MRSA in Pets: A Public Health Concern?

By Susan Sanchez, BSc, MSc, PhD, MIBiol, CBiol

Staphylococci are Gram-positive, facultative anaerobic cocci. The genus is subdivided into many different species and most of them are host adapted. For the most part, staphylococci are skin commensals of healthy individuals where they can be carried transiently or permanently and only cause disease when there is a breech of epithelial integrity. The most common clinical presentations are skin abscesses. The species of staphylococci most frequently isolated from both healthy and sick dogs and cats is Staphylococcus intermedius (SI). Staphylococcus aureus (SA), although isolated from animal infections, is not as common as SI. The susceptibility profiles of both staphylococci species is changing, with clear increases in the number of isolates that are multidrug resistant (MDR). Furthermore, the MDR profile of SA isolated from pets mirrors that of human methicillin-resistant Staphylococcus aureus (MRSA) isolates. MRSA is on the rise among our pets here in Georgia (see Figure 1). SA has coevolved with people and is a well-adapted commensal of human skin. Hence, SA in pets is viewed by many veterinary epidemiologists as an anthropozoonosis, or a reverse zoonosis.

Community-acquired MRSA (CA-MRSA) in people is currently one of the most common and dreaded pathogens in outpatient care. These organisms are clinically and epidemiologically distinct from health care–associated MRSA (HA-MRSA). The number of people affected by CA-MRSA is increasing. It was estimated that 94,360 invasive MRSA cases occurred in the United States in 2005, with 18,650, or about 1 in 5, of those cases resulting in death. The GA Division of Public Health confirms that there were 13 deaths due to invasive MRSA in 2005, eight of them involving young, previously healthy individuals. Most interestingly, we have seen that this increase in MRSA among people in Georgia mirrored an increase in MRSA in the animal population.
Studies conducted at the Athens Veterinary Diagnostic Laboratory (AVDL) into the epidemiology of MRSA show that CA-MRSA and HA-MRSA are currently circulating among people in our local community with no ties to health care institutions.

These same strains seem to be also established among our pets in which they are causing recalcitrant infections. In parallel, we are seeing in our clinical cases an increase in the numbers of SI that are MDR, including methicillin-resistant (MRSI) and coagulase-negative SA that are also MDR. The AVDL is working in partnership with faculty members from the UGA Department of Genetics and the UGA College of Public Health to elucidate the epidemiology of antibiotic resistance as well as the co-host evolution of MRSA. Most of the resistance genes that are responsible for MDR are located in mobile elements that can be transferred from bacteria to bacteria. The bacteria-acquiring resistance genes in turn emerge as the dominant population with the use of antimicrobials. Intense selection for MRSA in human hospitals is known to occur when fluoroquinolones are used empirically for treatment of infections.

These findings bring up several important questions:

  • Are pets reservoirs of MRSA and/or MRSI?
  • Is there anything we can do to avoid the spread of MRSA and MRSI in pets?
  • Are we aiding in the selection of these MDR strains?

Our continued research at the AVDL is aimed at answering these questions. If you have any questions, please contact Dr. Susan Sanchez at [email protected]

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