Epidemiology, Antimicrobial Resistance, and Genetic Determinants of Methicillin-Resistant Staphylococcus aureus...

Methicillin-resistant Staphylococcus aureus (MRSA) is a serious zoonotic pathogen that poses significant threats to both public health and the poultry industry due to its multidrug resistance and virulence. In broiler chickens, MRSA infections are commonly associated with arthritis, resulting in impaired mobility, reduced productivity, and economic losses. This study aimed to investigate the epidemiology, antimicrobial resistance profiles, and genetic determinants of MRSA in broiler chickens presenting with clinical arthritis in the Kafr El-Sheikh Governorate, northern Egypt. In this study, 100 broiler farms were randomly selected from a list provided by the local veterinary authority in Kafr El-Sheikh Governorate. Of these, 20 medium-sized farms (5000-10,000 birds per cycle) had clinically arthritic chickens. A total of 140 synovial fluid samples (six to eight chickens per farm) were aseptically collected from affected birds. The affected farms represented approximately 4.8% of the total broiler farms in the governorate and were distributed across different districts, indicating that MRSA-associated arthritis was not confined to a single locality. Bacterial isolation was performed using selective culture media and confirmed via biochemical tests and PCR targeting the nuc gene. Antimicrobial susceptibility testing was conducted using the Kirby Bauer disk diffusion method against 16 antibiotics following CLSI 2020 guidelines. Molecular screening for resistance and virulence genes (mecA, vanA, hig) was performed by uniplex PCR. Phylogenetic analysis of 16S rRNA sequences was carried out using the Maximum Likelihood method. Clinical signs included joint swelling, deformity, and impaired locomotion. Epidemiological analysis revealed that MRSA-associated arthritis was detected in (3/20) 15% of infected farms, with 7.67 ± 1.53% of birds affected within infected flocks. Staphylococcus aureus was isolated from 60% of footpad abscesses and 65% of hock joint swellings. All isolates were resistant to multiple β-lactam antibiotics, including oxacillin, cefoxitin, penicillin G, and piperacillin. High resistance was also observed to clindamycin, cephradine, and cephalexin. The most effective agents were ampicillin-sulbactam and vancomycin. The mecA and nuc genes were detected in all isolates, and vanA and hig genes were detected in 5% and 25% of isolates, respectively. Phylogenetic analysis confirmed the close genetic relatedness of the local isolate (PQ879639.1) to global S. aureus strains. MRSA is an emerging pathogen in broiler flocks with a marked capacity for antimicrobial resistance and zoonotic transmission. The detection of multidrug-resistant strains carrying key resistance genes highlights the urgent need for enhanced biosecurity, prudent antimicrobial use, and routine monitoring programs to mitigate its impact on poultry and public health.