CA-MRSA may be increasing in incidence but decreasing in virulence

The proportion of methicillin-resistant Staphylococcus aureus blood stream infections due to USA300 strains increased from 2000 to 2005 but decreased through 2007, according to findings presented at the 2009 meeting of the Society for Healthcare Epidemiology of America, held in San Diego.

Kyle Popovich, MD, of Rush University/Stroger Hospital of Cook County in Chicago presented data on 2,178 methycillin-resistant Staphylococcus aureus skin and soft tissue infections and 141 MRSA blood stream infections.

USA300 strains accounted for 77% of blood stream infections and 91% of skin and soft tissue infections.

The rate of skin and soft tissue infections due to USA300 strains increased significantly from 2000 to 2007. Blood stream infections due to USA300 strains increased until 2005 and then decreased in 2006-2007. “Up through 2005, the ratio of blood stream infections to skin and soft tissue infections was stable,” Popovich said. “However, in 2006 and 2007, there were significant decreases in this proportion.”

The researchers speculated that the decrease in blood stream infections due to USA300 strains may be attributed to earlier recognition and treatment of community-associated MRSA and/or a decrease in the virulence of some USA300 strains. “Later in the study period, more incision and drainage may have been occurring, which may have led to a lower incidence of invasive disease,” Popovich said. – by Rob Volansky

For more information:

Popovich K. #258. Presented at: Annual Meeting of the Society for Healthcare Epidemiology of America; March 19-22, 2009; San Diego.

This study demonstrates how the USA300 CA-MRSA strain has displace traditional, HA-MRSA in some hospitals, becoming the primary cause of invasive MRSA infection, including bloodstream infections. Although there were decreases in CA-MRSA in 2006 and 2007, not enough data is provided to conclude whether this decrease was do to changing in MRSA epidemiology in the community and hospital or to changes in management of CA-MRSA infection or virulence of CA-MRSA.

– Keith Kaye, MD

Infectious Disease News Editorial Board member

"May" is the operative word in the title. For the last decade we have read about the increasing problem of CA-MRSA, mostly USA300 strains. Now we are beginning to see reports of long-term studies that may indicate changes in secular trends. It should be no surprise to anyone that long-term secular trends may change the current situation, just as they always have. Consider, if you will, what became of epidemic streptococcal pharyngitis and rheumatic fever.

Several other reports presented at the recent SHEA meeting support the possibility that such changes are occurring. Notably, a report by Kallen et al, from CDC's Active Bacterial Core Surveillance project suggested that HA-MRSA infections are decreasing in incidence during the last several years. Just as phage type 80/81staphylococci disappeared during the 1960s we are left wondering why; are our MRSA prevention practices in hospitals causing this? Are other factors at work here as well? I hope further studies will eventually lead to answers.

– Theodore C. Eickhoff, MD

Infectious Disease News Chief Medical Editor