The Changing Epidemiology of Methicillin-Resistant Staphylococcus aureus in the United States

Seasonal and age variations seen in hospital MRSA infection rates.

Hospitals already dealing with high levels of hospital-associated (HA) methicillin-resistant Staphylococcus aureus (MRSA) now also are finding that patients are being infected with community-associated (CA) strains.

These researchers used national data (2005–2009) on hospitalizations and surveillance data on drug susceptibility to understand MRSA epidemiology patterns in hospitals.

The total number of S. aureus hospitalizations did not change significantly over the study period. In 2009, using the new MRSA-specific diagnosis code that was added in 2008, they calculated MRSA infections were 11.74 per 1,000 hospitalizations. The proportion was constant at 55 percent HA-MRSA and 45 percent CA-MRSA. Hospitalizations for blood stream infections were more likely to be caused by HA-MRSA than CA-MRSA (64% versus 36%) as were hospitalizations for pneumonia (74% versus 26%). “Other” infections, however, increased over time for HA-MRSA (44% in 2005 to 49% in 2008), while falling for CA-MRSA (56% to 51%).

Infections that led to hospitalizations in younger patients were more likely caused by CA-MRSA (74% for patients 0–19 years; 55% for 20–64; and 37% for over 65). Hospitalizations for older adults were more likely caused by HA-MRSA.

CA-MRSA infection rates peaked in the summer, especially for younger patients, while HA-MRSA peaked in the winter.