MRSA rates in the U.S. appear to be decreasing, study indicates

Rates of methicillin-resistant Staphylococcus aureus (MRSA) appear to be decreasing in the U.S., according to a new study.

Researchers used data from U.S. Department of Defense beneficiaries to examine incidence and trends of community- and hospital-onset S. aureus bacteremia and skin and soft-tissue infections (SSTIs), including the proportion due to MRSA. Beneficiaries included active duty members, retirees, guards and reservists, and their immediate family. S. aureus blood, wound or abscess cultures were classified as community- or hospital-onset infections and as methicillin-susceptible S. aureus or MRSA. Main outcome measures were unadjusted incidence rates per 100,000 person-years, proportion of infections due to MRSA, and annual trends. The study results appeared in the July 4 Journal of the American Medical Association.

The Department of Defense databases included 62,326 positive blood cultures and 181,317 positive wound or abscess cultures from 2005 through 2010. Among these, 12% of blood cultures and 62% of wound or abscess cultures yielded S. aureus isolates. Over 56 million person-years (47 million in nonactive duty and 9 million in active duty), 2,643 blood cultures and 80,281 wound or abscess cultures tested positive for S. aureus. Annual incidence rates were 3.6 to 6.0 per 100,000 person-years for S. aureus bacteremia and 122.7 to 168.9 per 100,000 person-years for SSTIs due to S. aureus. From 2005 to 2010, a decrease was seen in annual incidence rates of community-onset MRSA bacteremia (1.7 per 100,000 person-years vs. 1.2 per 100,000 person-years, respectively; P=0.005 for trend) and hospital-onset MRSA bacteremia (0.7 per 100,000 person-years vs. 0.4 per 100,000 person-years, respectively; P=0.005 for trend). Community-onset SSTIs due to MRSA reached a peak of 62% in 2006 but decreased each year thereafter to 52% in 2010 (P<0.001 for trend).

The authors noted that no pre-2005 data were available and that they were therefore unable to determine trends in S. aureus bacteremia and SSTIs before community-acquired MRSA emerged. Data on race, ethnicity and clinical outcomes were also unavailable, among other limitations. However, the authors concluded that while S. aureus bacteremia and SSTIs continue to place a substantial burden on the U.S. military health system, rates of community-onset MRSA and methicillin-susceptible S. aureus bacteremia and hospital-onset MRSA bacteremia decreased from 2005 to 2010, along with the proportion of community-onset SSTIs due to MRSA.

“These observations, taken together with results from others showing decreases in the rates of health care-associated infections from MRSA, suggest that broad shifts in the epidemiology of S. aureus infections may be occurring,” the authors wrote. “Additional studies are needed to assess whether these trends will continue, which prevention methods are most effective, and to what degree other factors may be contributing.”