Carriers as likely to spread MRSA as clinical patients

May 2008

ORLANDO, Fla. — Methicillin-resistant Staphylococcus aureus carriers identified by active surveillance were equally likely to transmit infection as people with infection or positive clinical cultures, according to a recent study.

As many as 85% of MRSA carriers are not identified by clinical cultures. Recent guidelines recommend active surveillance to identify patients, but considerable controversy surrounds the recommendation.

“A number of concerns have been raised about this mandate by infection control practitioners working in the trenches,” said Shelley Chang, a researcher in the division of epidemiology at Case Western Reserve University in Cleveland. “One of the concerns is illustrated in our study.”

Chang presented study results at the 18th Annual Scientific Meeting of the Society for Healthcare Epidemiology. The study was conducted by Case Western Reserve University and Louis Stokes Cleveland Veterans Administration Medical Center.

Study objectives were to test the idea that MRSA carriers identified only by active surveillance have a low frequency of skin and environmental contamination compared with patients with MRSA infection or positive cultures.

The researchers also sought to identify risk factors for skin and environmental contamination.

“Infection control strategies to limit MRSA transmission must address colonized patients as well as those identified by clinical culture,” Chang said.

Contamination

The researchers enrolled 82 patients with MRSA-positive nares cultures in a seven-month prospective study at Louis Stokes Cleveland VA Medical Center, an acute care hospital where active surveillance is performed on admission for all patients. Thirty-eight patients were detected only by active surveillance. Forty-four patients had current or previous MRSA infection or positive cultures.

The frequency of MRSA contamination for both skin and environmental sites were compared in carriers identified by active surveillance with patients with current and previous MRSA infection or positive clinical cultures. The frequency of skin and environmental contamination was equivalent among carriers detected by active surveillance (47%) and those with MRSA infection or positive clinical culture (45%).

Frequently contaminated skin sites included chest, abdomen, forearms and hands. Skin contamination was present in approximately 70% of patients in both groups. Frequently contaminated environmental sites included bedrails, bedside tables, call buttons and phones. Environmental contamination was present in approximately 55% of patients.

The environment was disinfected at baseline and then sampled to assess ongoing MRSA shedding. MRSA acquisition via hands was assessed by hand imprint cultures. Nares colonization density also was measured. Relatedness of isolates was determined by pulsed-field gel electrophoreses and predictors of contamination were determine by logistic regression.

Decreased mobility and increased density of nares MRSA were independently associated with skin and environmental contamination. The odds of skin and environmental contamination were approximately four times higher among patients with low mobility than those with normal mobility.

Although there was a clear difference in the frequency of skin and environmental contamination, approximately 20% of MRSA carriers with normal mobility had both skin and environmental contamination, the researchers said.

Patients with MRSA who were identified clinically had a trend for longer hospital stays. They also were more likely to receive antibiotics with activity against MRSA, to have boils or abscesses and to have two or more comorbid illnesses. These patients were more likely to live in nursing homes, to have been hospitalized within the past year and to be bedridden.

“Overall, these data suggest that patients identified clinically had more exposure to hospitals or nursing homes and were sicker or more debilitated,” Chang said.

Recent chlorhexidine bathing or use of antibiotics with activity against MRSA were protective and were associated with a 74% decrease in odds of MRSA contamination.

More than 90% of isolates from skin and environmental samples were identical to nares isolates in the same patients. Hand imprint cultures demonstrated MRSA was easily acquired by hands in both groups. Patients with nasal density of 100 colony forming units or greater had a seven-times greater risk of skin and environmental contamination.

For more information:

• Chang S, Sethi AK, Eckstein BC, et al. Patients with unsuspected carriage of methicillin-resistant Staphylococcus aureus are an important source for transmission in hospitals #382. Presented at: The 18th Annual Scientific Meeting of the Society for Healthcare Epidemiology of America; April 5-8, 2008; Orlando, Fla.