The emergence and prevalence of MRSA, MRSP, and MRSS in pets and people - Veterinary Medicine
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The emergence and prevalence of MRSA, MRSP, and MRSS in pets and people
Should you be concerned about getting a form of this stubborn infection from your patients—and vice versa? Here's an overview of this increasingly common and concerning bacteria—consider it MRS 101.


MRSA in pets

Methicillin-resistant staphylococci have been documented since the 1970s in large animals but was not reported in dogs until the 1990s.10 The prevalence of S. aureus colonization in dogs and cats is low and, when present, is usually assumed to have originated from an in-contact human.11-17 Epidemiological studies generally show that MRSA in pets belongs to the same strains as the dominant regional hospital-acquired and community-associated MRSA (in contrast to MRSA from livestock animals, which usually belongs to unique animal-associated lineages).18

Most small animals exposed to MRSA do not develop clinical disease; some become temporarily colonized, and most eliminate the organism. Colonization involves survival of MRSA on the body without any adverse effect. As in people, colonized animals usually show no adverse effects unless risk factors allow for development of clinical infection, including surgery, trauma, skin wounds, and immunosuppression. Optimal sites for MRSA screening in small animals have not been identified, but most references cite the use of nasal, rectal, and perineal swabs for bacterial culture.11,12,15,16,19

MRSA in dogs and cats is most frequently cultured from wounds, abscesses, otitis, and pyodermas.20 In one review of 40 dogs infected with MRSA compared with 80 dogs with methicillin-susceptible S. aureus (MSSA), risk factors identified were receipt of antimicrobial drugs (especially fluoroquinolones and beta-lactams, which increased risk of MRSA by five and three times, respectively) and intravenous catheterization.21 In a study comparing carriage of S. aureus in 50 dogs with normal vs. 59 dogs with inflamed skin, 16% of healthy dogs carried MSSA, and none carried MRSA. Staphylococcus aureus was cultured in 12% of dogs with inflamed skin, and 17% of these cases were methicillin resistant.20

Unlike MRSA in people, which is often associated with increased morbidity and mortality, there does not appear to be a significant difference in patient outcome between dogs infected with MSSA vs. MRSA, possibly because most infections are superficial (pyodermas and otitis) and not invasive. This is an important point to consider when counseling owners of pets with MRSA.21

Do pets contract MRSA from their owners?

By the numbers: The MRSA link between people and pets
Potential risk factors for acquisition of MRSA colonization by pets include contact with children and contact with human hospitals (especially if pets are allowed to lick patients or be fed treats by patients).22 Ownership of a pet by a person in the healthcare field was found to be a risk factor for MRSA colonization of the pets in one study,15 but not in another.2 Other studies have investigated the link between people and pets as well (see the sidebar "By the numbers: The MRSA link between people and pets").

1. Erythema multiforme complicated by MRSA in a dog. The owner was also affected by a nonhealing wound that was found to be infected with MRSA.
Although pets that are colonized or infected with MRSA most likely contracted the bacteria from people (Figure 1), pets may have the capability to be carriers of MRSA and subsequently pass it back to in-contact people, though most of the evidence is circumstantial and cannot definitively prove the direction of transmission.12,16 In a study from the United Kingdom, MRSA carriage was 7.5% in 120 owners of MRSA-infected pets—much higher than the 1.5% carriage in the general U.K. population.23 In another report, 47 dogs and 52 cats were sampled in 66 households in which a MRSA-infected human patient resided; 11 of 99 pets (11%) representing nine (13.6%) households were MRSA-positive.24 In six of these households, the human and animal source strains were genetically concordant. For each day of delay in sampling the pet after the person's MRSA diagnosis, the chance of isolating MRSA from the pet decreased by 13.9%, suggesting that MRSA carriage in pets is temporary.24


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