Nasal swabs were provided by 417 attendees from 19 countries. MRSA was isolated from the nasal cavities of 27 (6.5%) of 417 persons: 15 (15.6%) of 96 in large-animal practice, 12 (4.4%) of 271 in small-animal practice, and 0 of 50 in industry or research (p<0.001) (Table 1). The prevalence of colonization was higher in men (13/139, 9.4%) than in women (14/265, 5.3%); however, this difference was not statistically significant (p = 0.09).

Colonized persons were from the United States (n = 23), United Kingdom (n = 2), Denmark (n = 1), and Germany (n = 1). Difference in the prevalence of colonization between countries was not significant (p = 0.18). Colonization rates in persons from the United States, Canada, and the United Kingdom—the 3 countries with the highest representation—were 6.0%, 0%, and 17%, respectively (p = 0.11). According to stepwise forward logistic regression, employment in a large-animal practice was the only variable independently associated with MRSA colonization (OR 2.9; 95% CI 1.2–6.6).

Characterization of MRSA isolates, using PFGE, identified 2 predominant clones: CMRSA-5 (ST8-MRSA-IV, similar to USA500) and CMRSA-2 (ST5-MRSA-II, similar to USA100) (8,31). CMRSA-5 was isolated from 13 (48%) of 27 colonized persons, all of whom were in large-animal practice. These persons were from the United States (n = 10), United Kingdom (n = 2), and Denmark (n = 1). CMRSA-2 was isolated from 13 (48%) of 27 colonized persons: 11 in small-animal practice and 2 in large-animal practice from the United States (n = 12) and Germany (n = 1). One other isolate, possibly related to CMRSA-2, was recovered from a US veterinarian in small-animal practice. Overall, CMRSA-5 was more commonly isolated from persons in large- than in small-animal practice (p<0.001). A cluster of 5 colonized persons was identified from a US school of veterinary medicine, where 4 persons who worked with large animals were colonized with CMRSA-5, and 1 veterinarian who worked with small animals was colonized with CMRSA-2. No isolates were identified as carrying the lukF or lukS genes. All isolates were susceptible to vancomycin; other antimicrobial drug susceptibility results are shown in Table 2.

This study represents the most comprehensive evaluation to date of MRSA colonization in veterinary personnel. Although a control group was not included, the prevalence in veterinary personnel (6.5%) was higher than previously reported rates for community-based colonization (6,12–14,32). When compared with results from a similar study in which only 0.3% of medical professionals at a conference were colonized (15), our results suggest an increased risk for veterinary professionals. However, geographic location and culture methods may have affected the differences in study results, and further investigation is required to more accurately define the occupational risk.

The PFGE type distribution provides support that MRSA is being transmitted between animals and humans. A significant difference was identified between clones recovered from those who worked with large animals (CMRSA-5) and those who worked with small animals (CMRSA-2). If these results merely represented the background level of CA-MRSA colonization in the general population, this difference would not be expected. In addition, CMRSA-5 has not been commonly identified in persons in the community, at least not in Canada (8,33), although it has accounted for most of the reported cases of MRSA infection or colonization in horses (16,24,27). These findings, along with the identification of CMRSA-5 in large-animal veterinarians from the United States, United Kingdom, and Denmark, provide further evidence that CMRSA-5 is widely disseminated in the horse population and may be transmitted between horses and humans. The CMRSA-5 isolate from the Danish veterinarian was further classified as sequence type 8 and Ridom spa type t064, which has been identified in humans in Denmark, Norway, Germany, Belgium, and Sweden (R. Skov, pers. comm.). CMRSA-5 predominance in large-animal veterinarians may infer an occupational health concern for veterinary professionals who have contact with equine patients. Furthermore, this study's finding of large-animal practice as the only variable associated with colonization and the striking prevalence of MRSA colonization in persons who work with large animals (15.6%) also support an occupational risk for MRSA exposure in large-animal practice. Because zoonotic infections have been associated with exposure to CMRSA-5 (26), further evaluation of interspecies transmission within large-animal veterinary practices by routine screening of patients and personnel, along with implementation of infection control practices, may be warranted.

Identification of CMRSA-2 primarily in small-animal veterinarians was consistent with previous reports that MRSA isolates from dogs and cats reflect the predominant human strains in the community (17,22). In Canada, CMRSA-2 is a hospital-origin clone that has emerged in the community as a major cause of CA-MRSA infections in people (33,34). The lower prevalence of colonization in persons who work with small animals and the predominance of a common community clone make it more difficult to implicate animal contact as the source of MRSA. However, because colonization in persons who work with small animals was higher than community prevalence rates and because MRSA transmission from household pets to humans has been reported (20,21), dogs and cats must be considered as possible sources of MRSA. A comprehensive evaluation of MRSA colonization in small animals and risk factors for interspecies transmission are required to determine the true occupational risk for MRSA colonization and infection in small-animal veterinary personnel (17,18).

Although CA-MRSA isolates that express the PVL genes are frequently a cause of severe skin and soft tissue infections in the community (8,9,35–37), this virulence factor was not identified in isolates from veterinary personnel. These results are similar to those of previous studies in which MRSA isolates from veterinary species or personnel have been negative for PVL genes (24,26,28,38). However, considering the recent dissemination of the PVL-positive USA300 clone throughout North America and that MRSA isolates in dogs and cats often reflect the predominant community clones, PVL-positive clones may emerge in small animals (36). Further monitoring of the dynamic epidemiology of CA-MRSA is required to determine whether animals will have any role in dissemination of this clone.

Previous contact with MRSA in a colonized or infected animal was reported by 6.9% of colonized persons but was not significantly associated with MRSA colonization in veterinary personnel. The number of personnel reporting previous contact with an infected animal was low, thereby limiting the ability of this study to identify an association. Intuitively, one would assume that contact with MRSA-infected animals in veterinary practice would represent a high-risk situation; further investigation is required to more accurately determine this risk.

This study has several limitations. First, the sample used was a convenience sample of attendees at the Forum, which allows potential sample bias. Second, a greater proportion of veterinary personnel sampled were in specialty practice, leading to results that, because of different patient populations, may not apply to general practitioners. Third, because this was a cross-sectional study, causality between risk factors and colonization could not be determined; only an association between variables and colonization could be implied. The small sample size from several countries also limited the statistical power to identify an association in these populations. Lastly, because attendees collected their own nasal swabs, sampling variation may have led to underestimation of the prevalence of colonization. The variable sensitivity (75%–93%) of using nasal swab screening alone (39) could also have led to false-negative results. Ideally, a randomized sample of general and specialty practitioners would be performed using >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.