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Methicillin-resistant Staphylococcus aureus infection

ISSN 2398-2969

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Synonym(s): MRSA infection


  • Methicillin-resistant 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 methicillin as well as all other beta-lactam antibiotics, including the penicillins, cephalosporins and carbapenems.
  • Methicillin (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 Hospital-associated infections 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 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 dogs in the community.
  • Methicillin resistance is also found in other species of staphylococci isolated from animals, in particular pigs, poultry, cattle and sheep.
  • Cause: methicillin-resistant Staphylococcus aureus (MRSA) Methicillin-resistant Staphylococcus aureus.
  • Signs: 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.
  • Diagnosis: 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.
  • Treatment: subsequent sensitivity testing can be used to determine the course of further antimicrobial therapy.
  • Prognosis: good: the majority of MRSA infections can be treated successfully.
Print off the Owner factsheet on Methicillin-resistant Staphylococcus aureus infection to give to your clients.

Presenting signs

  • Not all animals that are infected with MRSA develop clinical signs.
  • The most commonly reported clinical signs are post-operative and wound infections, with less reported incidences including urinary tract infections, pneumonia, and skin and ear infections.
  • Clinical signs can vary from mild to severe and in some cases fatal bacteremia may result.

Acute presentation

  • Acute bacteremia is possible, especially in compromised patients with hospital-acquired infections Hospital-associated infections.
  • CA-MRSA tends to produce more virulent clinical signs than HA-MRSA.

Geographic incidence

  • Worldwide.
  • The prevalence of human S. aureus isolates that are methicillin-resistant varies between countries, being lower in northern Europe than in southern and western regions (range 1 to >40%), where successful control policies have been implemented in hospitals. In the USA some reported prevalence figures exceed >57%, particularly in patients within intensive care facilities.
  • In areas such as the Netherlands, certain strains of human MRSA have been found to be closely related to those found in pigs and cattle.
  • Canine MRSA infections have been reported from multiple locations it is likely disseminated widely internationally. Prevalence figures of MRSA in dogs range from 0.6% in healthy dogs to 23% in hospitalized dogs in the UK.

Age predisposition

  • No epidemiological studies have been reported in rabbits.
  • In humans the prevalence of nasal carriage of S. aureus is higher in children than in adults.

Breed/Species predisposition

  • Little data is available for rabbits.

Public health considerations

  • The natural habitat of S. aureus is the human nares (nostrils), but it can also be found in sites such as the groin, perineal area, armpit and throat. Nasal carriers also often carry S. aureus on the skin, and it may be transmitted via the hands.
  • Although prevalence figures vary, it is thought that approximately 20% of the human population are permanently colonized, and up to 60% transiently colonized with S. aureus.
  • Community-associated MRSA (CA-MRSA) infections tend to be associated with different strains of the bacterium and appear to be resistant to more classes of antimicrobials than the more frequently isolated hospital-associated MRSA (HA-MRSA).
  • The prevalence of MRSA carriage in community surveys of healthy people is around 1-3%.
  • Certain risk groups have higher carriage rates, for example in hospital staff, care workers and hospital outpatients MRSA prevalence can be up to 22%. In the UK and Ireland MRSA prevalence has been reported as 7-8%.
  • MRSA ST398 (livestock-associated MRSA, LA-MRSA) was isolated from an industrial rabbit holding in Europe. Farm workers and a worker's relative were found to be carrying MRSA, and surface and air samples were also positive. In this report, clinical signs of disease were not present. Spa typing suggested porcine origin of the MRSA isolates detected.
  • Dog owners and small animal practitioners also have higher carriage rates, with up to 12% MRSA prevalence reported in some studies.
  • The proportion of human S. aureus isolates that are resistant to methicillin has increased greatly in recent years, eg the proportion of MRSA in cases of S. aureus bacteremia in the UK has risen from 12% in 1990-1992 to 40% in 2000. However, the rate of resistance varies between geographical areas.
  • There are potential zoonotic implications for MRSA infections in companion animals, given the close social interaction between pets and humans.
  • Healthy animal carriers have been associated with transfer of MRSA to humans. This has been documented in the general community, and is becoming increasingly documented in health-care settings and veterinary clinics.
  • Typing studies have shown that MRSA isolated from dogs are identical to hospital-acquired MRSA in humans
  • The majority of canine infections appear to be caused by strains common in the general human population, the most predominant being EMRSA strain 15 and to a lesser extent EMRSA strain 16.
  • The first documented case of animal-to-human transmission was in 1994 when a husband and wife, who were both nurses, were re-infected with MRSA following initial clearance. Screening of the family dog confirmed the presence of the same MRSA strain. It is therefore likely that one or both were infected at work and originally transmitted the MRSA to the dog.
  • Other domestic animals such as pigs have also been suggested as a source of human MRSA infection, since identical strains have been isolated from humans and pigs.

Cost considerations

  • The cost of treating human cases of MRSA is increasing. Estimates suggest that MRSA infections cost the NHS approximately one billion pounds per year.
  • It is likely that costs for treating companion animal MRSA infections are also increasing, due to increased hospitalization and antimicrobial therapies.



  • S. aureus resistant to the beta-lactam antibiotic methicillin (meticillin, oxacillin).
  • Gram-positive cocci, catalase-, mannitol- and maltose-positive.
  • Reported canine MRSA strains are coagulase-positive.

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 dogs include: number of antimicrobial courses (more than 3 courses within 6 months prior to isolation of MRSA), prolonged hospitalization, immunosuppressive treatment, ongoing infection, post-operative 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.


  • 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 fuoroquinilones has been shown to contribute to the selection of MRSA strains in humans and dogs.
  • 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 are 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 dogs may be a source of infection for themselves or others via both direct and indirect (eg environmental) contamination.


  • In dogs, 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, and CMRSA-2/USA100 in Canada and the US. Community-associated MRSA infections in North America are largely caused by USA300/CMRSA10.
  • Most reports of MRSA in dogs have involved EMRSA strain 15.
  • Carriage of S. aureus is less common in most animals than in humans. Colonization rates of have been reported in dogs attending referral hospitals, whilst MRSA comprises approximately 3% of submissions to veterinary laboratories.
  • S. pseudintermedius is the most common coagulase-positive staphylococcus found in dogs, and has itself been found to be zoonotic, ie transmitted between dogs and their owners.
  • MRSA isolates have been obtained from cattle, horses, cats, dogs, chickens, sheep, and pigs, described in reports published from as long ago as 1972. Fewer reports exist for isolates from rabbits, but MRSA isolates have been obtained from wild, pet and farmed rabbits.
  • A survey of university veterinary clinics found that 14% of patients with S. aureus had MRSA, and this was most common in dogs.
  • Persistent infection in dogs has been reported.
  • 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 companion animals.
  • 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.
  • Canine MRSA infections have occurred in clusters in veterinary hospitals in the UK, and also in the USA, Ireland and Austria.
  • Isolates of MRSA from animals 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 dogs 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.
  • Abbott Y, Leonard F C, Markey B K (2010) Detection of three distinct genetic lineages in methicillin-resistant Staphylococcus aureus (MRSA) isolates from animals and veterinary personnel. Epidemiol Infect 138 (5), 764-771 PubMed.
  • Faires M C, Traverse M, Tater F C et al (2010) Methicillin-resistant and susceptible Staphylococcus aureus infections in dogs. Emerg Infecti Dis 16 (1), 69-75 PubMed.
  • Hunter P A, Dawson S, French G L et al (2010) Antimicrobial-resistant pathogens in animals and man: prescribing, practices and policies. J Antimicrob Chemo 65 (Suppl 1), i13-17 PubMed.
  • Loeffler A, Pfeiffer D U, Lindsay J A et al (2010) Lack of transmission of methicillin-resistant Staphylococcus aureus (MRSA) between apparently healthy dogs in a rescue kennel. Vet Microbiol 141 (1-2), 178-181 PubMed.
  • Loeffler A, Pfeiffer D U, Lloyd D H et al (2010) Methicillin-resistant Staphylococcus aureus carriage in UK veterinary staff and owners of infected pets: new risk groups. J Hosp Infect 74 (3), 282-288 PubMed.
  • Soares Magalhães R J, Loeffler A, Lindsay J et al (2010) Risk factors for methicillin-resistant Staphylococcus aureus (MRSA) infection in dogs and cats: a case-control study. Vet Res 41 (5), 55 PubMed.
  • Faires M C, Tater K C, Weese J S (2009) An investigation of methicillin-resistant Staphylococcus aureus colonization in people and pets in the same household with an infected person or infected pet. JAVMA 235 (5), 540-543 PubMed.
  • Hanselman B A, Kruth S A, Rousseau J et al (2009) Coagulase positive staphylococcal colonization of humans and their household pets. Can Vet J 50 (9), 954-958 PubMed.
  • Loeffler A, Kearns A M, Ellington M J et al (2009) First isolation of MRSA ST398 from UK animals: a new challenge for infection control teams? J Hosp Infect 72 (3), 269-271 PubMed.
  • Hanselman B A, Kruth S A, Weese J S (2008) Methicillin-resistant staphylococcal colonization in dogs entering a veterinary teaching hospital. Vet Microbiol 126 (1-3), 277-281 PubMed.
  • Leonard F C & Markey B K (2008) Meticillin-resistant Staphylococcus aureus in animals: a review. Vet J 175 (1), 27-36 PubMed.
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