Phylogenetic Tracking of LA-MRSA ST398 Intra-Farm Transmission among Animals, Humans and the Environment on German Dairy Farms.

Methicillin-resistant   (MRSA) are a major threat to human and animal health, causing difficult-to-treat infections. The aim of our study was to evaluate the intra-farm transmission of livestock-associated (LA) MRSA sequence type (ST) 398 isolates on German dairy farms. A total of 115 LA-MRSA ST398 isolates originating from animals, humans and the environment of six dairy farms were analyzed by whole-genome sequencing and core genome multilocus sequence typing. Phylogenetic clusters of high allelic similarity were detected on all dairy farms, suggesting a MRSA transmission across the different niches. On one farm, closely related isolates from quarter milk samples (QMS), suckers of calf feeders and nasal cavities of calves indicate that MRSA may be transferred by feeding contaminated milk to calves. Detection of related MRSA isolates in QMS and teat cups (4/6 farms) or QMS and human samples (3/4 farms) pointed out a transmission of MRSA between cows during the milking process and a potential zoonotic risk. In conclusion, LA-MRSA ST398 isolates may spread between animals, humans and the environment on dairy farms. Milking time hygiene and other internal biosecurity measures on farms and pre-treatment of milk before feeding it to calves may reduce the risk of MRSA transmission.