Spread of LA-MRSA CC398 in Danish mink (Neovison vison) and mink farm workers.

More than 55 million mink skins were produced globally in 2017. As a consequence, a large number of people are employed in mink production worldwide. In Denmark, farmed mink were found to constitute a reservoir of methicillin-resistant Staphylococcus aureus (MRSA) clonal complex (CC) 398 and 6000 mink farm workers in Denmark are potentially exposed to LA-MRSA CC398. The study aim was to elucidate the source of LA-MRSA CC398 in mink farms and to investigate possible transmission to humans. In total, 161 LA-MRSA CC398 isolates from mink (n = 65), mink feed (n = 16) and humans (n = 80) with reported contact to mink, were whole-genome sequenced and compared to 183 LA-MRSA CC398 isolates from Danish pigs and an international collection of 89 S. aureus CC398 isolates. Most of the mink-associated isolates clustered within the predominant LA-MRSA CC398 lineages circulating in the Danish pig production, supporting that pigs are a source of LA-MRSA CC398 in mink feed, mink, and mink farmers.

LA-MRSA CC398 in Dairy Cattle and Veal Calf Farms Indicates Spillover From Pig Production.

The possible spillover from pigs into other production animals incites concern for unresolved reservoirs of human exposure. The present investigation was therefore initiated, to elucidate if Danish veal and dairy farms constitute a reservoir of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) CC398 and to potentially identify the source of introduction. We collected nasal swab samples from 17 Danish veal farms, 2 slaughterhouses, and received bulk tank milk samples from 286 dairy farms. All samples were analyzed by culturing and screening on MRSA selective plates and presumed MRSA was verified by MALDI-TOF and PCR. MRSA isolates were subjected to spa typing and whole-genome sequencing. LA-MRSA was found on two veal farms in one and three calves, respectively, with subsequent follow-up samples found negative. Eight of 286 dairy farms (2.8%) were found LA-MRSA positive and follow-up samples, from five farms showed intermittent detection of LA-MRSA. The spa types, t034 and t011, were the most common while a single isolate from a dairy farm belonged to spa type t843 associated to mecC-MRSA CC130 and is the first report of mecC-MRSA in the Danish dairy production. A phylogenetic analysis showed that some of the isolates grouped within or close to the dominant Danish pig clusters, suggesting spillover into cattle farms. Other isolates clustered outside the dominant pig clusters suggesting that other routes of introduction cannot be excluded. Results of the investigation indicated a contamination of veal farms while some dairy farms seemed to be a permanent reservoir. Thus, Danish cattle represent a low prevalence reservoir of LA-MRSA CC398, which at present, is not of major human health concern.

Copresence of tet(K) and tet(M) in Livestock-Associated Methicillin-Resistant Staphylococcus aureus Clonal Complex 398 Is Associated with Increased Fitness during Exposure to Sublethal Concentrations of Tetracycline.

The tetracycline resistance gene tet(K) was shown to be integrated within the predominant staphylococcal cassette chromosome mec (SCCmec) element of Danish livestock-associated methicillin-resistant Staphylococcus aureus CC398 (LA-MRSA CC398). These LA-MRSA CC398 isolates already possessed tet(M), but the acquisition of tet(K) significantly improved their fitness at sublethal concentrations of tetracycline. Because tet(K) is genetically linked to SCCmec, the use of tetracycline in food animals may have contributed to the successful spread of LA-MRSA CC398.