The mystery of the Staphylococcus in retreat

Evidence keeps accumulating that Staphylococcus aureus disease, for whatever reason(s), is declining in incidence. The latest massive study to illustrate this trend is courtesy of the U.S. Military Health System and is published this week in JAMA. Using their integrated electronic medical record, military epidemiologists demonstrate that both community- and hospital-onset S. aureus bloodstream infections (MRSA and MSSA) declined in incidence between 2005-2010, and that the proportion of S. aureus skin/soft tissue infections due to MRSA declined beginning in 2007. While it might not be generalizable, the Military Health System is very large, encompassing active duty military, retirees, reservists and their families, and this study included over 56 million person-years of observation.

One strength of this study is that it shows not only healthcare-associated but also community S. aureus disease to be declining, a decline that is difficult to attribute to hospital-focused interventions. The reports now from NHSN, the Active Bacterial Core Surveillance program, the United Kingdom, the EARSS system, and now the military all speak to our ignorance of the complex interplay between S. aureus and its human reservoir. Successive long cycles of waxing and waning epidemic spread have been, and will be, the rule. 

For a nice example, check out this study from Oxfordshire hospitals demonstrating how their MRSA incidence began to drop well in advance of intensified infection control interventions, and how this drop coincided with the rise and fall of two competing strain types.

Image from Planet Science