Methicillin-Resistant Staphylococcus aureus: Where Is It Coming From and Where Is It Going?

Despite the availability of effective preventative and therapeutic strategies, methicillin-resistant Staphylococcus aureus (MRSA) remains amajor cause of health care–associated and, more recently, community-acquired infections. Since the introduction of methicillin in 1959, epidemics due to different clones ofMRSAhave occurred in diverse geographic regions.1 The result has been the persistence of MRSA as an important pathogen worldwide. In the United States, themost recentMRSA epidemic has been due to the clone USA300. USA300 has been responsible for themajorityof communityandhealth care–associatedoutbreaks since the early 1990s.1 Until recently, thenumber of infections has continued to rise, especially in urban centers. While themajority of these have been skin and soft-tissue infections (SSTIs), a notable minority have been invasive and life-threatening.1 Two articles in this issueof JAMAInternalMedicineprovide additional insight into both the incidence and the risks associatedwithdevelopmentofMRSA infections.Danteset al2provide evidence that the overall incidence of invasiveMRSA infections nationally is declining. Casey et al3 perform an ecological investigation; they find that individuals livingnear high-density farming operations (ie, farms having an animal density of 2 animals or 1000-lbs live weight per acre) or near crop fieldsusingmanure exported from thehigh-density livestock farms are at increased risk of developingMRSA or other bacterial SSTIs. Both studies raise questions thatwarrant further investigation. Dantes et al2 useddata from theActiveBacterial Core Surveillance Program (ABCSP) to compare the national incidence rate of invasive MRSA infections for the years 2005 through 2011. The findings are of special importance because of the surveymethods that areusedand the size of the sample population (approximately 15 million). The ABCSP is an active surveillance program that is population based and includes a reviewofmedical records. It serves as amodel for this type of surveillance. The results show that the incidence of health care–associated community-onset and hospital-onset invasive MRSA infections have declined by 27.7% and 54.2%, respectively. The findings are consistent with several earlier reports including one from the same group. This impressive declinemaywellbeduetothe implementationof infectioncontrol efforts at thehospital level; however, it is alsopossible that the epidemic MRSA strains circulating in the health care setting have evolved, becoming less virulent. In contrast to the decline inhealth care–associatedMRSA, the estimated reduction in incidence of community-associated infections was a modest 5%. In addition to the evidence that the incidenceofMRSA infections is declining, the study highlights 2 important points. The first is the observation that there is an increased risk of recurrent infection among recentlyhospitalizedpatientswith healthcare–associatedcommunity-onset infections.Thesepatients had a 64% risk of infection at 3 months or less followingdischarge.Others have also reported ahigh risk of reinfection in discharged patients either infected or colonized with MRSA.4 Possible explanations for the patients’ increased risk include underlyingmedical comorbidities, persistent colonizationor recolonizationwithMRSA,orexposure tosimilar risks of MRSA infection following discharge. This subset of patients would appear to be candidates for studies directed at reducing their high risk of recurrent infection. The second finding is the limited change in the incidence of communityassociated infections. This is in marked contrast with the reduction in the incidence of health care–associated infections. As noted by the authors, the study highlights the need for a greater understanding of how these epidemic MRSA strains spread and initiate infection within the community. Related articles pages 1970 and 1980 Research Original Investigation National Burden of Invasive MRSA Infections