MRSA in pig farming: the emerging role of flies in antimicrobial resistance: a cross-sectional study

Abstract

Background: The bacterium Staphylococcus aureus, particularly its methicillin-resistant form (MRSA), presents significant public health challenges due to its resistance to β-lactam antibiotics and frequent multidrug resistance. Livestock-associated MRSA (LA-MRSA), especially clonal complex 398, has become a concern in intensive pig farming, with emerging evidence suggesting flies may act as vectors. This study investigates the occurrence and molecular characterisation of MRSA in house flies (Musca domestica) and stable flies (Stomoxys calcitrans) on Austrian piglet-producing farms to understand their role in MRSA transmission and resistance dissemination. Results: MRSA was detected in 41.7% of 24 pig farms, with isolates identified in house flies (53.2%), stable flies (19.1%), boot sock samples (17.0%), and dust wipe samples (10.6%). All isolates were cefoxitin-resistant and belonged to CC398, carrying resistance genes such as mecA, erm(A), erm(B), erm(C), lnu(G), lsa(E), vga(E), dfrG, and in ciprofloxacin-resistant isolates, mutations in the quinolone resistance-determining regions (QRDRs) of the genes gyrA and grlA were observed. Beside resistance to β-lactams, resistance to tetracycline (100%), erythromycin (74%), clindamycin (74%), and ciprofloxacin (32%), trimethoprim-sulfamethoxazole (17%) was observed. Multidrug resistance (MDR) was observed in 94% of isolates. House flies (26%) were more frequently associated with MRSA carriage than stable flies (9.4%), indicating their potential as significant vectors. Environmental samples (boot sock and dust wipe samples) further confirmed widespread barn contamination. Conclusions: This study highlights the prevalence of LA-MRSA in Austrian pig farms and identifies flies as vectors contributing to its spread. These findings emphasise the importance of robust biosecurity measures, including effective fly control and stringent hygiene protocols, to mitigate MRSA risks in farming environments. Public health strategies should focus on prudent antimicrobial use and a One Health approach to curb the dissemination of antimicrobial resistance across humans, animals, and the environment.