Felis ISSN 2398-2950

MRSA infection

Synonym(s): Methicillin-resistant staphylococcus aureus infection; Meticillin-resistant staphylococcus aureus infection

Contributor(s): Sarah Binns, Karen Coyne, J Scott Weese


  • Methicillin resistance staphylococcus aureus (MRSA Methicillin-resistant Staphylococcus aureus) are strains of the gram positive bacterium, Staphylococcus aureus Staphylococcus aureus, that are resistant to the beta-lactam antibiotic methicilin as well as all other beta-lactam antibiotics, including the penicillins, cephalosporins and carbapenems.
  • Methicilin (also known as meticillin and more recently replaced by oxacillin), is a narrow-spectrum antibiotic of the beta-lactamases. It has been widely used to treat human infections caused by gram-positive bacteria, since the 1950s.
  • Its use was primarily against Staphylococcus aureus bacteria that were resistant to most penicillins.
  • The earliest recorded MRSA was identified in staphylococci isolated from humans in the UK in 1961.
  • MRSA was first reported to be a major problem in human hospitals in the USA in the 1970s, and in the 1990s it became recognized as an important cause of hospital-associated infection in human hospitals all over the world. These hospital-acquired strains of MRSA (HA-MRSA) cause the majority of human infections.
  • Recently, MRSA has become an important cause of infection in people in the general population (community-associated MRSA CA-MRSA). Although these are less prevalent than HA-MRSA they are thought to be more virulent.
  • In the UK, the most common strains of MRSA isolated from humans are epidemic MRSA (EMRSA) strains 15 and 16. These are both associated with HA-MRSA infections.
  • The first isolates of MRSA from domestic animals were detected in milk from cows with mastitis in the early 1970s.
  • MRSA is becoming an increasingly recognized problem in companion animal medicine. Over the past few years it has become increasingly apparent that MRSA infection in the UK is much more widespread in companion animals than previously thought.
  • MRSA tends to occur as sporadic cases or outbreaks in veterinary hospitals and appears to be relatively uncommon in cats in the community.
  • Methicillin resistance is also found in other species of staphylococci isolated from animals, in particular pigs, poultry, cattle and sheep.
  • Colonization of nasal passages may occur with or without clinical signs. When clinical infections are apparent they can range from mild skin infections to severe or fatal bacteremia.
  • Samples from suspected clinical cases, as well as samples from asymptomatic individuals (usually from nasal passages) can be submitted for bacteriological culture and species identification
  • Subsequent sensitivity testing can be used to determine the course of further antimicrobial therapy.
  • In cats, methicillin resistance within S. aureus, has been documented, but its occurrence has been more extensively studied in dogs.
    Print off the owner factsheet Antibiotic resistant bacterial infections Antibiotic resistant bacterial infections to give to your client.



  • S. aureus resistant to the beta-lactam antibiotic methicillin (meticillin, oxacillin).
  • Gram-positive cocci, catalase-, mannitol- and maltose-positive.
  • Both coagulase-positive and coagulase-negative species have been isolated from healthy feline skin and from cats with skin lesions.

Predisposing factors


  • Risk factors for hospital-associated MRSA in humans include proximity to other patients with MRSA, long-term antibiotic treatment, long-term hospitalization, intensive care, immunosuppression and surgery.
  • Risk factors for the isolation of MRSA in cats include number of antimicrobial courses (more than 3 courses within 6 months prior to isolation of MRSA), prolonged hospitalization, immunosuppressive treatment, ongoing infection, postoperative or other wound infections, surgical implant (eg orthopedic) and contact with at least one human admitted to hospital.
  • Infections may be seen in healthy individuals in the absence of recognized predisposing factors.
  • The presence of a cat in the home has been found to be a strong predictor for the isolation of MRSA from environmental surfaces in the home.


  • Penicillinases (often beta-lactamase) are synthesized by at least 80% of isolates of S. aureus. These can confer resistance to some beta-lactam antibiotics but are not associated with methicillin-resistance.
  • Infections with S. aureus have therefore often been treated with antibiotics that are able to resist penicillinase action, such as methicillin.
  • Most strains of MRSA exhibit multi drug resistance; ie they are resistant to many other classes of antimicrobial agents.
  • Use of cephalosporins and fluoroquinilones has been shown to contribute to the selection of MRSA strains in humans and cats.
  • Resistance to methicillin and other beta-lactam antimicrobials is caused by possession of the mecA gene, which codes for a penicillin-binding protein in the bacterial cell wall, PBP2a, that does not allow beta-lactam antibiotics to bind effectively.
  • This gene is part of the larger staphylococcal chromosomal cassette (SCC) mec, and may have been acquired from other species of coagulase-negative staphylococci.


  • MRSA colonizes the nasal passages of healthy individuals. It can also be found in the intestinal tract and on the skin and oral mucous membranes.
  • Infections generally often associated with exposure to one or more of the predisposing factors listed above but many infections occur in the absence of any identifiable risk factors.
  • Colonized cats may be a source of infection for themselves or others (cats and dogs) via both direct and indirect (eg environmental) contamination.


  • In cats, clinical infection has been reported to occur days to weeks after colonization.


  • Human hospital-associated MRSA is usually associated with dissemination of an epidemic clonal lineage of S. aureus, eg MLST ST254, EMRSA-15 (CC22) in the UK, CMRSA-2/USA100 in Canada and the US. Community-associated MRSA infections in North America are largely caused by USA300/CMRSA10.
  • Although there are less reports of MRSA in cats than in dogs, most reports of MRSA in cats have involved EMRSA strain 15, the same as that present in dogs and humans.
  • Carriage of S. aureus is less common in most animals than in humans. MRSA comprises approximately 3% of submissions to veterinary laboratories.
  • In one study of 148 samples from healthy cats, methicillin resistance was found in three of 14 S. aureus isolates, eight of 26 S intermedius, six of 11 Staphylococcus simulans and five of 37 S felis isolates.
  • MRSA isolates have been obtained from cattle, horses, cats, dogs, chickens, sheep, and pigs, described in reports published from 1972-2005.
  • A survey of university veterinary clinics found that 14% of patients (both dogs and cats) with S. aureus had MRSA, however, the prevalence was most common in dogs.
  • Environmental contamination is thought to be a significant source of MRSA infections in veterinary hospitals, but transmission is thought mainly to occur via human hands. MRSA can also exist in airborne bioaerosols but the relevance of this for transmission is unclear. 
  • Increasing numbers of veterinary personnel have been found to carry MRSA strains that have also been isolated from both dogs and cats.
  • There is accumulating evidence that strains of MRSA can be transmitted in both directions between humans and companion animals. It is probable that domestic pets become colonized with human MRSA strains, and then become a reservoir for re-infection of in-contact humans.
  • Feline MRSA infections have occurred in clusters in veterinary hospitals in the UK, and also in the USA, Ireland and Austria.
  • Isolates of MRSA from cats in different countries appear to be of different (but typically related) types, according to molecular epidemiological studies, ie typing studies from various countries have shown that MRSA isolates from cats are typically indistinguishable from HA-MRSA lineages dominant in each particular country.
  • It is likely that MRSA rates are increasing, however, this could be a reflection of the increased awareness of the bacterium in different species.
  • There is limited data to demonstrate the prevalence and persistence of MRSA in different species of animal, the ease of transmission, or the effectiveness of control procedures in these species.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
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