagnostic standardization for real‐time genogroup surveillance, and integrated control strategies tailored to P. salmonis genomic heterogeneity.

Recently, a strain of Streptococcus agalactiae serotype Ia sequence type 7 clonal complex 1 (SaIa ST7 CC1) has emerged in Latin American tilapia aquaculture as an international threat. This study evaluated outbreaks of acute streptococcosis occurring between 2021 and 2025 on commercial Nile tilapia (Oreochromis niloticus) farms in six Latin American countries, aiming to integrate molecular, clinical, pathological, and environmental data. In total, 360 moribund or recently dead fish at various production stages (larvae/fry, pre-grow-out, and grow-out) were examined, and 25 S. agalactiae isolates were serotyped and subjected to real-time PCR analysis, multilocus sequence typing (MLST), virulence and antimicrobial resistance gene profiling, and antimicrobial susceptibility testing. All isolates belonged to SaIa and shared the same ST7 CC1 MLST profile, forming a highly homogeneous cluster with reference SaIa ST7 CC1 strains previously isolated from tilapia farms in Asia. These results are consistent with the regional spread of a single clonal line. At the larval and fry stages, SaIa ST7 CC1 was associated with hyperacute septicemia, gastrointestinal hemorrhage, and frequent intestinal intussusception, whereas in pre-grow-out and grow-out fish, neurological signs were more prominent, followed by ocular signs, systemic hemorrhages, and coelomic lesions. Histopathological examination showed profuse colonization of the brain, spleen, liver, and intestine by Gram-positive cocci, accompanied by marked acute circulatory and inflammatory lesions and few chronic granulomatous responses, consistent with a rapidly progressing, highly aggressive infectious process. All outbreaks occurred during extended periods of warm water (>32 °C), with large day–night thermal gradients and reduced dissolved oxygen, suggesting that thermal stress may exacerbate disease expression in affected systems. All SaIa ST7 CC1 strains exhibited phenotypic susceptibility to florfenicol and amoxicillin, whereas 84% (21/25) and 100% (25/25) exhibited intermediate susceptibility to oxytetracycline and enrofloxacin, respectively. In total, 5 of the 21 isolates (23.8%) with intermediate susceptibility to oxytetracycline carried tetracycline resistance genes (tetM, tetO). These findings identify SaIa ST7 CC1 as a clinically significant emerging threat associated with thermally facilitated and geographically expanding streptococcosis in tilapia production in Latin America. Immediate priorities include screening imported broodstock using MLST or whole-genome sequencing (WGS), harmonized regional molecular surveillance, climate-adaptive farm management practices, prudent antimicrobial use, and serotype-matched vaccination and breeding strategies that improve both disease and heat resilience.

Recently, a strain of Streptococcus agalactiae serotype Ia sequence type 7 clonal complex 1 (SaIa ST7 CC1) has emerged in Latin American tilapia aquaculture as an international threat. This study evaluated outbreaks of acute streptococcosis occurring between 2021 and 2025 on commercial Nile tilapia (Oreochromis niloticus) farms located in six Latin American countries, with an aim to combine molecular, clinical, pathological and environmental data. In total, 360 moribund or recently dead fish at various production stages (larvae/fry, pre grow-out and grow out) were examined, and 25 S. agalactiae isolates were serotyped, subjected to real time PCR analysis multilocus sequence typing (MLST), virulence and antimicrobial resistance gene profiling and antimicrobial susceptibility testing. All isolates belonged to SaIa and had the same ST7 CC1 MLST profile, which created a highly homogeneous cluster that grouped with reference SaIa ST7 CC1 strains previously isolated from tilapia farms in Asia. These results are consistent with the regional spread of a single clonal line. At larval and fry stage, SaIa ST7 CC1 was associated with hyperacute septicemia, gastrointestinal hemorrhage and frequent intestinal intussusception; while in pre grow out and grow out fish neurological signs were more prominent followed by ocular signs, systemic hemorrhages and coelomic lesions. Histopathological examination showed profuse colonization of brain, spleen, liver, and intestine by Gram positive cocci accompanied by significant acute circulatory and inflammatory lesions and few chronic granulomatous responses consistent with a fast-progressing high aggressive infectious process. All outbreaks occurred during extended episodes of hot water (>32°C) with large day–night thermal gradients and reduced dissolved oxygen, suggesting that thermal stress may exacerbate disease expression in systems affected. All SaIa ST7 CC1 strains exhibited phenotypic susceptibility to florfenicol and amoxicillin, but 84% (21/25) and 100% (25/25) of them exhibited intermediate susceptibility to oxytetracycline and enrofloxacin, respectively. Five of the 21 isolates (23,8%) with intermediate susceptibility to oxytetracycline carried tetracycline resistance genes (tetM, tetO). These findings identify SaIa ST7 CC1 as a clinically relevant threat of emerging thermally facilitated and geographically expanded streptococcosis for tilapia production in Latin America. Immediate priorities include screening of imported broodstock using MLST or whole genome sequencing, harmonized regional molecular surveillance, climate adaptive farm management practices, prudent antimicrobial use and serotype matched vaccination and breeding strategies that improve both disease- as well as heat-resilience.

Antimicrobial resistance in bacteria associated with aquaculture, such as Edwardsiella spp., represents an emerging challenge because of their relevance to fish health and their potential impact on animal, environmental, and human health. In this study, we primarily investigated the antimicrobial susceptibility profiles of Edwardsiella anguillarum isolated from farmed Nile tilapia (Oreochromis niloticus) in Brazil. Based on our findings, herein, we propose provisional local epidemiological cut-off values (pECVs) using the normalized resistance interpretation method, with data for Edwardsiella tarda included as an exploratory context. Fifty isolates (31 E. anguillarum and 19 E. tarda) collected between 2017 and 2025 were tested against 28 antibacterial agents using the disk diffusion method. Based on the pECVs, isolates were classified as wild type (WT) or non-WT (NWT), and the multiple antibiotic resistance (MAR) index was calculated. Most E. anguillarum isolates remained susceptible to several classes, although NWT and multidrug-resistant profiles were detected with a MAR index of 0.68, suggesting selective pressure in intensive tilapia farming systems. These findings support the use of local, species-specific pECVs for resistance surveillance in aquaculture, highlighting the importance of continuous antimicrobial resistance monitoring in aquaculture from a One Health perspective.

Piscine orthoreovirus (PRV) infection is widespread in the global salmonid aquaculture industry. This study aimed to investigate the presence or absence of PRV genotypes in rainbow trout Oncorhynchus mykiss farmed in Peru. The Peruvian National Authority for Fisheries and Aquaculture Health and Safety (SANIPES) carried out a disease surveillance plan between October and December 2023. In this context, 48 fish exhibiting clinical signs of circulatory disorders were screened for PRV genotypes. Quantitative PCR analysis showed that 12.5% of pools (2 out of 16) were positive for PRV-3 but negative for PRV-1 and PRV-2. The C _T value was 23.48 for tissue pool no. 14 and 27.03 for tissue pool no. 15, but both pools contained tissues from fish farmed in Lake Titicaca. Concurrently, Weissella ceti was isolated from these fish. PRV-3-positive fish showed mild mononuclear myocarditis in the ventricular spongy layer (2 out of 6). The sequences of the S1 and M2 segments of the Peruvian isolates belonged to the PRV-3 gene group, subgroup PRV-3b. These sequences showed 100% identity with the Danish PRV-3b isolates, suggesting possible European origin. Overall, the results of this study confirm, for the first time, the presence of PRV-3b in the Peruvian rainbow trout farming industry and highlight the need to implement biosecurity measures for the import of embryonic eggs to minimize the risk of pathogen entry and dissemination in the industry, as well as a permanent epidemiological surveillance plan for all PRV genotypes.

Piscine orthoreovirus (PRV) is a virus that is widely distributed among global aquaculture populations of salmonid species. The coho salmon (Oncorhynchus kisutch) is a species of increasing productive and economic importance in Chile. The presence of PRV has generated concern about its impact on the health and welfare of this species. The objective of this study was to comparatively describe the clinical manifestations, pathological changes, and pathogenesis associated with PRV infection in two different farms of farmed coho salmon in Chile through a prospective longitudinal descriptive observational study. The results demonstrated that PRV-1b and PRV-3a are independently associated with the same clinical and pathological presentation in farmed coho salmon. Microscopic pathology of the disease associated with PRV-1b and PRV-3a was primarily characterized by degenerative and inflammatory findings in the heart and liver. Hematological and blood biochemistry biomarkers in fish exhibited alterations, manifesting as hemolytic anemia and prehepatic jaundice likely due to indirect hyperbilirubinemia. Pathogenesis of infection associated with both PRV-1b and PRV-3a would indicate a specific tropism for erythrocytes and cardiomyocytes of the spongy myocardium. It is noteworthy that despite a notable reduction in viral load of both PRV subgroups in tissues, the frequency of macroscopic lesions increased during the final phase of the study. In conclusion, the results indicate a strong correlation between infection by both PRV subgroups and the proposed orthoreoviral cardiomyopathy and hemolytic jaundice (OCHJ) disease. Further research on the pathogenesis and surveillance of PRV-1b and PRV-3a subgroups is pivotal to develop effective strategies for the control of OCHJ in farmed coho salmon.

Real‐time PCR (qPCR) testing is an essential component of early detection surveillance systems for Piscirickettsia salmonis infection in Atlantic salmon farms in Chile. Currently, all 11 laboratories in the authorised diagnostic laboratory network use assays based on published protocols. Compared with other P. salmonis qPCR assays, these assays have the advantage of targeting two different genes, that is, the 16S ribosomal gene and the internal transcribed spacer (ITS), potentially allowing for higher diagnostic accuracy. However, variation and lack of harmonisation of qPCR testing systems (e.g., primers/probe, RNA/DNA as target, extraction methods, etc.) may contribute to among‐laboratory variation in qPCR results and an increased frequency of false‐negative and false‐positive results. The purpose of the ring trial reported herein was to compare qPCR results from 11 laboratories in Chile routinely testing Atlantic salmon for P. salmonis as part of a national control program. The panel of 14 samples included duplicates of three concentrations of P. salmonis in a homogenised head kidney, LF89 and EM90 bacteria and two negative controls (blank and a suspension of Flavobacterium psychrophilum). The sample order was randomised across labs, samples were tested blinded and analysed without knowledge of the source lab. Of the laboratories, 8 (72.7%) had at least one incorrect result out of 14 tested samples. Low‐concentration samples (C_t of about 30) were more often incorrectly classified by reverse transcription‐qPCR (RT‐qPCR) (3/6 labs) than by qPCR (0/5). Six (54.5%) labs had at least one false‐positive result indicating that cross‐contamination was likely during sample processing. Affected laboratories are advised to conduct internal investigations to confirm the causes of false‐positive results and recommendations for design and implementation of future ring trials are discussed.

Brazil is one of the world’s leading producers of Nile tilapia, Oreochromis niloticus. However, the industry faces a major challenge in terms of infectious diseases, as at least five new pathogens have been formally described in the last five years. Aeromonas species are Gram-negative anaerobic bacteria that are often described as fish pathogens causing Motile Aeromonas Septicemia (MAS). In late December 2022, an epidemic outbreak was reported in farmed Nile tilapia in the state of São Paulo, Brazil, characterized by clinical signs and gross pathology suggestive of MAS. The objective of this study was to isolate, identify, and characterize in vitro and in vivo the causative agent of this epidemic outbreak. The bacterial isolates were identified as Aeromonas veronii based on the homology of 16S rRNA (99.9%), gyrB (98.9%), and the rpoB gene (99.1%). A. veronii showed susceptibility only to florfenicol, while it was resistant to the other three antimicrobials tested, oxytetracycline, enrofloxacin, and amoxicillin. The lowest florfenicol concentration capable of inhibiting bacterial growth was ≤0.5 µg/mL. The phenotypic resistance of the A. veronii isolate observed for quinolones and tetracycline was genetically confirmed by the presence of the qnrS2 (colE plasmid) and tetA antibiotic-resistant genes, respectively. A. veronii isolate was highly pathogenic in juvenile Nile tilapia tested in vivo, showing a mortality rate ranging from 3 to 100% in the lowest (1.2 × 10⁴) and highest (1.2 × 10⁸) bacterial dose groups, respectively. To our knowledge, this study would constitute the first report of highly pathogenic and multidrug-resistant A. veronii associated with outbreaks and high mortality rates in tilapia farmed in commercial net cages in Brazil.