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Veterinary Forum August 2007 (Vol 24, No 8)

Clinical Report: "Methicillin-resistant Staphylococcus aureus"

by Sophia Yin, DVM, MS (Animal Science)

    Although most pyodermas, ear infections and abscesses can be resolved by implementing proper cleaning protocols and dispensing appropriate antibiotics, some cases simply fail to go away. These challenging cases often are associated with an underlying allergy or endocrine disorder. However, a third cause of growing concern is the presence of methicillin-resistant Staphylococcus aureus (MRSA), which can be transmitted from infected humans to pets and back again.

    According to Carlo Vitale, DVM, DACVD, a dermatologist at San Francisco Veterinary Specialists, "I have seen 50 to 60 cases of methicillin-resistant Staphylococcus aureus during the past 3 years. These cases present with skin problems and a history of chronic resistant infection." Vitale is particularly interested in this bacterial invasion because his professional pursuits focus on immune- mediated dermatologic conditions.

    Diagnosis of infection with MRSA is based on culture and sensitivity testing to reveal resistance to oxacillin, a marker for methicillin. (See image .) Pets with MRSA become most difficult to manage because of resistance to all other penicillins and cephalosporins, regardless of its susceptibility reported to other β-lactam antibiotics. "Such multidrug resistance often forces the need for vancomycin, which is an expensive drug that can cause deafness and renal problems," Vitale explains.

    Mechanism of resistance

    The mechanism of resistance for MRSA is somewhat unique. β-Lactam antibiotics, such as the penicillins and cephalosporins, work by binding to the active site of bacterial proteins that synthesize the bacterial cell wall. When antibiotics bind to these penicillin-binding proteins (PBPs), they inhibit cell wall synthesis, leading to bacterial death from cell lysis. Some bacteria are resistant to these antibiotics because they produce β-lactamase, which destroys the β-lactam ring portion of the antibiotic, rendering the antibiotic ineffective.

    When resistant bacteria started appearing in the 1950s, second-generation penicillins, such as methicillin, were introduced. The structure of second-generation penicillins can prevent destruction by bacterial β-lactamase.

    However, shortly thereafter, bacteria developed a second mechanism of resistance. Some bacteria produced a form of PBPs that still assembled cell walls effectively but didn't bind the β-lactam antibiotics well. Thus, the bacteria became resistant to methicillin as well as to all other penicillins and cephalosporins. Today, methicillin is no longer used, even for culture and antibiotic sensitivity testing. Instead, oxacillin is used as a marker for methicillin resistance.

    Serious human health implications

    Problems associated with MRSA infection extend beyond the health of individual pets. The real problem is the implications regarding human health. "In the past 30 years," Vitale says, "there have been a number of documented outbreaks of S. aureus in human hospitals, especially in pediatric and oncology wards."

    Pets can develop MRSA, as well as methicillin-resistant S. intermedius (MRSI), just as humans can. When pets have MRSA, the infection becomes a human health concern because pets can act as a reservoir and transmit the MRSA organisms back to humans.

    MRSA also can be transmitted by direct contact and, because it can live on inanimate objects for weeks, can be transmitted through contact with contaminated objects. For instance, Vitale describes one scenario involving a client who used a home-based office to train some physicians associated with the local hospital. "This client owned a cat that developed an abscess and subsequent MRSA. We suspect that the source of infection involved one of the physicians carrying his laptop from the hospital to the client's home." Culture of the wound confirmed the diagnosis.

    This cat could have acted as a source of infection for other humans. In fact, both animals and humans can become bacterial carriers.

    Vitale recalls another case in which a babysitter developed a MRSA infection at a tattoo site. She then transmitted MRSA organisms to the family for whom she was babysitting, and all the family members developed MRSA infections. Although all of the family members apparently have recovered, Vitale indicates that one child, who slept with the family dog, continued to be affected with recurring bouts of MRSA infection. Vitale decided to obtain nasal and rectal samples from the dog for culture and sensitivity testing, and the results were positive for MRSA organisms.

    Because MRSA infections can be transmitted through scratches or abrasions of the skin, Vitale says it is possible that the child had such entry points on the skin. After clearance of the nasal carriage, the child's recurrent MRSA abscesses ceased.

    What's the take-home message? Vitale suggests that if a pet presents with recurrent pyoderma, culture and sensitivity testing for S. aureus should be conducted to identify isolates that are resistant to oxacillin. Furthermore, if staphylococcal infections are resistant to oxacillin, he advises veterinarians to identify whether the animal is infected with MRSA or MRSI organisms. Vitale also recommends that veterinarians contact their nearest dermatology specialist or veterinary teaching hospital for guidance on how to treat pets with MRSA or MRSI infection and how to handle the sensitive issue of finding the human source of infection.

    See Web Exclusive Contact Precautions for Handling Patients Infected with Methicillin-Resistant Staphylococcus aureus.

    NEXT: Contact precautions for handling patients infected with methicillin-resistant Staphylococcus aureus


    Did you know... Methicillin-resistant Staphylococcus aureus infection is not always life-threatening. Most affected horses respond to appropriate treatment.Read More

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