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5 Things Veterinarians Should Know About Methicillin-Resistant Infections

Karen A. Moriello, DVM, DACVD University of Wisconsin–Madison

Infectious Disease

|July 2012|Peer Reviewed|Web-Exclusive

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The prevalence of methicillin-resistant Staphylococcal spp infections in animals is increasing, and this is no longer an “uncommon” finding in clinical practice.

Excellent information for clinicians is available in the recent edition of Infectious Diseases of the Dog and Cat, and University of Guelph's Worms and Germs Blog is a great resource for clients. Until you have time to read these sources, here are answers to some of the most commonly asked client questions:

1. This is the “doggie” MRSA, right?
It cannot be stressed enough that a distinction needs to be made between methicillin-resistant infections in dogs and cats and in humans. The primary pathogen of concern for this type of infection in humans is Staphylococcus aureus, and resistant infections are referred to as methicillin-resistant Staphylococcus aureus or MRSA. These infections can be hospital or community acquired. Hospital-acquired or nosocomial infections are contracted while the patient is in a hospital; in contrast, community-acquired infections refer to those that develop in patients with no known exposure to a healthcare setting. The primary pathogen of dogs is Staphylococcus pseudintermedius, and this is referred to as methicillin-resistant Staphylococcus pseudintermedius or MRSP. MRSA and MRSP are two different organisms with different biological behaviors. The similarity shared between these two organisms is their resistance pattern to antibiotics (ie methicillin resistance). These organisms share the mecA gene that confers resistance to all beta-lactam antibiotics (all classes and all generations of penicillins and cephalosporins). Methicillin resistance does not mean the organism is more virulent, and most infections can be treated successfully.1

Related Articles: Methicillin-Resistant Staphylococcal Infections

2. Can this infection be transmitted to humans?
The short answer is that transmission to humans is highly unlikely and very rare. This organism is of low zoonotic risk. When discussing this aspect, it is often helpful to first define the difference between “colonization” and “infection” for clients. Briefly, colonization refers to the presence of bacteria that are causing no harm (tissue invasion or damage), whereas infection refers to bacterial invasion and infliction of  clinical signs. Unlike in humans, in which S aureus colonization is common, colonization with S pseudintermedius is unlikely even among humans that have frequent contact with animals. In one study in which 144 healthy veterinary staff members were cultured, only one showed colonization.2 When hospital records of 3397 cultures were reviewed, only 2 S pseudintermedius isolates were identified.2

However, the following needs to be noted: dog bite wounds are a risk factor for infection, with one report from the UK identifying S (pseud)intermedius in 6/34 bite wounds.2 Transient colonization can occur in humans that are treating dogs with deep pyoderma due to MRSP. Two groups of owners were compared: owners with contact with a dog with deep pyoderma and owners without contact. S pseudintermedius was isolated from 6/13 owners of dogs with deep pyoderma compared to 1/13 in the non-deep pyoderma group. Typing revealed that the strains were identical between human and canine isolates. When owners were re-cultured once the deep pyoderma was resolved, they were found to no longer be carriers.3 Contact with purulent material was the most likely source and exposure to this and/or contact during bathing may be risk factors for colonization. Extra attention to good hygiene is always recommended when in contact with patients with MRSP infections, especially in anyone that is at increased risk for infection in general.

Related Article: A Brief History of Staphylococcus

Clinician's Brief
3. What precautions are needed at home during treatment?
The most important precaution is frequent and thorough hand-washing with soap and water after touching or handling the pet or its food bowls or bedding. Avoid contact with active lesions. It is especially important to teach children hand washing techniques (ie, with soap and water), as there have been anecdotal reports of toxicity in small children from exposure to alcohol hand sanitizers (eg, playing with and unsupervised overuse of sanitizer).The other pets in the home do not need to be screened or decolonized. There is no information about when and how to decolonize MRSP from colonized pets. This organism evolved to live on pets, and this makes it difficult if not impossible to decolonize pets. The infected pet does not need to be quarantined; however, until the infection is resolved the dog should not attend playgroups, dog day care, dog parks, training classes, or pet therapy activities. The intent is to limit contact and transmission to animals that may be susceptible to transmission and colonization (eg, pets with wounds, chronic skin diseases).Regarding home contamination, a recent study did show that MRSP could be present in the environment when there is an MRSP-positive pet in the home.4 Again, practical hygiene and cleaning are recommended. It is important to wash the pet bowls daily and clean any regular sleeping site. Regularly wash collars, leashes, toys, and bedding. Bathing of pets is part of the treatment plan and it is prudent to be especially attentive to hand washing and changing of clothes after bathing. Wash potentially contaminated bedding, towels, or clothes separately, and dry on hot settings. MRSP-positive dogs should not sleep on beds or with children.

Related Article: Roundtable: Current Thinking in Antimicrobial Resistance

Clinician's Brief
4. How is this treated?
Treatment is two-fold—systemic and topical therapy. Systemic antibiotic therapy is based upon culture and susceptibility to determine if there is a non-beta-lactam antibiotic treatment option. The two most common treatment options are potentiated sulfonamides (30 mg/kg PO q12h) and chloramphenicol (50 mg/kg PO q8h); however, many infections may be susceptible to clindamycin and/or minocycline.1 (It is important to note that susceptibility to minocycline must be determined by using testing; susceptibility to tetracycline is not a predictor.) The use of vancomycin and linezolid in veterinary patients is controversial, due to ethical concerns; these drugs are reserved for use in MRSA patients. Topical therapy begins with good coat hygiene; remove shed hairs and mats and clip the hair coat to facilitate bathing. Daily or every-other-day bathing with an antimicrobial shampoo (eg, chlorhexidine for 5 to 10 minutes is recommended). Topical 2% chlorhexidene solution can be used as a leave-on spray if owners cannot bathe pets that frequently.   Mupirocin ointment can be used to treat focal areas of infection.

5. How does a pet contract an MRSP or MRSA infection?
The factors that led to the increase in prevalence of methicillin resistance are not known, but there is evidence that one cause is the increased use of antibiotics in the last decade.5 Methicillin-resistant S pseudintermedius (MRSP) can be found in or on clinically healthy dogs and cats. Reported prevalence of colonization ranged from 1.5% to 17% in healthy dogs and in healthy cats was 1.2%.6 Typically, pets acquire MRSP via animal-to-animal infection; healthy colonized pets are the most likely source. If an MRSA is isolated as the primary pathogen from a pet, the most likely source of exposure is from a human. Studies have shown that fewer than 4% of healthy dogs or cats are colonized with MRSA. Risk factors for MRSA in pets include surgery, hospitalization, previous antibiotic use, or exposure to humans with MRSA. Therapy pets in hospitals are also at increased risk. 


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