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GENEVA, April 15 -- One in every 20 healthcare workers carries methicillin-resistant Staphylococcus aureus (MRSA), researchers here said.

But the vast majority is without symptoms and only 5.1% have full-blown clinical infections, according to Stephan Harbarth, M.D., of the University Hospitals of Geneva, and Werner Albrich, M.D., of University Hospital Bern.

One implication is that screening efforts aimed at symptomatic infections are likely to miss a large proportion of colonized healthcare workers who might transmit the bacteria, they wrote in a literature review in the May issue of Lancet Infectious Diseases.

Instead, they said, "aggressive screening and eradication policies" should be used in an outbreak and in situations where MRSA has not reached highly endemic levels.

The researchers looked at 127 studies published from January 1980 through March 2006, to see how likely healthcare workers are to be infected or colonized by MRSA and to assess their role in MRSA transmission.

On the basis of the published evidence, they said, healthcare workers are usually vectors, rather than the main sources of MRSA transmission, implying that "good hand hygiene practices remain essential to control the spread of MRSA."

Among 33,318 workers screened in the studies, 4.6% on average were carrying MRSA, the researchers found -- usually in the nose, although other sites were found. Most (94.9%) had no symptoms.

Risk factors included chronic skin diseases, poor hygiene practices, and having worked in countries with endemic MRSA.

Poor infection control practices were linked to both acquisition and transmission of MRSA, the researchers said, but even good adherence to infection control and hand hygiene did not entirely prevent transmission to patients.

In fact, several studies were unable to identify any risk factors for MRSA colonization, the researchers said.

Of the 127 studies that screened healthcare workers, 106 looked at transmission to patients, Drs. Harbarth and Albrich said. Of those, 27 reported clear molecular and epidemiological evidence of transmission, and another 52 found that transmission was likely.

But the transmission was not dependent on a symptomatic infection in a healthcare worker, they said. In one reported case, a healthy worker was carrying MRSA in the nose-infected patients in a newborn nursery, leading to patient-to-patient transmission.

"Our search revealed 18 studies with proven and 26 studies with likely transmission to patients from healthcare workers who were not clinically infected with MRSA, the researchers said.

That finding suggested that a recent recommendation that screening efforts focus on healthcare workers with symptomatic infection is likely to miss the boat, they said.

"Staphylococcal dispersal is mainly dependent on whether the person is a nasal carrier," they said, so that "screening of infected healthcare workers only will likely miss a large number of asymptomatic personnel capable of transmitting MRSA to patients.

Instead, they suggest that institutions should screen workers as part of a pre-employment exam "irrespective of the presence of risk factors or purulent infections."

They might even periodically -- and without making an announcement -- conduct such screening before a work shift, Drs. Harbarth and Albrich said.

But they added that MRSA screening -- and treatment to eradicate colonies -- "should always be part of a comprehensive infection control policy including staff education and emphasizing high compliance with hand hygiene and contact precautions."

It's also important to avoid "feelings of guilt or stigmatization" among those found to be colonized, they said. "In analogy to needle-stick injuries, MRSA carriage or infection in a healthcare worker should be considered an occupational hazard," they said.

The researchers did not report any outside source of financing. Dr. Harbarth reported consulting fees from 3M, BioMerieux, and Roche Diagnostics. Dr. Albrich reported no potential conflicts.

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