Transmission and persistence of livestock-associated methicillin-resistant Staphylococcus aureus among veterinarians and their household members

Abstract

After the first isolation of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) in 2003, this MRSA variant quickly became the predominant MRSA obtained from humans as part of the Dutch national MRSA surveillance. Previous studies have suggested that human-to-human transmission of LA-MRSA, compared to that of other MRSA lineages, rarely occurs. However, these reports describe the transmission of LA-MRSA based on epidemiology and limited molecular characterization of isolates, making it difficult to assess whether transmission actually occurred. In this study, we used whole-genome maps (WGMs) to identify possible transmission of LA-MRSA between humans. For this, we used LA-MRSA isolates originating from a 2-year prospective longitudinal cohort study in which livestock veterinarians and their household members were repeatedly sampled for the presence of S. aureus. A considerable degree of genotypic variation among LA-MRSA strains was observed. However, there was very limited variability between the maps of the isolates originating from the same veterinarian, indicating that each of the veterinarians persistently carried or had reacquired the same LA-MRSA strain. Comparison of WGMs revealed that LA-MRSA transmission had likely occurred within virtually every veterinarian household. Yet only a single LA-MRSA strain per household appeared to be involved in transmission. The results corroborate our previous finding that LA-MRSA is genetically diverse. Furthermore, this study shows that transmission of LA-MRSA between humans occurs and that carriage of LA-MRSA can be persistent, thus posing a potential risk for spread of this highly resistant pathogen in the community.

FIG 1

(A) Minimum spanning tree depicting the genotypic diversity among LA-MRSA isolates originating from veterinarians (n = 110). Each node in this minimum spanning tree represents the WGM of a single LA-MRSA isolate. Nodes with identical colors represent isolates from the same veterinarian. The halos represent groups based on a similarity cutoff value of ≥98% for indistinguishable WGMs. (B) The demographic location of each veterinarian is represented in the map of the Netherlands. (C) In the smaller version of the minimum spanning tree, the colors of the nodes represent the spa types.

FIG 2

Minimum spanning tree displaying transmission of LA-MRSA between veterinarians and household members. Each node in this minimum spanning tree represents the WGM of a single LA-MRSA isolate. Nodes with identical colors represent isolates from the same household. The isolates obtained from the household members of each veterinarian are indicated with “H.” The halos represent groups, as defined for the minimum spanning tree of Fig. 1.

FIG 3

Whole-genome maps of isolates obtained from household (VET226) displaying colonization and transmission with different LA-MRSA strains at multiple sampling moments. The blocks in the maps represent the restriction fragments, and blocks that were considered to have the same size carry the same color. Blocks with reduced height were ignored in the comparison of the profiles. The upper panel of the figure displays the complete WGMs, and the lower panel depicts a zoomed-in region of the maps to display the variation in the restriction fragments. The origin of the sample, the sampling moment, and the anatomic location of the sampling is indicated on the right. The dendrogram on the left displays two clusters, A and B, and the similarity between the most distant members of the clusters.