Coming to a practice near you? community-acquired meticillin resistant staphylococcus aureus (ca-mrsa)

Introduction

Meticillin resistant Staphylococcus aureus (MRSA) is classically associated with healthcare interventions. Consequently, hospital admission represents a prominent risk factor for the acquisition of certain epidemic strains of MRSA, leading to the term ‘hospital-acquired infection’ (HAI). This was modified to ‘healthcare associated infection’ acknowledging patient movement between healthcare and community settings, despite the MRSA strains remaining identical.

New strains termed community-associated MRSA (CA-MRSA), with distinct phenotypic and genotypic characteristics, emerged in the last ten years and are now widespread in the USA. As cases have been reported in the UK and the incidence is likely to increase,1 the dental team should have an understanding of this emerging pathogen and their role in preventing further spread. This article provides a background on the origins of this bacterium, discusses its occurrence in the oral cavity and highlights current treatment and prevention strategies. Dental healthcare workers should have sufficient knowledge to manage patients colonized or infected with any strain of MRSA competently.

Background

Staphylococcus aureus is a Gram-positive coccus that colonizes the nose, skin and oral cavity of approximately 20–30% of healthy individuals. Although S. aureus may exist as a commensal, it is responsible for a variety of infections, ranging from superficial skin infections to invasive life-threatening conditions, including septicaemia, endocarditis, osteomyelitis, pneumonia and toxic shock syndrome. There is political pressure to reduce the incidence of this important nosocomial pathogen2 and Figure 1 demonstrates an overall reduction in the number of S. aureus bacteraemias reported in Scotland between 2005 and 2009 as part of a mandatory surveillance scheme.3

Emerging resistance

Prior to the 1940s, virtually all S. aureus isolates were susceptible to penicillin. Following the introduction of this therapeutic agent in the mid 1940s, widespread resistance appeared as a result of the enzyme penicillinase hydrolysing the beta-lactam ring, rendering the drug inactive. Meticillin, a semi-synthetic penicillinase stable derivative, became available in the late 1950s and the first MRSA was described around the same time.4 Meticillin is no longer used clinically with flucloxacillin, the preferred agent for a meticillin sensitive S. aureus (MSSA). This pattern of emerging resistance has been repeated following the introduction of other antibiotics (Figure 2).5

MRSA refers to a strain of S. aureus that has acquired the mecA gene coding for an altered penicillin-binding protein (PBP2A) that renders the organism resistant to all current beta-lactam antibiotics, including penicillin, amoxicillin, flucloxacillin and the cephalosporins. Many HA-MRSA isolates demonstrate resistance to other antibiotic classes, such as the macrolides (eg erythromycin), lincosamides (eg clindamycin) and fluoroquinolones (eg ciprofloxacin). Serious infections with MRSA are treated with intravenous glycopeptides, such as vancomycin or teicoplanin. Newer oral agents, such as linezolid, have a role in special circumstances, although there are concerns regarding bone marrow toxicity.

Hospital and community-acquired MRSA – what is the difference?

In 1999, one of the first cases of CA-MRSA was reported in the USA, causing the death of four previously healthy children.7 Cases of CA-MRSA have continued to increase in the USA, particularly amongst young people; often running a rapid and devastating course. Worldwide prevalence, however, remains low.8 There are important phenotypic and genotypic differences between HA- and CA-MRSA, which are used to distinguish them (Table 1). Skin and soft tissue infections are among the most common manifestations of CA-MRSA and patients may report recurrent abscesses. Less common, but more severe, infections include necrotizing pneumonia and severe sepsis.

An array of pathogenic determinants may facilitate host colonization and disease in S. aureus, including:

Panton-Valentine Leucocidin (PVL) toxin production is an important virulence factor in CA-MRSA, causing destruction of white blood cells and leading to massive tissue necrosis. The precise role of PVL in disease remains unresolved, although epidemiologically associated with SSTI and believed to have a role in the pathogenesis of severe necrotizing. While almost all CA-MRSA strains carry the PVL toxin, it is also found in MSSA.9

Staphylococcus aureus in the oral cavity

Infection

Staphylococcus aureus is implicated in a spectrum of oral diseases, including angular cheilitis (Figure 3), parotitis, osteomyelitis and mucositis (especially in the immunocompromised).

A 3-year study of laboratory data from Glasgow found a 5% incidence rate of MRSA from the 5,005 specimens analysed. Patients with MRSA were more commonly seen in primary care settings, such as nursing homes, hospices and general dental practice, rather than the dental hospital where the majority of MSSAs were isolated.16

Antibiotic prescribing and MRSA

An important role in the control of MRSA by dentists is responsible prescribing of antibiotics. Frequent antibiotic use has been linked to increased MRSA acquisition.18 One study showed that, among professional football players, the risk of contracting MRSA was almost eight times higher if players had taken antibiotics in the previous year versus those who had not.19 A study of general dental practitioners in England found that, of the 2,951 prescriptions issued over a time period, only 29% were considered justifiable using published guidelines.20 Another study,21 examining prescribing practices of general dental practitioners in England, found 69% of dentists prescribed antibiotics prior to drainage and 45% when it was established, despite research demonstrating that drainage of an infection is the only treatment necessary for the majority of infected swellings without systemic symptoms. As S. aureus may colonize various body sites, systemic antibiotics prescribed inappropriately for other conditions will affect the endogenous flora and potentially drive resistance.

Prevention of MRSA transmission

The dental team has a responsibility for the prevention of spread of MRSA. Therefore, it is essential that high standards of infection control are adhered to for every patient in the practice, as most patients are unaware of their MRSA status. Furthermore, it is not feasible to screen for MRSA carriage in dental practice. Table 2 outlines the standard infection control procedures (SICPs) that must be followed. No additional precautions are required to manage a patient known to have MRSA in the dental setting.

There have been documented cases of MRSA oral infection transmitted by a dentist.22 A study in Japan23 of nosocomial transmission of MRSA, via the surfaces of the dental surgery, found that 6% (8/140) of patients treated in an oral and maxillofacial surgery were found to be colonized after treatment, after identical strains were isolated on the dental chair and air syringe. A further study of S. aureus environmental contamination in an 89 chair dental clinic in a dental hospital in Brazil identified MRSA colonized surfaces in the emergency dental clinic and that clinical procedures increased the dispersal of S. aureus into the environment.24 Both these studies highlight the need for strict adherence to SICPs in dental settings.

Conclusions

CA-MRSA is a significant problem in the USA and is of growing concern in the UK. These new strains appear to have a predilection for damaged skin and airways, and may be PVL toxin positive. CA-MRSA strains are transmissible, especially amongst family members, staff/children in nurseries and in sports teams. Therefore, it is essential that the dental team follow SICPs, practise prudent antimicrobial prescribing and do not discriminate against MRSA positive patients.

The incidence of documented oral S. aureus infections is low, but this may be masked by an equally low number of diagnostic samples taken by dentists. We strongly encourage all dentists to consider using diagnostic microbiology laboratories to facilitate in the diagnosis of oral infections.