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I. DEFINITION

Infection with methicillin-resistant Staphylococcus aureus (MRSA) (clustered gram-positive cocci) causes a variety of localized and invasive purulent infections and toxin-mediated syndromes such as toxic shock syndrome and staphylococcal scalded skin syndrome (SSSS). MRSA infections used to be limited to healthcare facilities and were strictly nosocomial; however, a significant increase in community-acquired MRSA (CA-MRSA) has been seen in the past 10 to 15 years. The separation between healthcare-related MRSA and CA-MRSA is becoming less distinct, as CA-MRSA is becoming more virulent and causing significantly more healthcare-associated infections.

II. INCIDENCE

The methicillin-sensitive S aureus normally colonizes the nose, umbilicus, and groin area by 1 week of age, with a colonization rate of 30% to 70%. Maternal anogenital colonization with MRSA ranges from 0.5% to 10.4%, with small risk for early-onset disease in the newborn. Case reports have described maternal vertical transmission of MRSA from maternal chorioamnionitis, by nasal colonization, and through breast feeding. Horizontal transmission from siblings and other family members has been documented as well. There are several documented outbreaks of invasive CA-MRSA that developed in healthy newborn infants discharged from normal newborn nurseries as well as neonatal intensive care units (NICUs). The majority of MRSA infections in the NICU are of late onset. According to neonatal data reported from the National Nosocomial Infections Surveillance System for the years 1995 to 2004, MRSA accounted for 23% of all hospital-associated S aureus infections. The incidence of MRSA infections increased by 308% during the study period (from 0.7 per 100,000 patient-days in 1995 to 3.1 in 2004). In a recent meta-analysis of 18 studies reporting data from 1999 to 2011, the prevalence of MRSA colonization at admission to a NICU or pediatric intensive care unit was 1.9%, and the acquisition rate was 4.1%. Neonates who were admitted to the NICU after discharge from the birth hospitalization were more likely to be colonized than those who had never left the NICU (5.8% vs 0.2%). The risk of MRSA infection during hospitalization is increased 24-fold among colonized versus noncolonized patients. Another study that involved 1320 NICU patients found a MRSA colonization rate of 4% within 7 days of NICU discharge. Hand carriage of MRSA in healthcare personnel working in NICUs is estimated at 8%.

III. PATHOPHYSIOLOGY

If the newborn infant is exposed to MRSA, whether from the community or the hospital, then he or she will be colonized with more virulent strains that are more likely to cause invasive disease. MRSA has specific virulence factors that make it more invasive than methicillin-sensitive S aureus. These include staphylococcal chromosome cassette (SCC) mecA, Panton-Valentine leukocidin (PVL), and staphylococcal enterotoxins. The SCC mecA has the genes that encode antibiotic resistance. PVL genes lead to the production of cytotoxins that form pores in the cellular membrane and cause tissue necrosis and cell lysis. Exfoliative toxins A and B secreted by S aureus are implicated ...

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