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An Overview of Livestock-Associated MRSA inAgricultureAbby L. Harper MPH a , Dwight D. Ferguson MS a , Kerry R. Leedom Larson DVMMPH b ,Blake M. Hanson MA c , Michael J. Male DVM d , Kelley J. Donham DVM b & Tara C. SmithPhD ca Department of Occupational & Environmental Health and the Center for EmergingInfectious Diseases, University of Iowa College of Public Health, Iowa City, Iowa, USAb Department of Occupational & Environmental Health, University of Iowa College ofPublic Health, Iowa City, Iowa, USAc Center for Emerging Infectious Diseases and the Department of Epidemiology,University of Iowa College of Public Health, Iowa City, Iowa, USAd Department of Epidemiology, University of Iowa College of Public Health, Iowa City,Iowa, USA

Available online: 19 Apr 2010

To cite this article: Abby L. Harper MPH, Dwight D. Ferguson MS, Kerry R. Leedom Larson DVMMPH, Blake M. Hanson MA,Michael J. Male DVM, Kelley J. Donham DVM & Tara C. Smith PhD (2010): An Overview of Livestock-Associated MRSA inAgriculture, Journal of Agromedicine, 15:2, 101-104

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Journal of Agromedicine, 15:101–104, 2010Copyright © Taylor & Francis Group, LLCISSN: 1059-924X print/1545-0813 onlineDOI: 10.1080/10599241003627110

WAGR

CONFERENCE FORUM

An Overview of Livestock-Associated MRSA in Agriculture

MRSA and Farming Abby L. Harper, MPHDwight D. Ferguson, MS

Kerry R. Leedom Larson, DVM, MPHBlake M. Hanson, MAMichael J. Male, DVM

Kelley J. Donham, DVMTara C. Smith, PhD

ABSTRACT. Researchers, veterinary and health care practitioners, and agricultural producers gath-ered in Johnston, Iowa, to attend the eighth annual Midwest Rural Agricultural Safety and HealthForum (MRASH), November 2009. Among several focus areas, four plenary talks were given on thecurrent research being conducted examining methicillin-resistant Staphylococcus aureus (MRSA) onswine farms in the United States. These focused on prevalence of MRSA on farms, both in swine andin human workers; the presence of MRSA in air samples and in swine barn shower facilities; and the

Abby L. Harper and Dwight D. Ferguson are affiliated with the Department of Occupational & Environ-mental Health and the Center for Emerging Infectious Diseases, University of Iowa College of PublicHealth, Iowa City, Iowa, USA.

Kerry R. Leedom Larson is affiliated with Department of Occupational & Environmental Health, University ofIowa College of Public Health, Iowa City, Iowa, USA.

Blake M. Hanson and Tara C. Smith are affiliated with the Center for Emerging Infectious Diseases andthe Department of Epidemiology, University of Iowa College of Public Health, Iowa City, Iowa, USA.

Michael J. Male is affiliated with the Department of Epidemiology, University of Iowa College of PublicHealth, Iowa City, Iowa, USA.

Kelley J. Donham is affiliated with Department of Occupational & Environmental Health, University of IowaCollege of Public Health, Iowa City, Iowa, USA.

Ethical considerations—protection of human subjects and animals: The authors obtained University ofIowa Institutional Review Board-01 (IRB) and Institutional Animal Care and Use Committee (IACUC)approval for this study prior to commencement. All subjects were provided with informed consent docu-ments and offered the chance to ask questions about the research prior to signing the consent document andproviding biological samples and questionnaire data.

This work was supported by start-up funds from the University of Iowa; the National Pork Board; and the Heart-land Center for Occupational Health and Safety. The funding source had no influence on study design; collection,analysis, and interpretation of data; the writing of the report; or the decision to submit the paper for publication.

Address correspondence to: Tara C. Smith, PhD, Assistant Professor, Department of Epidemiology,Center for Emerging Infectious Diseases, 200 Hawkins Drive, C21F GH, University of Iowa, Iowa City, IA52242, USA (E-mail: tara-smith@uiowa.edu).

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102 MRSA AND FARMING

presence of methicillin-resistant and methicillin-sensitive S. aureus in retail meats. These findingsbegin to elucidate the overall picture of livestock-associated MRSA in the Midwestern United States.

KEYWORDS. Bioaerosol sampling, MRSA, Staphylococcus aureus, ST398, swine, zoonosis

Staphylococcus aureus is a common type ofbacteria that normally lives in the nasopharyn-geal passages and sometimes on the skin.Although approximately one third of the popu-lation carries some strain of S. aureus, coloni-zation with methicillin-resistant S. aureus(MRSA) is much less common.1 In the generalpopulation, approximately 1.5% of people arecolonized with MRSA.2 Although many peoplemay carry the organism without associated ill-ness, it can result in clinical infection in certainsituations. In 2005, MRSA was associated with94,000 infections and 18,000 deaths.3,4 Deathsfrom MRSA infections in the United Stateshave exceeded mortality from many otherinfectious diseases, including human immuno-deficiency virus/acquired immunodeficiencysyndrome (HIV/AIDS).3,4

Although MRSA was originally recognizedas a concern in hospitalized patients, since themid-1990s, additional sources of MRSA havebeen recognized in the community outside ofhospitals (community associated, or CA-MRSA).CA-MRSA infections typically occur in healthypeople who have not had recent health carefacility contact and are caused by strains of bac-teria distinct from those that typically causehospital-based infections. These bacteria havecaused outbreaks among athletes who shareequipment, shower facilities, or personalitems.5 Additional CA-MRSA at-risk popula-tions include children in daycare facilities6,7

and prisoners.8 Further, a newly recognizedMRSA type is livestock-associated MRSA(LA-MRSA), making livestock producers anewly identified risk group.9

The Netherlands was the first to report theprevalence of LA-MRSA among swine farmersin 2005.10 Later studies showed farmers had anincreased risk of infection with ST398 than thegeneral population,10 and that this strain nowaccounted for 20% of all human MRSA casesin the Netherlands in 2007.11 Multilocussequence typing (MLST) showed that these

isolates belonged to sequence type ST398. Thisstrain was found in both pigs and pig farmers,indicating that the MRSA had been sharedbetween animals and their caretakers in thefarming environment, a finding confirmed byadditional research.12–14 This documents theimportance of considering livestock and otheranimals when examining the epidemiology ofMRSA. Despite the research in other countries,only one pilot study has been carried out in theUnited States.14

In a rural state such as Iowa, which produces31% of the swine raised in the United States,transmission of MRSA on swine farms couldcomplicate efforts to reduce MRSA transmis-sion.14 With pork being a $5 billion a yearindustry,15 it is important to gather informationto assess the potential problem and determineways to control occupational and public healthproblems if they seem present. Therefore, thefaculty and staff of the University of Iowa’sCenter for Emerging Infectious Diseases(CEID) lab are conducting a study to examinethe prevalence of MRSA in swine and swineworkers from the Midwestern United States.

Collaborating with Ohio State Universityand the University of Minnesota, identical stud-ies are being conducted to determine the preva-lence of MRSA in their respective region. Eachstate investigated a total of 18 farms—9confinement operations and 9 antibiotic-free.Samples were collected from both pigs andhuman caretakers. Producers and employeesalso filled out questionnaires in order to collectinformation on demographics, potential riskfactors, and farm production practices. To date,samples from swine have been collected on 17different farms (8 confinements, 9 antibiotic-free) in Iowa and Illinois.

No MRSA has been found on antibiotic-freefarms in Iowa to date. Overall MRSA preva-lence in swine (confined and nonconfined pro-duction) was found to be 11%, whereas MRSAin confinement swine was higher. These results

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Harper et al. 103

indicate that colonization of swine by MRSA iscommon on the farm systems examined, butthere is variation according to the type of pro-duction system. This adds to the concern aboutdomestic animal species serving as a reservoirof this bacterium. Humans working in confine-ment operations, when compared to individualsnot working in confinements, had higher preva-lence MRSA nasal carriage.

A subset of isolates examined was negativefor the Panton-Valentine Leukocidin (pvl) geneand found to be ST398 by MLST.14 This wasthe first identification of ST398 in the UnitedStates. Although many human cases of ST398have been recorded in Europe, there have beenno published cases of clinical infections withST398 in the United States. However, fromthese data, it was concluded that individualsexposed to swine, especially in confinementoperations, may carry ST398, and are atincreased risk for ST398 MRSA clinical infec-tions. Further research is needed to define theclinical significance of this organism in theUnited States.

Another concern in confinement operationsis how this organism may be passed amongworkers and pigs. In addition to direct contact,we suspected the organism may be in the airwithin the buildings, creating an inhalationsource of infection. In order to test this hypoth-esis, we conducted viable bioaerosol samplingfor MRSA. To evaluate the presence and partic-ulate size distribution of MRSA, a six-stageviable Andersen Cascade Impactor was used tosimulate how MRSA particles can be depositedin the respiratory system. Additionally, theAndersen Cascade Impactor was used to quan-tify the concentration of airborne MRSA insideand outside a swine confinement building.

Potential health effects of aerosolized MRSAwere assessed by size distribution of particlesinto the nonrespirable and respirable sizeranges. Although the microbial concentration ofMRSA detected inside the confinement opera-tion was similar for the respirable and nonrespi-rable ranges, the concentration of MRSA insidethe confinement operation was higher than theconcentration detected outside. These results indi-cate MRSA can be deposited in the lower respira-tory airways and workers inside a confinement

operation are at a higher risk of exposure toaerosolized MRSA. How the deposition ofMRSA in the lower respiratory airways physi-cally affects confinement operators is not yetknown.

Because MRSA has been found in confine-ment air samples outside the building, concernhas arisen about MRSA as a potential environ-mental hazard and public health hazard.Biosecurity is intended to stop introduction ofpathogens into a system, prevent productionlosses due to existing diseases, and is key toprotecting pigs and workers. However, com-mon biosecurity measures, such as showeringin and out of the facility, may expose workersto risk factors for infection. Shower facilitieshave been known to be fomites for transmissionof MRSA between athletes. Therefore, wehypothesized that showers in swine facilitiescould also act as fomites, facilitating transmis-sion between pork production facility workers.

We addressed the question of MRSA pres-ence in pork production shower facilities byswabbing farming facilities where swine wereknown to be colonized with MRSA. Prelimi-nary results indicate that in production systemswith MRSA-positive swine, shower stalls canalso be contaminated with MRSA. This pre-sents a potential challenge to biosecurity mea-sures.

As well as reporting the first prevalence rateof MRSA among swine farmers, TheNetherlands16 and Italy17 reported 11.9% and0.45% MRSA colonization of raw retail meat,respectively. As this has been found in live-stock, questions emerged as to MRSA preva-lence in meat. In Iowa, convenience sampleswere taken from 22 grocery stores across Iowa,and included pork, chicken, beef, turkey, bison,veal, hen, and lamb. Isolates that tested positivefor methicillin-susceptible Staphylococcusaureus had a prevalence of 16.1%. MRSAprevalence was 1.1%.

The emergence of MRSA in livestock repre-sents a novel reservoir of this pathogen, but cur-rently the implications of this phenomenon areunclear. Additional research needs to be carriedout in order to determine the occupational andpublic health risk of livestock-associatedMRSA.

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104 MRSA AND FARMING

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