Describing the methicillin-resistant Staphylococcus aureus epidemic: a public health challenge

Methicillin-resistant Staphylococcus aureus (MRSA) infections have been documented in hospitals for the past few decades. MRSA among intensive-care unit patients in the USA has increased dramatically in recent years [101]. In addition, in the last few years, the epidemiology of MRSA has changed to include skin and soft tissue infections in the community among individuals without any healthcare-related risk factors [1,2]. Occasionally, individuals in the community may develop more severe MRSA infections. Bacteremia, pneumonia and pyomyositis are among several severe infections that are attributed to MRSA in the community [3–5]. Although there is a large volume of recent academic papers in the medical literature that describe the increase in MRSA in healthcare and community settings, MRSA has not been a bacteria that has been traditionally reportable to public health.

One of the most significant challenges in monitoring trends of different types of MRSA infections is deciding on a definition for ‘community-MRSA’ and ‘healthcare-MRSA’. Traditionally, MRSA in the healthcare setting has been referred to as healthcare-associated MRSA (HA-MRSA). HA-MRSA infections are often acquired by patients who are immunocompromised or who have had an invasive procedure. These strains of MRSA often have varying susceptibility patterns and may cause severe disease in susceptible patients. Even more of a challenge is deciding on a definition that describes MRSA in persons who have no traditional healthcare-associated risk factors. Community-acquired, community-associated (CA) and community-onset are some of the terms used in the medical literature to describe these types of infections. Furthermore, there is no standard, accepted definition used by academics who publish on the epidemiological and clinical aspects of MRSA in the community. Complicating this issue further are the recent reports of ‘CA’ MRSA that have been described as circulating in the hospital setting [6,7]. Describing the change in MRSA epidemiology has become a difficult challenge for public health. Thus, it is difficult to make clear statements that accurately describe the impact of MRSA in the community or in the healthcare setting.

Distinguishing between CA-MRSA and HA-MRSA has been subject to interpretation by public health professionals, academics, clinicians and microbiologists. Three different perspectives are represented by the following definitions of CA-MRSA. One epidemiological definition of CA-MRSA is MRSA recovered from a clinical isolate from a patient who had no established risk factors for MRSA infection. The established risk factors are MRSA isolated more than 48 h after hospitalization, history of hospitalization, surgery, dialysis, residence of a long-term care facility within 1 year of the identification of the MRSA isolate, the presence of a permanent indwelling catheter or percutaneous medical device at the time of culture and previous history of MRSA isolation [8]. Variations in this particular epidemiological definition have been used by many researchers and public health professionals to describe MRSA in the community. Results of the corresponding publications have led to different conclusions concerning MRSA and have made it difficult to compare various studies and investigations. Understanding the emergence of CA-MRSA using different but overlapping epidemiological definitions may be potentially misleading.

CA-MRSA has also been defined from a laboratory perspective in various publications. Particular pulsed-field gel-electrophoresis (PFGE) patterns have been used to help describe MRSA in the community. PFGE patterns of MRSA strains have been described previously [9]. PFGE patterns USA 300 and 400 have been identified as being patterns of known CA-MRSA strains [10]. USA 300 has often been described as a CA-MRSA strain circulating in the USA and the predominant clone that causes skin and soft tissue infections [1]. Furthermore, CA-MRSA strains are known to carry a staphylococcal chromosomal cassette mec element (SCCmecIV). The SCCmecIV element has a lower molecular weight compared with SCCmec elements from HA-MRSA strains. One notable feature of the smaller SCCmecIV element is that it contains fewer antibiotic-resistance genes than the larger SCCmec elements common in HA-MRSA strains [11]. In addition, CA-MRSA strains are known to carry genes that encode the virulence factor Panton-Valentine leukocidin (PVL). PVL is known to be a leukocyte-killing exotoxin linked to the development of furuncles, cutaneous abscesses and severe necrotic infections. Importantly, PVL has been associated with severe disease, including pneumonia, necrotizing fasciitis, infective pyomyositis and myositis and aggressive infections of the eye and orbit [4,5,12,13]. Molecular diagnostics have been used to identify and define CA-MRSA strains; these techniques have allowed researchers to distinguish CA-MRSA from HA-MRSA according to laboratory features.

Clinical infections caused by MRSA in the community have been reported extensively in the past few years. Most commonly, MRSA infections in the community are described as various skin and soft tissue infections. Initially, as MRSA infections in the community emerged, the skin and soft tissue manifestations were misdiagnosed as spider bites [14]. Lesions have also been described as folliculitis, abscesses, furuncles, carbuncles, boils and cellulitis. The similarity between MRSA skin lesions and spider bites remains a new diagnostic challenge. In addition, recent clusters of MRSA infections identified in healthy newborns reflect another presentation of MRSA skin lesions. Lesions in the newborn have been described as blisters, vesicles or pustules. These lesions have appeared to clinicians as being similar to the vesicles caused by the herpes simplex virus [15]. The new clinical syndromes attributed to CA-MRSA have allowed clinicians to create new differential diagnoses for spider bites and herpes simplex virus infections in neonates.

Even with the inherent problem of distinguishing CA-MRSA from HA-MRSA by using epidemiological, laboratory and clinical descriptions, lines are further being blurred by investigations that demonstrate CA-MRSA transmission in the healthcare setting. If CA-MRSA can be defined clinically or by laboratory methods, then it is possible for those strains to circulate in healthcare institutions. This phenomenon has been described previously [6,7]. Of course, if using the epidemiological definition of CA-MRSA, this would be impossible. This discussion leads to the following questions: does distinguishing between CA-MRSA and HA-MRSA matter? Which specific data should be captured by a public health surveillance system? How do we create consistent, acceptable ways to describe MRSA or Staphylococcus in 2007? What are the main goals of Staphylococcus or MRSA surveillance?

The answers to all of these questions are related to the last question. The goals of the surveillance system should dictate the type of data collected and ensure that the data are described consistently by all reporting entities and public health. The goals of MRSA surveillance should include: disease data that will help guide clinical decisions made by healthcare providers, quantitative assessment of the burden of MRSA that will help influence policy and funding and identification of clusters of MRSA infections that require public health intervention to halt further transmission. To accomplish these goals, it will first be necessary to establish a standardized language that will be simple and consistent between reporting healthcare entities and public health. Since there are a variety of case definitions that describe CA-MRSA and HA-MRSA, these terms should no longer be used. Instead, MRSA or methicillin-susceptible S. aureus infections should be described in the context of types of clinical infections, whether or not the organism was acquired nosocomially and the laboratory features of the particular isolate. The second consideration in accomplishing this goal-oriented surveillance system is to realistically assess the resources available and the likelihood of the system’s sustainability. Owing to the lack of targeted public-health funding for this issue in most locales, trying to obtain the most valuable information with few resources is crucial. Currently, there are no comprehensive data sets that reflect the status of MRSA in healthcare settings in the USA. The CDC’s National Nosocomial Infections Surveillance system has produced limited data regarding MRSA, however, it comprises mostly intensive care unit data from a small number of hospitals [16]. Involving local stakeholders, such as hospital infection control professionals, microbiologists in hospitals and commercial laboratories, and community healthcare providers in surveillance plans is also important for recognizing existing resources. Public-health officials and local partners must work together to create a feasible surveillance system that will optimize the data collected and the available resources.

Public health working with key stakeholders will hopefully lead to a surveillance system that will provide useful information for healthcare providers, healthcare institutions, policy-makers and the public. Community healthcare providers who are diagnosing and treating skin and soft tissue infections need to be presented with clear, consistent data that accurately describe the emergence of MRSA in the community setting. Changing medical practices to include more culturing of skin lesions to identify not only the organism but also the susceptibility profile has become more important in the age of community MRSA. Healthcare institutions need to examine cost-effective practices aimed at screening patients and laboratory testing. Policy-makers need data that will explain the impact of MRSA on healthcare institutions and communities. Defining the MRSA epidemic is important for assuring funding for much-needed studies aimed at therapeutic approaches and prevention of transmission strategies, such as decolonization methodologies. In addition to providing clear information to community clinicians, it is imperative that the public receive clear information regarding this new communicable disease threat in the community. Educating the public about transmission of MRSA is especially important in certain settings, such as group living situations, sports teams and daycare centers. Collecting data regarding MRSA infections will allow healthcare providers, healthcare institutions, policy-makers and the public to develop new strategies to combat this worsening epidemic.