Methicillin-resistant Staphylococcus aureus (MRSA) is a problematic pathogen in human medicine and appears to be an emerging problem in veterinary medicine. Historically, hospital-associated MRSA infections have predominated in humans and contributed to significant illness and death ( 1 – 4 ). Recently, a shift in the epidemiology of MRSA infection has been documented, whereby community-associated (CA)-MRSA infections have become more common ( 5 – 9 ). CA-MRSA may arise from hospital-origin clones that are carried into the community and then transmitted between persons or from de novo development of resistance through acquisition of resistance factors (mecA) by methicillin-sensitive strains of S. aureus ( 10 ). Asymptomatic colonization with MRSA represents a major risk factor for infection or for transmission among persons within hospitals or the community ( 11 ). While CA-MRSA infections are becoming more widely reported, the prevalence of MRSA carriage overall remains low in healthy persons in the community ( 6 , 12 – 14 ). Reported prevalence of MRSA colonization in the community has been variable; the study population has a major effect on MRSA carriage rates. In the absence of recognized risk factors, the prevalence of colonization tends to be low. In a 2003 study, Salgado et al. identified MRSA colonization in 1.3% of persons overall but in only 0.2% of persons with no identified healthcare-associated risk factors ( 12 ). A study from Switzerland reported MRSA colonization in 0.09% of persons at the time of hospital admission ( 6 ). The prevalence of MRSA carriage was 0.3% in a 2005 study that Nulens et al. conducted at a European conference for physicians and others involved in clinical microbiology and infectious disease ( 15 ). MRSA infection and colonization have been reported in horses, dogs, cats, birds, and cattle ( 16 – 19 ). Transmission of MRSA between animals and humans has been reported ( 20 – 23 ) as have human MRSA infections from animal contact ( 16 , 21 , 24 ). Recent studies have identified high colonization rates in humans who have close contact with animals. MRSA colonization of persons who work with horses in Canada and the United States was 13% (14/107); on every farm where MRSA was identified in a horse, at least 1 person was colonized ( 25 ). In another study, 10 (9.7%) of 103 tested veterinary hospital personnel in a large-animal clinic were colonized with MRSA, and clinical skin infections were reported in 3 ( 26 ). Isolates from horses and humans in each of these studies were indistinguishable by pulsed-field gel electrophoresis (PFGE) and were typed as Canadian epidemic MRSA (CMRSA)-5 (ST8:MRSA:SCCmecIV, also known as USA500), which suggests transmission between horses and humans ( 27 ). A study at a small-animal referral hospital in the United Kingdom reported MRSA colonization in 17.9% of veterinary personnel. Investigation of clinical infection in 5 dogs and 3 cats found colonization in 14 (16%) of 88 household contacts or veterinary personnel ( 28 ). In all of the above reports, a screening bias for MRSA colonization may have been present if an outbreak had been ongoing in the population. Whether these results would accurately reflect the prevalence of MRSA in the general veterinary population, and therefore the occupational risk of MRSA exposure for veterinarians, is unclear. Our objective was to determine the prevalence of MRSA colonization in veterinary personnel attending an international veterinary conference and to characterize recovered MRSA isolates. Materials and Methods Study Population This study was performed at the annual American College of Veterinary Internal Medicine Forum held in Baltimore, Maryland, USA, June 3–5, 2005. The conference was attended by 3,240 persons: 2,744 practicing veterinarians, 354 technicians, and 142 other veterinary personnel involved in industry or research. Most (86%) attendees were from the United States; however, 43 other countries were represented. An information and sampling booth attended by the investigators was used to enroll adult volunteers; all attendees were eligible. This study was approved by the University of Guelph Research Ethics Board. Sample Collection Participants provided a single nasal swab sample each, which they collected themselves according to instructions to insert a cotton-tipped swab ≈1 cm into each nostril. The swabs were placed in liquid Stuart medium and maintained at 4°C until processing. Participants completed a brief questionnaire designed to identify potential risk factors for MRSA colonization: nationality, occupational position, type of clinical practice, veterinary patient contact, known exposure to MRSA in veterinary practice, previous hospitalization (within 30 days), previous MRSA infection, and residence with a healthcare worker. Practice types were small-animal (primarily dogs and cats), large-animal (primarily horses but also ruminants), and mixed (combination of large and small animals). We defined CA-MRSA colonization as MRSA isolation from a person with no history of healthcare-associated risk factors. MRSA Identification, Characterization, and Typing Swabs were placed into 2 mL of enrichment broth consisting of 10 g/L Tryptone T (Oxoid Inc., Nepean, Ontario, Canada), 75 g/L sodium chloride, 10 g/L mannitol, and 2.5 g/L yeast extract and incubated for 24 h at 35°C. Approximately 100 µL of broth was spread onto mannitol-salt agar with 10 g/L cefoxitin and incubated at 35°C for 48 h. Isolates were identified as S. aureus on the basis of colony morphologic features, gram-positive stain, catalase-positive reaction, positive tube coagulase test result, and positive latex agglutination test result (Pastorex Staph Plus, Bio-Rad Laboratories Ltd, Mississauga, Ontario, Canada). Methicillin-resistance was confirmed by demonstration of penicillin binding protein 2a with a latex agglutination antibody screening kit (Denka Seinken Co. Ltd, Tokyo, Japan). Antimicrobial susceptibility was performed by Kirby-Bauer disk diffusion according to the Clinical Laboratory Standards Institute (CLSI) guidelines ( 29 ); mupirocin MIC was determined by using E-Test gradient strips (AB Biodisk, Solna, Sweden). MRSA isolates were typed by SmaI PFGE and categorized as different CMRSA types as described previously ( 8 ). Real-time PCR was used to detect the lukF and lukS components of the Panton-Valentine leukocidin (PVL) gene previously described ( 30 ). Statistical Analysis Categorical comparisons were performed using χ2 analysis or Fisher exact test. A p value 1 sampling site to further characterize the prevalence of MRSA colonization in veterinary personnel. As MRSA expands into the community, changes in its epidemiology are inevitable. The lives of humans and animals, and their microflora, are closely intertwined. MRSA is now a pathogen of domestic animals that can be transmitted between animals and humans. Accordingly, further scrutiny of the roles of animals in MRSA infection and colonization is required. While occupational and recreational exposure to horses may be a risk factor for MRSA colonization, the effect of routine contact with household pets on the global epidemiology of MRSA is still unknown.