By Brenda L. Mooney
SAN ANTONIO — The Military Health System is winning some key battles, but the outcome of the war against methicillin-resistant Staphylococcus aureus (MRSA) is still in question.
That’s according to a new DoD analysis of more than 9 million servicemembers and TRICARE beneficiaries. In a report published recently in JAMA, researchers found that the rates of both community- and hospital-onset bacteremia decreased from 2005 to 2010. Skin and soft-tissue infections (SSTI) caused by MRSA also declined but later in the period than bacteremia.1
“These observations, taken together with results from others showing decreases in the rates of healthcare-associated infections from MRSA, suggest that broad shifts in the epidemiology of S aureus infections may be occurring,” the authors write. “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.”
|Under a very high magnification of 50,000x, this scanning electron micrograph shows a strain of Staphylococcus aureus bacteria taken from a vancomycin intermediate resistant culture (VISA).|
The study, led by Michael L. Landrum, MD, of the San Antonio Military Medical Center, Fort Sam Houston, TX, reviewed the incidence rates of community- and hospital-onset S aureus bacteremia and SSTIs, including the proportion due to MRSA. Records of all TRICARE beneficiaries were reviewed from January 2005 through December 2010, with all annual first-positive S aureus blood and wound or abscess cultures identified and classified as methicillin-susceptible S aureus (MSSA) or MRSA, and as community- or hospital-onset infections (isolates collected >3 days after hospital admission).
In 2005, 2,643 blood and 80,281 wound or abscess annual first-positive S aureus cultures were included in the study for further analyses among a study population that was 52% male and 84% non-active duty.
Infections from S aureus bacteremia were more likely to be community onset, 79%, and the highest rates were in those 65 years or older and in men. Of hospital-onset bacteremia, 54% were due to MRSA.
Nearly all, 99%, of the S aureus SSTI were community-onset, and 58% of them were due to MRSA, significantly higher than for either community-onset bacteremia, 39%, or hospital-onset SSTIs, 53%.
According to the study, annual incidence rates varied from 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 S aureus SSTIs. Community-onset MRSA bacteremia decreased in annual incidence from 1.7 per 100,000 person-years (95% CI, 1.5-2.0 per 100,000 person-years) in 2005 to 1.2 per 100,000 person-years (95% CI, 0.9-1.4 per 100,000 person-years) in 2010 (P =0.005 for trend).
From 2005 to 2010, the annual incidence rates for hospital-onset MRSA bacteremia also decreased from 0.7 per 100,000 person-years (95% CI, 0.6-0.9 per 100,000 person-years) to 0.4 per 100,000 person-years (95% CI, 0.3-0.5 per 100,000 person-years).
The authors note that the peak year for community-onset SSTI due to MRSA was 2006 when rates were 62%, dropping annually to 52% in 2010.
“The bacteria epidemiology is changing,” said one of the authors, Army Maj. Clinton K. Murray, MD, of the San Antonio Military Medical Center. “The prevalence of MRSA skin and soft-tissue infections is decreasing, while the MSSA skin and soft-tissue infections are not increasing, really meaning, overall, there’s a decrease in MRSA and MSSA across the U.S.”
During a video interview made available by JAMA, Murray added, “some of the infection control that we do in the hospital is working, but there’s also a change in the community, and that’s not where we put in aggressive infection-control measures.”
The highest rates of community-onset S aureus bacteremia occurred in the very old and the very young, according to the study, while community-onset S aureus SSTI rates were highest in young adults aged 18 through 24 years, men, active-duty servicemembers and Southerners.
The authors question whether the characteristics are independent risk factors, adding, “The geographic distribution of community-onset SSTI rates may reflect both the influence of climate on SSTIs and the primary locations of military training, where rates of SSTIs are known to be high.”MRSA Infections Down Significantly in Military
The unique characteristics of the military-related study group prompted the authors to raise questions about its applicability across the general population.
One issue noted was the special situation of active-duty servicemembers, who are generally young, healthy, have open access to healthcare, yet have increased risk of SSTIs due to military training and related exposures. In fact, the study found that the rates of SSTIs and both bacteremia and SSTIs due to MRSA were higher in active duty personnel than others in the study group.
Because of the makeup of the TRICARE population, non-active-duty beneficiaries made up about 85% of the patients studied. Rates of bacteremia and SSTIs declined in both active-duty and non-active-duty groups, however, and the authors note that, “while the overall rates of disease were likely influenced by the characteristics of the Department of Defense population, the observed trends were consistent with investigations in other U.S. populations.”
Another way this study differed from previous research was the diverse locations of MTFs – urban, suburban and rural. In addition, TRICARE beneficiaries, because of the typically younger age of servicemembers and their families, tend to have lower rates of comorbid medical conditions, which are associated with increased S aureus bacteremia risk. Other large studies have tended to be conducted at urban tertiary-care centers, where many patients may have chronic kidney disease requiring hemodialysis and/or intravenous drug use.
In addition, most TRICARE beneficiaries have ready access to healthcare services and a reliable income, either while on active duty or following retirement, the study points out, explaining, “such factors may mitigate socioeconomic differences that have been associated with increased risk of MRSA infections.”
An advantage to looking at this population, however, was the ability to look at hospital-onset and community- infections simultaneously, according to the authors.
“The lines between the hospital and community settings have become less distinct in recent years,” they write. “Whether the changes in S aureus epidemiology, overall, were driven by changes within either the hospital or community settings, or both, remains uncertain. Improved infection-control practices may be affecting rates of both hospital-onset and healthcare-acquired community-onset MRSA bacteremia.”
Despite the positive trends, background information in the article notes that SSTIs have become a significant issue for the military. It states that, during training, 4% to 6% of troops deal with skin and soft-tissue infections and that S aureus has been isolated from 91% of such cases, with MRSA accounting for 70% of S aureus isolates.
Because of that and other resistant infection threats, the authors caution that vigilance on prevention and treatment needs to continue.
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1: Landrum ML, Neumann C, Cook C, Chukwuma U, Ellis MW, Hospenthal DR, Murray CK. Epidemiology of Staphylococcus aureus blood and skin and soft tissue infections in the US military health system, 2005-2010. JAMA. 2012 Jul 4;308(1):50-9. PubMed PMID: 22760291.