MRSP: A MORE COMMON CULPRIT IN PETS
While S. aureus is not considered to be a normal flora on dogs, companion animals do normally carry other species of Staphylococcus bacteria that can become pathogenic—S. pseudintermedius (previously known as Staphylococcus intermedius) most commonly and, less commonly, S. schleiferi (see below).16,19,20,25,26 In recent years, MRSP has emerged as a clinically important pathogen causing infections in dogs and cats.
Like MRSA, methicillin-resistance in S. pseudintermedius is mediated by PBP2a, which is encoded by the mecA gene. In addition to beta-lactam antibiotic resistance, most MRSP strains
are also resistant to other classes of antibiotics.12,27
MRSP in people
Staphylococcus pseudintermedius is not considered to be a human pathogen, but it has occasionally been reported to cause severe infections in people,28 and people may be colonized by S. pseudintermedius, in some cases by the same strain of S. pseudintermedius that infects their pets.29-35 (See the sidebar "By the numbers: MRSP in people.")
By the numbers: MRSP in people
MRSP in pets
The prevalence of MRSP colonization varies depending on the population studied, with rates of 1.5% to 2% in dogs in the community
and dogs admitted to veterinary hospitals16,20 and up to 7% of dogs with inflammatory skin disease (Figures 2A & 2B).20 The prevalence of MRSP in healthy cats was found to be 4%.36 As in MRSA in people, risk factors for the development of MRSP in animals include prior antibiotic use and hospitalization;
emergence of methicillin-resistance in S. pseudintermedius may be due to selection pressure from antimicrobial use or horizontal spread of resistance factors from MRSA within the community.16,37-40
2A & 2B. An atopic Lhasa apso with erythematous and lichenified skin due to secondary spreading superficial pyoderma caused
Most clinical reports of methicillin-resistant infections have included superficial and deep pyoderma, otitis, and wound infections.
Unlike MRSA in people, there is no indication that MRSP is more virulent than methicillin-susceptible S. pseudintermedius (MSSP), and most reported infections have been treated successfully, though possibly with a longer time to resolution.41
For example, treatment outcomes in dogs with methicillin-resistant pyoderma compared with outcomes in methicillin-susceptible
cases were evaluated in a study of 123 MSSP and 93 MRSP clinical cases.42 The study found that most cases resolved (with topical treatment, systemic antimicrobial therapy, or both), regardless of
methicillin sensitivity, although some MRSP cases took longer to resolve compared with the MSSP dogs. Corticosteroid administration
was significantly associated with lack of resolution of all pyodermas at the first three- to four-week recheck examination.42
In a study comparing 56 dogs with MRSP infection with 112 control dogs with MSSP infections (most patients had pyoderma),
mortality rate was not significantly different between study groups, and systemic administration of antimicrobials within
30 days before diagnosis of infection was significantly associated with risk of MRSP.40
Clinical cure of MRSP does not necessarily equate to microbiologic cure. In a sampling of dogs that initially had an MRSP
pyoderma, 26 of 42 (61.9%) were still colonized with MRSP at follow-up, even though the pyoderma had clinically resolved.
Additionally, in the same study of 60 dogs with pyoderma that did not have MRSP initially, MRSP was isolated from the skin
after treatment in 17 dogs (28.3%).43
MRSS: A NEW PLAYER ON THE FIELD
Staphylococcus schleiferi subspecies coagulans is a relatively newly described coagulase-positive staphylococcal pathogen in dogs.13,20
Staphylococcus schleiferi subspecies schleiferi is a coagulase-negative variant that has been reported to be a normal component of human preaxillary flora and also has been
implicated in human nosocomial infections, postsurgical infection, osteomyelitis, and endocarditis.44 This variant can also be pathogenic in animals and is increasingly found in methicillin-resistant infections.44
Although S. schleiferi is isolated less commonly in veterinary patients, one study found that S. schleiferi has a higher incidence of methicillin resistance then either S. aureus or S. pseudintermedius.45 In a study of 225 dogs with S. schleiferi infections (usually of the skin and ears), the most common underlying cause was atopy, and prior treatment with beta-lactam
antibiotics was a risk factor for methicillin resistance. Of the 225 isolates (117 coagulase-negative and 95 coagulase-positive;
13 had no coagulase status reported), 129 (57%) were methicillin resistant, and coagulase-negative isolates were more likely
to be methicillin resistant than coagulase-positive isolates.44
Now that you know more about how common methicillin-resistant staphylococci are in pets and the risks of infection to both
people and pets in your veterinary practice, see next month's issue for a guide to diagnosing and treating these stubborn
infections in your patients and steps you can take to curtail their occurrence.
Kimberly S. Coyner, DVM, DACVD
Dermatology Clinic for Animals of Las Vegas
5231 W. Charleston Blvd.
Las Vegas, NV 89146