Methicillin-resistant Staphylococcus aureus (MRSA) ST398 associated with clinical and subclinical mastitis in Belgian cows

Methicillin-resistant Staphylococcus aureus (MRSA) is infrequently reported in mastitis. Yet, as in many other countries, the prevalence of methicillin resistance among S. aureus from mastitis is currently unknown in Belgium. To elucidate this, the presence of mecA was investigated in 118 S. aureus strains originating from diagnostic mastitis milk samples from 118 different farms experiencing S. aureus mastitis. MRSA strains were characterized by disk diffusion susceptibility testing, spa-typing, MLST and SCCmec-typing. In an additional study, four MRSA-positive farms were selected to assess the in-herd prevalence of MRSA, by sampling all cows in lactation. Isolated MRSA strains were similarly characterized. The mecA gene was detected in 11 (9.3%) of the 118 S. aureus isolates, indicating that nearly 10% of the Belgian farms suffering from S. aureus mastitis have an MRSA problem. The in-herd prevalence varied between 0% and 7.4%. Characterization of the MRSA strains showed that they were all resistant to tetracycline. Additional resistances to macrolides, lincosamides and aminoglycosides were frequently detected. The strains were ST398, spa-types t011 or t567 and had SCCmec-type IVa or V, proving that they belong to the emerging livestock-associated MRSA (LA-MRSA) strains of CC398. Our study shows that after detection in Belgian pigs, horses and poultry, LA-MRSA has also attained Belgian cattle. It is the first report on frequent isolation of LA-MRSA from bovine infections. As the in-herd isolation rate resembles that of regular S. aureus in farms experiencing S. aureus mastitis, the multi-resistance of LA-MRSA strains may cause future treatment problems.