Frequency unknown

Methicillin Resistant Staphylococcus Aureus

Introduction

Staphylococcus aureus (S. aureus) is a common commensal bacterium that colonizes the skin and muscosa of healthy people. It is well recognized that the anterior nares serve as the main source of colonization, with 20% of healthy people presenting as persistent and 60% as intermittent carriers. 1

Methicillin resistant Staphylococcus aureus (MRSA) is the name used for bacteria of the Staphylococcus aureus group (S. aureus) that are resistant to antibiotics for standard treatment of infections with this and similar organisms. In the past MRSA has stood for methicillin resistance, but recently the term refers to a multi-drug resistant group whose members have resistance to many antibiotics traditionally used against S. Aureus. The resistance to methicillin is due to the presence of the mec gene in the bacteria, altering the site at which methicillin binds to kill the organism and rendering methicillin binding ineffective.

Epidemiology

MRSA is found worldwide, mainly in hospitals, where it emerged rapidly after the introduction of methicillin.2 MRSA is also found in patients with no contact with the hospital environment; these isolates are termed community-associated MRSA (CA- MRSA). The global epidemiology of CA-MRSA is consistent with the domination of a single clone in the USA, whereas in other regions multiple clones have been identified.3 The prevalence of methicillin-resistance among S. aureus isolates in intensive care units in the US is 60 percent 4 and more than 90,000 invasive infections due to MRSA occurred in the United States in 2005 5. The CA-MRSA and HA-MRSA classifications are no longer distinct, since patients can develop MRSA colonization in one realm and develop manifestations of infection in another.6

Incidence and prevalence of vulvar MRSA are not known, but MRSA is the most common pathogen among vulvar abscesses that require incision and drainage.7 Many clinicians do not know that the vagina is an important reservoir of MRSA.8 Studies have shown that MRSA is often present in cultures of the perineum taken during routine obstetric care and that MRSA is the most common cause of skin and soft tissue infections at other body sites. 9 10 11

Etiology

Colonization increases the risk for MRSA infection. Colonization can occur in the following ways:12

  • Contact with contaminated wounds or dressings of infected patients
  • Contact with another individual’s colonized intact skin
  • Contact with contaminated inanimate objects
  • Inhalation of aerosolized droplets from chronic nasal carriers.

It is important to remember that, as discussed, while the anterior nares is the most common site of MRSA colonization,13 the vagina is also a common site.14

MRSA on the vulva may develop in a colonized woman as a skin and soft tissue infection or may represent a secondary invader in severe vulvar dermatitis or dermatosis. 15 Host, environmental, and bacterial factors trigger transformation of a commensal S. aureus strain into a pathogen capable of inducing life-threatening infection.16 A damaged skin barrier, as seen with eczema, open wounds, and innocent abrasions after shaving, facilitates abscess formation and skin infection.17 Heterosexual transmission also occurs.18

Vulvar abscesses, including Bartholin gland abscesses, are often mixed polymicrobial infections, consisting primarily of methicillin-resistant Staphylococcus aureus (MRSA) enteric gram-negative aerobes, and female lower genital tract anaerobes.

Symptoms and clinical features

The clinical presentation of CA-MRSA is similar to that of methicillin-sensitive S. aureus (MSSA) infection, and is characterized by dominant skin and soft tissue infection including furuncles ( boils), and abscesses.19 The role of MRSA in cellulitis is less well known, because cultures are rarely obtained in such infections. A wide clinical spectrum ranges from mild, superficial infection with spontaneous appearance of a red, raised lesion (classically presenting as a “spider bite”) to deep, soft tissue abscesses requiring hospital admission, surgery, and intravenous antibiotics.

Vulvar abscess presents as a painful vulvar mass, also sometimes described by patients as a “pimple” or a “spider bite.” In addition, they may also complain of vulvar fullness or pressure, or pain with walking, sitting, or sexual intercourse. 20

Secondary infection with MRSA would manifest as an erythematous field surrounding papules, plaques, fissures, excoriations, or erosions of eczema, lichen simplex chronicus, and other dermatoses.

Diagnosis

Presence of erythematous papules and pustules representing furuncles, boils and abscesses or erythema surrounding classic lesions of dermatoses are characteristic. Cultures will be positive for MRSA.

Pathology/Laboratory

Positive cultures for S. aureus or MRSA.

Differential diagnosis

Furuncles, hidradenitis, Bartholin abscess, necrotizing fasciitis

Treatment/management

Standard treatment for skin lesion such as furuncles, boils, and abscesses includes incision and drainage (I & D) with or without antibiotics. I & D is frequently the only treatment needed. 21

Antibiotics are given for fever or signs of systemic disease, in patients with rapidly progressive skin and soft tissue infection and cellulitis, in patients at both extremes of age, those in immunosuppressed states (including diabetes mellitus and neoplastic disease), and those failing I & D. Cultures should always be obtained before initiating antibiotics.

Currently, CA-MRSA strains have a narrow spectrum of resistance, mainly to β-lactam antibiotics (penicillin and cephalosporin) and macrolides/azalides (erythromycin, clarithromycin, azithromycin). Resistance to other drug groups has been reported and needs consideration if partial improvement occurs while on antibiotics. Reasonable antibiotic choices include clindamycin, tetracyclines, trimethoprim-sulfamethoxazole, rifampin (used only in combination with other agents), and Linezolid.22 Each drug group has its spectrum of adverse outcomes and prohibitions, mainly pregnancy for tetracyclines and sulfa, multiple drug interactions for rifampin, thrombocytopenia for Linezolid. Vancomycin has been the drug of choice for the treatment of MRSA for years; recently its efficacy has been compromised by resistance.23

It is essential to understand that treatment of a MRSA infection should aim at both treating the current infection and reducing risks for re-infection. Consequently, standard practices such as washing hands and avoiding sharing of personal items that come in contact with infected skin areas (e.g. bed sheets, towels, and clothes) are crucial. Attempts to reduce skin abrasions by avoiding shaving and covering skin lesions to decrease the spread of the resistant strain are fundamental. These measures are particularly relevant to the common practice among women of shaving the pubic area. Given that most S. aureus skin infections occur from endogenous strains that colonize the infected individual, eradication of the reservoir (i.e. de-colonization) is expected to reduce recurrence. 24

In the past, the vagina has not been considered a common reservoir, 25 but studies referenced above suggest that vaginal colonization with S. aureus is common and should be suspected in patients with buttock and genital area MRSA lesions. 26 27 28 Extra-nasal colonization sites (e.g. the vagina) cannot be eradicated by intranasal mupirocin alone, and systemic or local therapy are necessary considerations. A combination of rifampin, doxycycline, intranasal mupirocin, and chlorhexidine gluconate reduced HA-MRSA colonization and infections, suggesting that multiple drugs in various routes are superior to single drugs given by one route only. 29 30

References

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