Despite a similar prevalence of carriage of S. aureus and MRSA among ILO and AFLO-exposed individuals, the distribution of S. aureus demonstrating characteristics of livestock association (tetracycline resistance, CC398, scn-negative) differed between groups. This is the first report of a livestock-associated strain of S. aureus CC398 (including MRSA CC398 and MDRSA CC398) in North Carolina. We observed that individuals who worked with livestock raised using intensive confinement and antibiotic inputs, including antibiotics administered non-therapeutically, carried MRSA and MDRSA with multiple markers of livestock-association (tetracycline-resistance, CC398, scn-negative). Individuals who worked with livestock raised without the use of antibiotics and confinement were not observed to be carrying MRSA and MDRSA demonstrating multiple markers of livestock-association. We also observed some indication that, regardless of markers of livestock association, there was an elevated proportion of S. aureus that was MDRSA among ILO compared to AFLO participants.
Previous work demonstrates the potential for transmission of S. aureus
between animals and humans on ILO and AFLO operations 
. A study conducted on Dutch dairy farms 
and another across livestock production sectors in Belgium 
observed nearly indistinguishable MRSA CC398 isolates in humans and various types of animals on the same property. Both of these studies focused on MRSA CC398, but also observed transmission of multidrug-resistant MRSA CC398. In the present study, we observed a higher prevalence of livestock-associated S. aureus
strains among ILO participants and a higher prevalence of human epidemic clones of S. aureus
among AFLO participants. This is consistent with previously published studies of S. aureus
carriage among workers and pigs at conventional 
and non-conventional 
farms in the United States. However, since we studied human carriage only, our ability to draw inferences about the directionality, frequency, or intensity of S. aureus
exchange between animals and humans in ILO or AFLO settings is limited.
CC398, which has been identified as an emerging livestock-associated S. aureus
strain in Europe 
, is commonly considered as a molecular marker of livestock-associated S. aureus
. Near universal resistance to tetracycline has been reported among S. aureus
CC398 isolates from livestock and humans in close contact with livestock 
and tetracycline-susceptibility has been reported among S. aureus
CC398 collected from humans with no known livestock exposure 
. These observations are consistent with findings reported by Price et al. (2012) that CC398 acquired resistance to methicillin and tetracycline after the introduction to livestock from humans 
. In the Price et al. (2012) study of historical isolates, the tetracycline resistance gene tet
(M) was found among 99% of isolates from livestock and absent from isolates obtained from humans 
. Thus, tetracycline resistance in conjunction with CC398 has been used as a marker of livestock-associated S. aureus.
Since the 1950s, tetracycline and its derivatives have been used extensively in US livestock production for non-therapeutic purposes such as growth promotion 
. In 2011, approximately 5.6 million pounds of tetracycline were sold and distributed in the United States, the greatest amount for any drug class approved for use in food-producing animals 
. Our observation of a greater prevalence of tetracycline-resistant S. aureus,
including MRSA and MDRSA, among ILO compared with AFLO participants is consistent with the historical and ongoing use of tetracycline in industrial livestock production. Although tetracycline resistance is important to note with regard to its use in animal production, it is also used in clinical settings to treat human infections, including skin and soft tissue S. aureus
Recent work has demonstrated that absence of the scn
gene may aid in the differentiation of animal origins of S. aureus
carried by humans exposed to livestock 
. In the present study, the majority of scn-
negative strains carried by ILO participants (70.6%), belonged to CC398. All other scn
-negative strains carried by participants also belonged to CCs that have previously been described in animals (CC97, CC45, CC30, CC9, CC8, CC5, CC1) 
. CC30 has recently been described in livestock in Europe 
and was the most prevalent scn
-negative strain among AFLO participants in our study. The majority of scn
-negative isolates from ILO participants (82.3%) were resistant to tetracycline while those from AFLO participants were not. Within the ILO group, substantial overlap occurred among markers of livestock-association (tetracycline resistance, CC398, scn
-negative). Within the AFLO group, little overlap of the markers of livestock-association was observed. One CC398 isolate was observed among the AFLO group, and while it demonstrated tetracycline resistance, it carried the scn
gene. Further, among the AFLO participants no scn-
negative isolates were tetracycline resistant or CC398. These findings suggest that while both groups may be exposed to MRSA and MDRSA, individuals working in the ILO setting may be exposed to a larger or more sustained livestock reservoir of MRSA and MDRSA, or that these livestock-associated MRSA and MDRSA are transmitted more frequently between animals and humans in the ILO as compared to the AFLO setting.
appears to be a promising marker, additional work is needed to establish its validity as an indicator of livestock-associated S. aureus.
It is not known how long scn
is harbored by human-origin S. aureus
strains once passed to livestock hosts, nor are the factors that might influence the speed of scn
bacteriophage loss fully understood, although previous work showed that feed-delivered antibiotics can induce prophage loss 
. Establishing the temporal dynamics of scn
presence/absence among S. aureus
passed between humans and animals will help determine its utility as a marker of livestock association.
When using all participants as the denominator – to facilitate population prevalence comparisons with other studies – the overall prevalence of S. aureus
(39%) and MRSA (3.4%) in our study were slightly greater than estimates of prevalence of S. aureus
(30.4%; 95% CI: 29.4, 31.5%) and MRSA (1.2%; 95% CI: 0.9, 1.5%) carriage in the general US population from 2001–2004 
. Additional comparison of S. aureus
and MRSA nasal carriage prevalence with populations similar to the present study is challenging as few other studies examine carriage among populations not receiving or working in medical care. There is little basis for making comparisons to other MDRSA population prevalence estimates because most other studies investigate MRSA only, or do not provide sufficient information to interpret study population denominators for MDRSA carriage.
Smith et al. examined MRSA carriage among 20 workers employed at two industrial swine production facilities in Iowa and Illinois 
. MRSA was detected among swine in one facility where 9/14 workers were also found to carry MRSA. All observed MRSA was tetracycline-resistant CC398. No MRSA was detected among swine or workers in the second facility, leading to an overall MRSA carriage prevalence of 45% among workers. Although the overall MRSA prevalence among ILO workers reported in the present study (3/80; 4%) was lower than that reported by Smith et al., the present study included workers from a larger number and a more diverse cross-section of ILOs. While we did not ask workers to reveal their place of employment for reasons of privacy and confidentiality, our sampling of ILO workers from a larger number of operations is based in part on knowledge of the average number of employees at each operation. Data from the 2007 Census of Agriculture conducted by the United States Department of Agriculture indicate that the reported industry-wide average number of employees on hog and pig farming operations in North Carolina was 6.8 among 938 out of 1619 operations that reported having hired labor (the remaining 681 operations reported having no hired labor) 
Smith et al. (2013) also examined carriage of MRSA among weaned pigs and 148 workers on ILOs and AFLOs in Iowa, Illinois, Minnesota, Ohio, and North Carolina 
. MRSA-positive herds were only found in Iowa and Illinois. MRSA was not found among AFLO pigs or among AFLO workers, whereas we found 3 of 92 AFLO workers (3.3%) to be MRSA-positive. Smith et al. (2013) observed 8.5% MRSA prevalence among ILO pigs. Overall, 31 ILO workers were positive for MRSA carriage and 27 of 31 (87%) MRSA-positive workers were employed at ILOs where MRSA was detected among pigs. Further comparison with our study’s findings is not possible because ILO and AFLO worker denominator information was not included in Smith et al. (2013) 
The present study would be strengthened by sampling animals and production facilities 
; however, we did not have access during the course of the study. Access to livestock operations by the public health research community in Europe and other countries has allowed a robust body of evidence to develop there regarding the extent of exposure to S. aureus
, MRSA, and MDRSA among ILO workers as well as the potential dissemination of livestock-associated strains into the broader community. Though our study may be viewed as small, to the best of our knowledge, it is one of the largest studies on this topic in the United States to date. Furthermore, it was not restricted to ILOs that have agreed to participate in research.
In principle, the difference in characteristics of S. aureus
observed among the ILO compared to the AFLO group could be related to livestock exposures, to other differences between the AFLO and ILO participant populations, or to a combination of the two. The ILO and AFLO study populations appeared different in their demographic make-up; 82% of ILO participants in our study self-identified as Hispanic whereas 79% of AFLO participants identified as non-Hispanic white. Industrial swine operations in North Carolina are disproportionately located in low income communities of color in the eastern portion of the state 
. Differences in ethnicity and education between ILO and AFLO workers reflect differences in the communities where they live, and differences in S. aureus
subtypes circulating in these populations could affect differences in carriage observed in this study. Although some factors (e.g. recent antibiotic use and hospitalization) associated with risk of carriage of antibiotic-resistant S. aureus
were similar between groups or greater among the AFLO group (pets in the home) 
; the latter would tend to offset any differences in the study groups due to occupational exposures.
Because we did not sample from an enumerated base population (e.g., employee rosters) it is unclear how generalizable these results may be to all ILO workers in North Carolina or the United States. Notwithstanding this limitation, our findings raise important questions about the frequency of potential occupational exposure to antibiotic-resistant S. aureus
among an estimated 292,000 livestock workers in the United States in 2012 
Despite current understanding of livestock-associated MRSA as a relatively rare cause of human infection in the United States (which may be limited due to a lack of systematic national surveillance) 
, there is public health concern about potential broad dissemination of drug-resistant S. aureus
to the general public. Although others have observed evidence of phenotypic and molecular strain concordance within households 
, we did not observe S. aureus
strain concordance between workers and household members. However, relatively few household members participated in this study and consequently this finding should be interpreted with caution. We also assessed only one S. aureus
colony per colonized individual for all outcomes presented. It is possible that some individuals carried multiple S. aureus
strains. We may not have captured all carriage states of some individuals and consequently may have missed potential occurrences of strain concordance within households. To advance understanding of patterns of potential household transmission, future studies should address these limitations.
The results reported here show that the proportion of S. aureus
identified as MDRSA and the proportion exhibiting phenotypic and molecular markers of livestock association was elevated among individuals exposed to the ILO environment compared to those exposed to the AFLO environment. Carriage of scn
-negative MRSA CC398 and scn
-negative MDRSA CC398 was limited to individuals with direct exposure to ILO production – all of these isolates were also resistant to tetracycline. Whether or not these livestock-associated S. aureus
strains (including MRSA and MDRSA) pose a health risk to workers and the broader public requires further investigation. Overall, our findings support growing concern about antibiotic use and confinement in livestock production, and raise questions about the potential for occupational exposure to an opportunistic and drug-resistant pathogen which in other settings including hospitals and the community is a major cause of morbidity and mortality in the United States 
and globally