Bug that came in from the cold: CA-MRSA in hospital

Emerging trend may increase threat to patients

Hardier and more virulent than traditional nosocomial strains, community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) now appears to be laying claim to the hospital.

Reports are increasing that CA-MRSA strains are becoming predominant in some health care settings, raising the possibility that staph infections — the bane of the infection control professional for decades — soon may pose an even greater threat to patients.

"The concern is that these seem to be more virulent organisms and they might cause more severe infections or might be transmitted easier from one person to another," says Lauren Miller, MD, assistant professor of medicine at the University of California Los Angeles (UCLA) School of Medicine. "It is concerning that we could have more infections or more serious infections."

Indeed, Miller was the lead author of a paper published last year in the New England Journal of Medicine that found that CA-MRSA is causing severe outcomes such as necrotizing fasciitis and toxic shock syndrome.1 Thus it was with understandable concern that he realized that the USA 300 strain of CA-MRSA was gradually setting up shop in his hospital.

"In 2002 and 2003, people started noticing a large amount of health care-associated MRSA infections that looked like they were being caused by these community acquired strains," Miller tells Hospital Infection Control.

The first clue

The first clue was finding that the antibiotic susceptibility profile of staph infections began to subtly change. "We know that the community-acquired strains tend to be susceptible to tetracycline, clindamycin and trimethoprim/sulfamethoxazole (TMP/SMX)," he explains. "So just by the antibiotic sensitivity of the strains we suspected they were community acquired. Suddenly, we were seeing people getting surgical site infections, pneumonias, and bloodstream infections caused by CA-MRSA. Recently, we did a formal analysis to see if these were indeed community-acquired strains and to quantify the trend."

Presented last December in Washington, DC, at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), the research verified the suspicion that CA-MRSA was coming in from the community.

All health care-associated MRSA isolates (those cultured more than 72 hours after admission) from patients at Harbor-UCLA Medical Center were retrospectively examined from 1999 to 2004.2 Isolates were considered to be community acquired ("CA-phenotype") if they were susceptible to gentamicin, TMP/SMX, and clindamycin. Miller and colleagues found 352 nosocomial MRSA infections for the period. The proportion of nosocomial MRSA infections caused by the CA-phenotype increased linearly from 17% in 1999 to 56% in 2003. It was confirmed: CA-MRSA isolates had become the predominant strain associated with nosocomial MRSA infections in the hospital.

"[We wondered whether] the community-acquired strains were simply causing more infections or are the strains really shifting or swapping?" Miller says. "Looking at the number of isolates, they remain relatively stable over time. It looks more like there is a shift in the strains rather than an additive [effect]. These community strains are basically becoming the MRSA strains in our hospital."

In what may not be good news for other hospitals, Miller says the CA-MRSA strains have proven capable of faster replication in the test tube than longstanding nosocomial strains. That hardy nature may translate to a new endemic nemesis, but the $64,000 question is whether more severe infections will be the result.

"They are more fit than the traditional health care MRSA strains so it makes sense that they would be able to take over a hospital," he says. "Our next project is to look at the clinical syndromes. It looks like [CA-MRSA] is causing more skin infections in the hospital, and that’s very consistent with these community strains in general. As to the seriousness of these infections, I don’t think anyone has a handle on it."

If there is anything encouraging about the trend — more of "silver cloud" than a lining, he notes — it is that the CA-MRSA strains are actually susceptible to more drugs than the established hospital strains. "We now have some more antibiotic choices, but most of the antibiotics that we could use — tetracycline, for example — are relatively poorly studied for MRSA infections in the hospital," Miller says. "There are very little data. Even though we have more choices, we are not sure how good these antibiotics are."

But the key point is that antibiotic therapy used for traditional MRSA would work, meaning patients should not succumb to treatment failure due to the wrong drug choice as CA-MRSA moves to the bedside. Regardless, ICPs may know much more soon enough because many other hospitals may face the same problem.

Mounting evidence

"We’re not the only center to report this," Miller says. "There was a French group that reported very similar findings at ICAAC. In Los Angeles, we are ahead of the curve, but my guess is that this will become the predominant strain soon in most hospitals."

Indeed, seven hospitals in the Duke University Infection Control Outreach Network in Durham, NC, reported similar findings at ICAAC.3 From January 2001 through April 2005, 1,523 patients with MRSA infection were hospitalized in the seven facilities. The proportion of all MRSA infections that were community-acquired pneumonia or skin infections rose from 17% to 37% from 2001 to 2005, the researchers reported. In another ICAAC study, investigators in Tel Aviv, Israel, traced a staph outbreak in a neonatal intensive care unit to a nurse colonized in the nares with CA-MRSA.4 The nurse was successfully decolonized with mupirocin and the outbreak ceased. The finding is telling, however, because it means CA-MRSA could come through the hospital doors via patients or workers.

"Whether it might be colonization — people bringing it into the hospital — the short answer is, nobody knows," Miller says. "Most people don’t have CA-MRSA 300 when they walk in the hospital but it’s possible that those that do might be able to spread it very easily. But we really don’t know, and at this point in time our infection control remains the same."

As the epidemiological evidence mounts, the trend may reinvigorate the debate about active surveillance approaches such as culturing patients on admission. Such "search-and-destroy" methods will be sorely tested if a pathogen so well established in the community continues to show designs on taking over the hospital.


  1. Miller LG, Perdreau-Remington F, Rieg G, et al. Necrotizing fasciitis caused by community-associated methicillin-resistant Staphylococcus aureus in Los Angeles. N Eng J Med 2005; 352:1,445-1,453.
  2. Maree KC, Daum RS, Boyle-Vavra S, et al. Rapid Temporal Increase in "Community-Acquired" Methicillin-Resistant S. aureus(MRSA) Strains Causing Nosocomial Infections. Abstract LB2-11. 45th ICAAC, Washington, DC; Dec. 16-19, 2005.
  3. McDonald JR, Engemann JJ, Anderson DJ, et al. Increas-ing Burden of Community-Acquired (CA) Pneumonia and Skin Infections Caused by Methicillin-Resistant Staphylococcus aureus (MRSA) in a Network of Community Hospitals. Abstract K-557. 45th ICAAC, Washington, DC; Dec. 16-19, 2005.
  4. Stein M, Sprecher H, Yossepowitch O, et al. An Outbreak of Staphylococcus aureus with Phenotypic Features of CA-MRSA in the NICU Introduced by a Staff Member: Early Intervention and Prevention of Further Spread. Abstract K-555. 45th ICAAC, Washington, DC; Dec. 16-19, 2005.