Person-to-person transmission within households serves as a major reservoir for methicillin-resistant Staphylococcus aureus (MRSA), often leading to significant infections and hospital visits, according to a two-year study of community-associated MRSA infections in Chicago and Los Angeles.
Once the bacteria gain admission, the researchers found, they managed to persist, spreading from one person to another for an estimated two to eight years before causing symptoms severe enough to require a doctor’s attention. Household members continued to show evidence of bacterial colonization for at least six months after hospital discharge.
Introduction of a new strain of MRSA into a household from outside sources was infrequent, the researchers found, but once introduced, “it can hang out there for years, ping-ponging around from person to person,” said study director Michael David, MD, PhD, assistant professor of medicine at the University of Chicago.
“Over time the bacteria evolve a genetic profile unique to that household,” he said. “We are concerned that such repeated person-to-person transmissions could act as a long-term reservoir, which could, over time, enhance MRSA’s ability to colonize humans and cause infections.”
“We want to find ways to disrupt this pattern,” he said, “to develop decolonization strategies and public health programs to control the spread of MRSA, such as treating every member of the household, not just one patient.”
Robert Daum, MD, (UChicago) and Loren Miller, MD, (UCLA) led the original project that recruited 350 patients who came to the University of Chicago Medicine (177) or Harbor-UCLA Medical Center in Los Angeles (173) between August 2008 and June 2010 for treatment of a skin or soft-tissue infection (SSTI) with Staph. aureus. After the patients were treated and discharged, the researchers visited each household three times—at about three weeks, three months and six months after the hospital visit.
During those home visits the researchers cultured bacterial samples from the nose, throat and groin of patients and their household contacts, a total of 1,162 people.
They found that 40 percent of the initial patients, and 50 percent of their household contacts, carried Staph. aureus in at least one of the sampled sites. The predominant type of Staph. aureus was a strain known as USA300, which has been the most common cause of community-associated SSTIs in the United States since 2004.
Co-authors Timothy Read, PhD, and Tauqeer Alam, PhD, of Emory University School of Medicine, in Atlanta, used whole genome sequencing on 146 of the MRSA samples to create an evolutionary tree depicting the relationships among the bacterial strains. They found that isolates within households clustered into closely related groups, suggesting a single common USA300 ancestral strain had been introduced into and repeatedly transmitted within each household.
They also used a technique called Bayesian evolutionary reconstruction to estimate, based on random genetic changes accumulated over time, how long the initial MRSA strain had persisted within each household.
“There is about a one in a million chance that any nucleotide will change in a year, and there are about 2.5 million base pairs in a single S. aureus genome,” David explained. “By that measure, our MRSA samples had inhabited our patients’ households for an estimated 2.3 to 8.3 years.”
The samples broke down into two big groups. The vast majority of isolates from Los Angeles differed genetically from those in Chicago. Fluoroquinolone-resistant USA300 clones, which emerged around 1995, were more widespread in Los Angeles than in Chicago.
“The study adds to the knowledge base of how USA300 MRSA has spread throughout the country,” said Read. “We’re also getting hints at how it evolves inside households. Decolonization of household members may be a critical component of prevention programs to control USA300 MRSA spread.”