Mercy, Mercy, MRSA

Colonies of methicillin-resistant Staphylococcus aureus.

In her 2010 book, Superbug: The Fatal Menace of MRSA, author and self-proclaimed “disease geek” Maryn McKenna charted the steady advance of a treatment-resistant organism which she referred to as a “crisis in many dimensions.” MRSA illustrates “failures of science, failures of the marketplace, and failures of support for research and innovation,” McKenna wrote.

Since she wrote that, there have been successes, but in the overall story of MRSA, things have not improved much. Long known only as a hospital-acquired infection often preying on the infirm, MRSA – short for methicillin-resistant Staphylococcus aureus – now sickens previously healthy people who have not been near a hospital. These community-associated MRSA infections, first described in 1998, are not susceptible to many standard antibiotics. Although most can be treated by in a doctor’s office or in an emergency department, many require hospitalization.

MRSA may not, at the moment, generate the wall-to-wall media coverage it routinely garnered a few years ago, but three recent studies from the University of Chicago Medicine show how this multi-dimensional crisis continues to unfold. The number of cases may still be on the rise. MRSA is costing society a fortune. And it can still baffle researchers who want to learn how to disrupt the chain of infection and make this persistent pathogen go away.


In the August 2012 issue of the journal Infection Control and Hospital Epidemiology, infectious disease specialists Michael David, Robert Daum and colleagues at the University of Chicago Medicine used information from the University HealthCare Consortium database — which includes 90 percent of all U.S. not-for-profit academic medical centers — to show that the burden of resistant Staph infections continues to mount.

Between 2003 and 2008, patients receiving care at U.S. academic medical centers for MRSA infections doubled, rising from about 21 out of every 1000 patients hospitalized in 2003 to about 42 out of 1000 in 2008, or almost 1 in 20 inpatients. As a result, by 2006 the number of people hospitalized with recorded MRSA infections exceeded the number hospitalized with AIDS and the number hospitalized with influenza…combined.

The researchers also found that this database tends to underestimate the number of MRSA infections among hospitalized patients by as much as one-half. “While the number of invasive MRSA infections may be decreasing inside of hospitals,” David said, “effective control measures outside of hospitals are needed.”

More Money

A second study, by researchers at the University of Chicago, Argonne National Laboratory, the University of Pittsburgh and UCLA, in Clinical Microbiology and Infection tried to measure the costs of community-associated MRSA (CA-MRSA). “Until CA-MRSA’s overall economic burden is better quantified,” they note, “it may be difficult for decision makers to determine where CA-MRSA should fall on public health, medical, and scientific priority lists.”

An extensive search for previous economic-impact studies found only two, one focused on the cost to insurance providers and the other on patients with MRSA-related pneumonia. So the researchers built their own model to estimate the costs to third-party payers — insurance companies or government programs — as well as lost productivity due to community-associated MRSA infections. They multiplied the average cost per case by the estimated number of outpatient and inpatient cases.

The models indicated that direct health care costs to third-party payers, primarily for hospital care, totaled about $560 million a year. The cost to society, mostly through lost productivity, came to $2.7 billion. The cost per case of a MRSA infection is two to five times the cost of influenza, three to ten times that of food-borne illnesses and more than 17 times the cost of a case of Lyme disease.

More Transmission

A third study, published in the June issue of Clinical Infectious Diseases, looked at how various strains of MRSA might be passed on in the home. Researchers in Chicago and Los Angeles visited 350 households of patients who had been treated for MRSA infections at the University of Chicago Medical Center or Harbor-UCLA Medical Center in Torrance, California.

They found that about 22 percent of the 812 household contacts for these former patients were colonized by MRSA, and 50 percent were colonized by MRSA or methicillin-susceptible S. aureus (MSSA).  Previous studies found a lower rate, from 20 percent to 35 percent, but those earlier studies examined only nasal passages; this study collected nasal, throat and inguinal samples. A nostril-only culture survey, the authors note, would have missed 51 percent of MRSA-colonized persons.

It’s increasing, it’s expensive, and it’s everywhere. When tested at home a few weeks after leaving the hospital, 40 percent of the discharged MRSA patients remained colonized with S. aureus; 50 percent of their household contacts were colonized, and 26 percent of colonized household contacts had a different strain of MRSA than the original patient.

More disturbing was the indication that one particular strain, known as USA300, appears to spread more easily from infected individuals than others. Household contacts of patients who had a USA300 infection were more likely to be colonized with this strain than the contacts of patients infected by other strains. The researchers worry that any attempt to “decolonize” households might “eradicate less pathogenic strain types, leaving the person vulnerable to recolonization with more pathogenic strain types.”

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