Prevalence of methicillin-resistant Staphylococcus aureus (MRSA) in Italian dairy herds

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) has been reported in human medicine as a cause of nosocomial and community-associated infections. In veterinary medicine, the microorganism has been identified in a wide range of animals and diseases, including dairy cows mastitis. Therefore, MRSA is considered an emerging threat with a high zoonotic potential. AIM - In the present study, we investigated the diffusion of MRSA in Italian dairy herds and the prevalence of infected animals at herd level. MATERIALS AND METHODS Quarter milk samples were aseptically taken from all lactating cows of 83 S. aureus infected herds and bacteriological analysis was performed by culture on blood-agar plates. The hemolytic, Gram-positiv and coagulase-positive colonies of growth were identified as S. aureus. At least four isolates morphologically representative for each herd were tested by oxacillin disk-diffusion method to detect methicillin-resistance. DNA of oxacillin resistant isolates was extracted and the presence of mecA gene was investigated by PCR. RESULTS - All oxacillin-resistant isolates were confirmed as MRSA by mecA positivity. Prevalence of S. aureus infected cows and frequency of MRSA isolation were registered at herd level. Overall prevalence of mammary infections by S. aureus ranged 0,7 - 62%: in 28 herds (33.7%) the prevalence was higher than 25%, while in 27 herds (32.5%) it was lower than 4.7%. Methicillin-resistant strains were detected in 11 farms (13,25%), all belonging to the low-prevalence group. In 5 out of such 11 herds, all S. aureus isolates were MRSA, while in the remaining 6 herds both MRSA and methicillin-susceptible S. aureus (MSSA) were present. CONCLUSIONS - Our data confirmed the potential risk of subclinical mastitis caused by MRSA and suggest a low diffusiveness of these strains in infected herds. This characteristic could reduce the zoonotic potential of bovine MRSA, but the real prevalence in dairy farms could be underestimated, since bacteriological analysis of milk is usually performed only when a clear problem of mammary infections affects the herd