Longitudinal study of Staphylococcus aureus colonization and infection in a cohort of swine veterinarians in the United States

In this study conducted by Dr. Sun during her PhD under the direction of Dr. Peter Davies, 66 swine veterinarians were sampled via nasal swabs every month for 18 months. Swabs were first cultured to detect Staphylococcus aureus and to determine the strains susceptibility (or resistance) to methicillin, and then were characterized by spa typing and MultiLocus Sequence typing (MLST). Click on the banner below to read the publication in open-access.

Sun longitudinal study Saureus in swine veterinarians 2017

S. aureus monthly prevalence ranged from 58% to 82% and MRSA ranged between 6% and 15%. Those results were higher than the ones for the US population. 52% of the swine veterinarians were classified as persistent carriers, meaning that their samples came back positive for S.aureus at least 80% of the time, 47% were intermittent carriers. 21% of the veterinarians were true persistent carriers meaning that the same spa type of S.aureus was recovered each time the sample was positive.

Staphylococcus aureus positive samples repartition
Number of veterinarians based on the carrier index (number of samples positive for S.aureus divided by number of samples taken per veterinarian)

Whole genome sequencing showed that strains isolated at the beginning and at the end of the study were similar genetically. Comparing spa types, 83% of all isolates belonged to a type also present in swine.

Abstract

BACKGROUND:

People working with pigs are at elevated risk of harboring methicillin resistant S. aureus (MRSA) in their nose, which is attributable to occupational exposure to animals harboring livestock adapted S. aureus. To obtain insight into the biological nature of occupationally related nasal culture positivity, we conducted a longitudinal study of 66 swine veterinarians in the USA.

METHODS:

The study cohort resided in 15 US states and worked predominantly with swine. Monthly for 18 months, participants self-collected nasal swabs and completed a survey to report recent exposure to pigs and other animals; the occurrence of work related injuries; and any relevant health events such as skin and soft tissue infections or confirmed staphylococcal infections. Nasal swabs were cultured using selective methods to determine the presence of MRSA and methicillin susceptible S. aureus (MSSA), and isolates were characterized by spa typing and MLST.

RESULTS:

Prevalences of S. aureus (64%, monthly range from 58 to 82%) and MRSA (9.5%; monthly range from 6 to15%) were higher than reported for the US population (30% and 1.5% respectively). Predominant spa types were t034 (ST398, 37%), t002 (ST5, 17%) and t337 (ST9/ST398 13%), a distribution similar to that found in a concurrent study in pigs in the USA. Veterinarians were classified into three groups: Persistent carriers (PC, 52%), Intermittent carriers (IC, 47%) and Non-carriers (NC, 1%). Persistent carriage of a single spa type was observed in 14 (21%) of participants, and paired (first and last) isolates from PC subjects had minor genetic differences. Swabs from PC veterinarians carried higher numbers of S. aureus. Among IC veterinarians, culture positivity was significantly associated with recent contact with pigs.

CONCLUSIONS:

Exposure to pigs did not lead to prolonged colonization in most subjects, and the higher numbers of S. aureus in PC subjects suggests that unknown host factors may determine the likelihood of prolonged colonization by S. aureus of livestock origin. Exposure to S. aureus and persistent colonization of swine veterinarians was common but rarely associated with S. aureus disease.

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