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Equine May/June 2007 (Vol 2, No 3)

The Editor's Desk: "The Emergence of Some Mighty Microbes"

by Katharina Lohmann, MedVet, PhD, DACVIM (Large Animal)

    My "exposure" to the issue of methicillin-resistant Staphylococcus aureus (MRSA) has been limited. At one point, I swabbed my nose for a research study and was admittedly glad to learn that I was not colonized. Then, more recently, I attended a seminar after MRSA was identified in two small animal patients at the University of Saskatchewan Veterinary Teaching Hospital. Although there were some humorous aspects to the seminar—the presenters from the local medical school mused about the challenge of "barrier precaution" in an environment where patients weren't beyond licking the clinician's face—it made me think about the impact of emerging diseases on our daily work and the veterinary profession.

    MRSA has been identified as an emerging pathogen and has received a lot of attention in the medical literature. Although the pathogen is not new—methicillin resis­tance of S. aureus was identified as early as 1961, and the first report of MRSA as a cause of mastitis in cows was in 1972—the perceived threat appears to be increasing. In human medicine, this may be largely due to the recognition of "community-associated" strains of the organism that vary distinctly from strains identified in cases of nosocomial infection. In addition, MRSA prevalence has reportedly increased dramatically in the past 30 years, and some authors1 report methicillin resistance rates higher than 50% among S. aureus strains.

    Information specific to veterinary patients is more limited. Although many questions remain unanswered, several aspects of the epidemiology of MRSA have been elucidated and are of interest and importance to the veterinary community.

    First, MRSA has to be considered as a cause of disease in veterinary patients. While this may be the least of our problems at the moment, it may become more important in the future. Aside from mastitis, reported infections range from metritis in mares to skin infections and infection of surgical implants and intravenous catheter sites in small as well as large animals. In some of these patients, treatment with appropriate antimicrobials resolved the condition, while in others, MRSA was implicated as a cause of death. As clinicians, therefore, we should consider MRSA in cases of nonresolving infection and should base antimicrobial treatment on sensitivity testing of isolates, when possible.

    Second, we must consider the significance of MRSA as a zoonotic pathogen and address the relevance of MRSA colonization in clinically healthy individuals. The carrier status has been identified in humans and animals, and it is likely that transmission in any direction (i.e., human to human, human to animal, animal to animal, animal to human) occurs. Several reports2,3 suggest that persistence of the carrier status in humans was linked to MRSA colonization of an animal, and animals have been implicated as sources of outbreaks of MRSA infection in humans. Conversely, skin infection with MRSA in one person working with infected horses has been demonstrated.4 Interestingly, MRSA isolates from dogs and cats have largely been identical to those in humans, while equine isolates in some studies represent distinctive clones of the organism. MRSA epidemiology in equine hospitals may further vary in that the organism seems to persist in the environment.

    I think that as a profession, we should invite and support further investigation of MRSA as a pathogen relevant to veterinary medicine and as a public health concern. In addition, we should all take responsibility in counseling our clients. Veterinarians play an important role in dealing with zoonotic disease, and we should rise to the challenge. With regard to hospital management, we must add MRSA (and other methicillin-resistant staphylococci) to the list of pathogens that we need to guard against through proper biosecurity measures. In recent years, closure of several teaching hospitals because of infectious disease outbreaks has painfully put this problem on the map, and it is likely here to stay. Some institutions have already adopted large-scale screening programs for infectious disease pathogens, added biosecurity fees for their clients, and employed infectious disease specialists to establish and monitor biosecurity protocols. Discussions about the approach to biosecurity typically revolve around cost:benefit ratios, compliance, and client reassurance, but many of us feel that we can no longer afford not to address the problem proactively. The good news, at least regarding MRSA, is that commonsense practices, such as washing or disinfecting hands between patients, go a long way in preventing transmission. Routine screening of the environment, hospital admissions, and personnel working with veterinary patients is a more sensitive issue but may become a reality at some point.

    And third, there are great concerns about MRSA contributing to antimicrobial resistance in both human and veterinary medicine, and we should not ignore our potential role in this dilemma. MRSA strains are generally multidrug resistant, and transmission of antimicrobial-resistant genes among organisms is suspected. While some may argue that veterinarians are already more conscious of proper antimicrobial use than physicians, we are beginning to face restrictions regarding drug use in our patients, and we do not have the larger lobby in this argument. We help our profession by acknowledging the problem and demonstrating our commitment to public health. Ensuring judicious use of antimicrobials in keeping with the approach proposed by the American College of Veterinary Internal Medicine,5 which is available at www.acvim.org, may be a beginning.

    1. Klevens RM, Edwards JR, Tenover FC, et al: Changes in the epidemiology of methicillin-resistant Staphylococcus aureus in intensive care units in US hospitals, 1992-2003. Clin Infect Dis 42(3):389-391, 2006.

    2. Manian FA: Asymptomatic nasal carriage of mupirocin-resistant, methicillin-resistant Staphylococcus aureus (MRSA) in a pet dog associated with MRSA infection in household contacts. Clin Infect Dis 36(2):e26-e28, 2003.

    3. Scott GM, Thomson R, Malone-Lee J, et al: Cross-infection between animals and man: Possible feline transmission of Staphylococcus aureus infection in humans? J Hosp Infect 12(1):29-34, 1988.

    4. Weese JS, Caldwell F, Wiley BM, et al: An outbreak of methicillin-resistant Staphylococcus aureus skin infections resulting from horse to human transmission in a veterinary hospital. Vet Microbiol 114:160-164, 2006.

    5. Morley PS, Apley MD, Besser TE, et al: Antimicrobial drug use in veterinary medicine (ACVIM Consensus Statement). J Vet Intern Med 19:617-629, 2005.

    References »

    NEXT: The Final Diagnosis: "Emergency Response"