S. aureus: The Nose Knows

Abstract & Commentary

By Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert.

Synopsis: In 2001-2002, an estimated 89.4 million individuals in the United States were nasally colonized with S. aureus and 2.3 million with MRSA.

Source: Kuehnert MJ, et al. Prevalence of Staphylococcus aureus Nasal Colonization in the United States, 2001-2002. J Infect Dis. 2006;193:172-179.

Samples obtained by swabbing both nares of almost 10,000 individuals > 1 year of age in the US National Health and Nutrition Survey (NHANES) in 2001-2002 were cultured on mannitol salt agar. S. aureus was identified in 32.4% of subjects and 0.8% were colonized with methicillin resistant S. aureus(MRSA). Extrapolated to the entire US population, it was estimated that approximately 89.4 million were nasally colonized with S. aureus and 2.3 million with MRSA.

The prevalence of S. aureus colonization was highest in individuals 6 to 11 years of age. MRSA colonization was associated with female gender and age > 60 years; health care exposure was not a significant risk factor. In unweighted analyses of a sample of isolates, one or more toxin genes (enterotoxins A-H, PVL, TSST-1 were sought) were identified in 52% of methicillin susceptible S. aureus (MSSA) and 76% of MRSA. TSST-1 was the most commonly identified virulence gene in MSSA, while enterotoxin D was most frequent in MRSA. Fifty-one percent of MRSA carried SCCmec type II and 49% carried SCCmec type IV. All MRSA isolates with the PVL gene carried SCCmec type IV. The PVL gene was detected in only 1% of MSSA.

Commentary

This extensive study gives us a snapshot (albeit at slow shutter speed) of the prevalence of nasal colonization with MSSA and MRSA in the United States in 2001 and 2002. Its overall results are consistent with those of previous studies, with approximately one-third being colonized at this site with S. aureus, representing an astounding estimated 89.4 million individuals. The prevalence of MRSA nasal colonization was only 0.8%, but this extrapolates to approximately 2.3 million people.

It is quite likely that a repeat of this study now would find a significantly greater prevalence of nasal colonization with MRSA, as community-acquired strains carrying SCCmec type IV continue their spread. An increased prevalence of MRSA colonization has recently been documented in homeless youth in San Francisco, as well as children in Nashville and in Houston.

It should also be noted that only one site—the nares— was evaluated for colonization. Studies have shown that a greater yield is obtained when additional sites, such as the axillae and inguinal areas, are sampled.