Suspected transmission and subsequent spread of MRSA from farmer to dairy cows.

In the present study we describe an outbreak where PVL positive MRSA belonging to spa-type t002 and multi-locus sequence type ST2659 persisted in a Swedish dairy herd for at least two years, despite efforts to hinder transmission between animals and between the farmer and his animals. This is the first description of persistence and spread of MRSA in a dairy herd in Sweden. Sampling of animals in the herd was initiated by the finding of MRSA in the farmer and was performed at eight occasions from November 2012 to September 2014. In total, MRSA was detected in 25 animals and in 16 of these MRSA was detected in milk samples. In addition, MRSA was also detected in bulk milk samples. Whole genome sequencing (WGS) of twelve isolates from farmer (n = 1), animals (n = 9) and bulk milk (n = 2) revealed high relatedness, implying a common source. MRSA may initially have been transmitted from humans to cows with further spread within the herd. WGS showed minor differences in one isolate (loss of phage ΦN315) which could indicate adaption of the strain to an animal host.