Population-based surveillance for MRSA

NHANES is an abbreviation that’s quite familiar to epidemiologists of all stripes: the National Health and Nutrition Examination Survey. This survey dates back to 1956 with the passage of the National Health Survey Act, providing legislative authorization for “a continuing survey to provide current statistical data on the amount, distribution, and effects of illness and disability in the United States.” Generally, information from these surveys has been used to look at the effect of nutrition, particularly micronutrients, on the health status of the population, or subgroups within the population. However, survey data and biological samples obtained from those enrolled in these studies can be used for other purposes as well—and one recent study took advantage of this. A new publication by Kuehnert et al. uses NHANES samples to examine colonization with Staphylococcus aureus. As with many pathogens, a significantly greater number of us are asymptomatic carriers of this bacterium than are sickened by the pathogen. Additionally, strains of S. aureus that is resistant to the antibiotic methicillin (methicillin-resistant S. aureus, MRSA) appear to be increasing in prevalence in the community. I say, “appear to be,” because no good, population-based study had been carried out to date. This new study begins to fill that void.

Samples from 9,622 people enrolled in the NHANES study in 2001-2 were examined for S. aureus colonization; almost a third (32.4%) were found to be positive. Extrapolating to the U.S. population, that means that almost 90 million people are colonized with this pathogen. Prevalence was found to be highest in the 6-11 year age group, and males were slightly more likely to be colonized than females.

75 of the S. aureus-colonized individuals were carrying MRSA–.8% of the population, or about 2.3 million Americans. In this analysis, MRSA was found more frequently in older age groups, and in females. However, when community-acquired MRSA (in contrast to nosocomially–or hospital-acquired–isolates) were analyzed separately, the groups most at risk again were young children and African-Americans. As previous studies have shown that those who are colonized are at a higher risk of subsequent MRSA disease, this puts a significant amount of our population–and particularly, our children–at risk of developing serious disease due to this bacterium, which is extremely difficult to treat.

Additionally, MRSA disease in individuals who have no nosocomial exposure has increased in the past several years. Since the data from this study comes from samples taken in 2001-2, these results can be compared to more recent data. For example, other studies conducted at various sites in 2001 found MRSA prevalence rates of .6-2.8%. However, by 2004, a study of 500 healthy children using identical methods found colonization rates of 9.2%, an increase of greater than 10-fold in that community. Likewise, a 2004 study of youths in San Francisco found a MRSA colonization rate of 6.2%, and a Texas study earlier this year reported a 22% MRSA colonization rate. Certainly with these numbers, the increasing find of community-acquired MRSA becomes much less surprising. The bad news is, we don’t currently have a way to keep this from increasing even further, and there are few options in the pipeline as far as treating MRSA. A vaccine is in the works, but clinical trials–for both vaccines and new drugs–take time and money. Meanwhile, thousands of people worldwide are dying from this pathogen each year. As usual, the best prevention is simply to wash your hands and generally practice good hygiene–something to keep in mind while you gather with your loved ones during this holiday season.

Cheers, and have a good one–I’ll be out for a few days with the kiddos and extended family myself.

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