Prevalence at herd-level of methicillin resistant Staphylococcus aureus in milk samples of dairy herds

Staphylococcus aureus is a frequent cause of mastitis in cows and heifers. Recently, the presence of methicillin-resistant Staphylococcus aureus (MRSA) has been demonstrated in clinical and subclinical mastitis in dairy cows. The strains isolated were similar to those found in pigs and other animal species, including humans (Animal-Associated MRSA). To estimate the spread of these strains in farms, we determined the prevalence of MRSA-infected cows within a previously proven positive herd. Likewise the burden of this strain on udder health can be estimated. In this study, all lactating cows from 5 herds (1 located in the Netherlands, 4 in Belgium), previously proven MRSA positive, were sampled at quarter level using a standardised method. The same day, milk samples were inoculated on agar plates with 5% sheep's blood (CSA-plates) and MRSA-ID plates. Colonies showing the typical morphology for S. aureus on the MRSA-ID-plates were tested by triplex PCR (mecA gene for methicillin resistance, part of the 16S rRNA gene for the Staphylococcus genus and the nucA gene for the speciation into S. aureus). Additionally, strains were typed by multi locus sequence typing (MLST) and spa typing. The percentages of MRSA positive cows for herds 1, 2, 3, 4 and 5 are 9.5, 14.3, 7.4, 3.9 and 0, respectively. Strains were submitted to spa-typing and this revealed spa-type t011. Multi Locus Sequence Typing showed them to be ST398. In conclusion, the spread of MRSA in positive farms is limited to less than 15% of the cows in the herd. Culling of infected animals is advisable seen the thread of this strain and the problematic treatment due to multiple resistances.