Community-Acquired Strains Cause Most MRSA

VIENNA — Community-acquired MRSA as a proportion of all MRSA infections tripled in a recent 10-year period in the United States, according to a large national study.

This rise in community-acquired MRSA (CA-MRSA) has occurred across all age groups, in all types of Staphylococcus aureus infections, all geographic regions of the country, and in both inpatient and outpatient settings, Dr. Robertino M. Mera reported at the annual European Congress of Clinical Microbiology and Infectious Diseases.

At the hospital level, the incidence of CA-MRSA has increased faster than hospital-acquired MRSA. The rate of hospital discharges in the United States for hospital-acquired MRSA infections has gone from 3.2 per 1,000 discharges in 1998 to 4.8 per 1,000 in 2007, a modest albeit statistically significant increase. Meanwhile, the rate of discharges for CA-MRSA has shot up eightfold, from 0.4 per 1,000 to 3.3 per 1,000.

The biggest changes in CA-MRSA trends, however, have occurred in children and in skin and soft tissue infections, Dr. Mera noted.

Although CA-MRSA accounted for less than 5% of all S. aureus infections in U.S. children under age 15 years in 1998, the proportion jumped to 45% a decade later. By 2007, 82% of all MRSA infections in U.S. children under age 15 years had the CA-MRSA phenotype.

“This is quite remarkably high,” observed Dr. Mera of GlaxoSmithKline, Durham, N.C.

Moreover, 86% of all MRSA isolates obtained from abscesses in 2007 were CA-MRSA, as were 75% of those from wounds. CA-MRSA has replaced hospital-acquired MRSA as the source of most skin and soft tissue hospital infections.

In contrast, by 2007 CA-MRSA accounted for 30% of all MRSA sputum samples and 44% of blood samples.

Dr. Mera presented an analysis of 824,307 S. aureus isolates obtained from across the United States in 1998-2007. The analysis utilized the Surveillance Network (TSN) database and National Hospital Discharge Survey data.

In 1998, MRSA accounted for 33% of all S. aureus infections. This proportion climbed steadily during the following decade, reaching 54% in 2007. Meanwhile, the proportion of CA-MRSA within all MRSA infections jumped from 22% in 1998 to 66% in 2007.

In 1998 there were large age-group differences in the proportion of S. aureus infections due to MRSA. Among patients older than 65 the figure was 47%, while in children younger than 15 years it was just 13%. By 2007, however, MRSA accounted for fully 47% of all S. aureus infections in children and 57% in the elderly. Among patients age 16-44 years, the proportion of S. aureus infections due to MRSA climbed from 23% to 55% during the decade.

In 1998, CA-MRSA accounted for a mere 7.3% of all S. aureus isolates. By 2007, this figure rose to 35.6%. This increase was achieved by CA-MRSA replacing hospital-acquired MRSA strains, and to an even greater extent by replacing methicillin-susceptible S. aureus, which accounted for 67% of all S. aureus infections in 1998 but only 46% a decade later.

Disclosures: This study was supported by GlaxoSmithKline.

The proportion of CA-MRSA within all MRSA infections jumped from 22% in 1998 to 66% in 2007.

Source DR. MERA

Risk Factors Double Likelihood of Treatment Failure in Diabetics With MRSA

Predictors of clinical failure in the treatment of complicated skin and skin structure infections in diabetic patients due to methicillin-resistant Staphylococcus aureus include involvement of a body area other than the lower extremities, according to an analysis combining the results of three large, prospective, randomized, phase III/IV clinical trials.

The other independent predictors of treatment failure in diabetic patients were comorbid peripheral vascular disease and polymicrobial pathogens, Dr. Benjamin A. Lipsky reported at the annual European Congress of Clinical Microbiology and Infectious Diseases.

The presence of these risk factors, each of which roughly doubled the likelihood of treatment failure, should serve to alert physicians to an elevated risk of poor outcome, added Dr. Lipsky of the Veterans Affairs Puget Sound Health Care System and the University of Washington, Seattle.

He reported on 845 patients with complicated skin and skin structure infections due to MRSA who participated in three open-label clinical trials involving randomization to treatment with linezolid or vancomycin. A total of 34% of participants were diabetic.

This is believed to be the largest-ever analysis of predictors of clinical failure in patients with skin infections caused by MRSA, he said.

Clinical failure was defined as persistence or progression of clinical signs and symptoms of active infection at the study's end, which came variously 6-28 days after the last dose of study medication.