Antimicrobial resistance in Salmonella Enteritidis and Typhimurium isolates from laying hens in Latvia

Salmonella is an important foodborne pathogen that causes gastrointestinal manifestations, with poultry remaining an important source of the pathogen. Laying hens are frequently colonized by antimicrobial resistant (AMR) Salmonella, hence posing concerns of introduction of the pathogen into the food chain. The aim of the present study was to detect the distribution of Salmonella Enteritidis and Typhimurium serovars and their antimicrobial resistance in laying hens in Latvia. Salmonella isolates from feces of laying hens from poultry farms in Latvia during 2025, within the frame of official control and self-checks sampling, were used for the detection of antimicrobial resistance. AMR to the following antimicrobials were tested: ampicillin, azithromycin, amikacin, ceftazidime, cefotaxime, chloramphenicol, ciprofloxacin, colistin, gentamycin, meropenem, nalidixic acid, sulfamethoxazole, tetracycline, tigecycline and trimethoprim with the Minimum Inhibitory Concentration (MIC) method. Results were evaluated according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) requirements. Genetic determinants of AMR resistance were detected with Whole Genome Sequencing (WGS). Salmonella spp. were identified in 45 out of 693 (6%) positive samples. Among Salmonella-positive samples, 26 (58%) were the European Union target serovars Enteritidis and Typhimurium. All S. Typhimurium isolates (4/4, 100%) but only 7 (7/22, 32%) of S. Enteritidis were sensitive to all antimicrobials tested. S. Enteriditis exhibited resistance only to nalidixic acid and ciprofloxacin (15/22, 68%), and one isolate to colistin (1/22, 5%). No multidrug-resistant isolates were identified. Genetic determinants associated with resistance to fluoroquinolones were identified in isolates resistant to nalidixic acid and ciprofloxacin. The high level of dissemination of fluoroquinolone-resistant S. Enteritidis isolates among laying hens in Latvia represents an important public health challenge.