Staphylococcus species are part of the normal mammalian and avian flora, but the worldwide increase in methicillin-resistant infections is likely a reflection of increased antibiotic use.
The Big Misnomer. Although methicillin resistance is the common term, today oxacillin is the antibiotic used for susceptibility testing, as methicillin is no longer manufactured and oxacillin is considered to be more stable.
- Staphylococcus bacteria (gram-positive, facultative, aerobic cocci) are opportunistic pathogens that can cause skin, body cavity, and soft tissue infections when inflammation is present and/or the host has compromised defense mechanisms.
- S aureus, S schleiferi, and isolates of the S intermedius group (SIG; ie, S pseudintermedius, S intermedius, S delphini) are important organisms in veterinary medicine.
- Molecular testing has determined that the primary pathogen of dogs is S pseudintermedius; previous S intermedius isolates are now believed to have been S pseudintermedius.
- The occurrence of methicillin-resistant staphylococcal infections is increasing.
- Methicillin resistance is encoded by the mecA gene, which confers intrinsic resistance to all b-lactam antibiotics (ie, penicillins, cephalosporins).
- The gene is located on the staphylococcal chromosomal cassette along with other resistance genes, allowing resistance to multiple antimicrobials.
- In humans, methicillin-resistant S aureus (MRSA) can be hospital- or community-acquired.
- Human or animal S aureus carriers without clinical signs are classified as colonized.
- Studies have shown that up to 30% of humans have nasal colonization of S aureus; approximately 0.5% of those isolates are MRSA.1,2 The percentage of isolates of the methicillin-resistant S intermedius group (MRSIG) is low (<0%–5%), depending on the study and where it was conducted.1
- Skin, soft tissue, and the ears are the most common sites of methicillin-resistant infection; however, any organ has the potential for infection.
- Methicillin resistance has been noted worldwide in humans and animals, but prevalence of MRSA and MRSIG varies widely with geographic distribution.
- 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%.3
- Methicillin resistance can occur in dogs and cats of any age, breed, or sex.
Transmission & Risk Factors
- Pets are most likely to contract S aureus or MRSA from humans.
- Studies have shown that less than 4% of healthy dogs and cats are colonized with MRSA.
- Risk factors for MRSA in pets include infections related to surgery, hospitalization, prior antibiotic use, or exposure to humans with MRSA. Therapy pets in hospital visitation programs are also at increased risk.
- Pets typically acquire MRSP via animal-to-animal contact; healthy colonized pets are the most likely source.
- Clinical signs of methicillin-resistant infections are identical to methicillin-sensitive infections. Clinical suspicion of methicillin resistance includes lack of response to appropriate empiric therapy, worsening of signs while receiving therapy, and/or recurrent infections (particularly of the skin and ears).
MRSA = methicillin-resistant S aureus, MRSIG = methicillin-resistant S intermedius group, MRSP = methicillin-resistant S pseudintermedius, SIG = S intermedius group
- Speciation of S pseudintermedius, S intermedius, and S delphini requires molecular testing; most laboratories will report SIG isolates.
- S schleiferi and S aureus can be identified by standard laboratory speciation test kits (eg, API Staph, biomerieux-diagnostics.com).
- Methicillin resistance requires culture and susceptibility testing via broth or agar-based tests.4
- Accurate detection of methicillin resistance can be difficult because of heteroresistance (ie, the presence of 2 subpopulations [one susceptible, the other resistant] that can coexist within a culture).
- These cells grow more slowly than oxacillin-susceptible populations and longer incubation periods are required to avoid missing the diagnosis.5
- PCR assay is considered the gold standard for detection of the mecA gene.
- Many methicillin-resistant staphylococcal infections in dogs and cats are complicated by secondary yeast overgrowth; cytologic examination of glass microscope slides or clear acetate tape is useful to identify organisms.
A dog was presented with a 3-month history of severe lameness and swelling of the lower leg. Culture of a draining tract between the foot pads (A) revealed a MRSIG infection susceptible to chloramphenicol. After 5 days of treatment, the paw is markedly improved (B). The dog made a full recovery. (View larger image)