MRSA goes airborne in unusual outbreaks

HICPAC mulling mask changes

Methicillin-resistant Staphylococcus aureus (MRSA) — historically the most troublesome nosocomial pathogen in U.S. health care settings — appears to be trying to grow wings.

An emerging body of research indicates the bacteria traditionally spread by contact can become aerosolized under certain conditions and inhaled by patients and workers. In light of such reports, the Centers for Disease Control and Prevention Healthcare Infection Control Practices Advisory Committee (HICPAC) may revise masking recommendations for MRSA patients. MRSA is not considered a true airborne pathogen (i.e., measles), but it can be spread by dispersed droplets, says Robert Sharbaugh, PhD, international director of infection control at Hill-Rom Inc. in Charleston, SC., and former liason HICPAC member representing the Association for Professionals in Infection Control and Epidemiology.

"HICPAC is now working on several revisions of previous CDC guidelines," he says. "One of those has to do with isolation precautions facilities. This particular issue is planning to be addressed. There is the possibility now if we have a patient with MRSA — particularly if it is in the sputum — then droplet precautions might not be a bad idea. [That would include] wearing a mask if you are very close to the patient, say 3 feet."

The case of the opened vent

An usual case of this phenomenon, which took a combination of shoe leather epidemiology and molecular typing to crack, occurred last year at Rhode Island Hospital in Providence. MRSA cases in a surgical intensive care unit that had been averaging 0.8 a month (three per 1,000 patient days) jumped to 2.4 a month (10/1000 patient days) in July to November 2000. Eleven of 13 MRSA cases were inpatients who were mechanically ventilated.

"We noticed an increase in MRSA in our surgical ICU," says Leonard Mermel, MD, hospital epidemiologist at the facility. "Then looking a little more closely at where these infections we’re coming from, [we found] the vast majority — 85% of the respiratory cultures — were obtained while patients were on a vent or shortly after they had come off the vent."

No environmental contamination was found after cultures were done on surfaces, bedrails, and bedside computers. Culturing the staff to look for MRSA carriers yielded only one nurse out of 42 nurses and physicians.

However, four of 72 respiratory therapists were MRSA colonized, suggesting that they might be linked to cases in patients. Interviews with the therapists indicated they were scrupulous hand washers who did not appear to be in violation of any infection control guidelines.

"I spent some time with them and noticed that they were giving lots of treatments," Mermel says. "I never realized [before] how often they were opening the ventilator circuit to give nebulizer treatments."

The system uses heat-moisture exchange (HME) filters, which are commonly used in ventilator circuits in U.S. hospitals, he noted. The circuits are opened very close to the HME filter to provide the medication. "When the system is open, the machine is still on, and most of our patients on vents these days are on positive pressure," Mermel explains. "The machine is blowing out, and you can actually hear it. It is on the backside of the HME filter, [where] the patients secretions have been building up."

Aerosol cultures grow MRSA

To test his theory that aerosol MRSA transmission was occurring from ventilated patients to health care workers, Mermel and colleagues held agar plates for 15 seconds 12 inches away from disconnected ventilator tubing facing the exposed inline HME filter. Two cultures from four patients’ ventilator aerosol grew MRSA.

"That suggests, for the first time that I am aware of, that there could be MRSA aerosolized from ventilator equipment in the course of respiratory therapy treatments," he tells Hospital Infection Control. "This suggest that when opening the system — particularly if you are downwind from this filter that has the patient’s cumulative secretions on it — there is a risk of aerosolization."

Indeed, three of 10 patients’ MRSA isolates were identical to a respiratory therapist’s isolate by pulse-field gel testing. "It is the chicken and the egg," Mermel says. "Did the patient get it from the therapist, or the therapist from the patient? It suggests epidemiologically that there is an association. If the therapist picked it up from a patient and then went to do other treatments, there is a risk of [secondary] transmission," he says.

The respiratory therapist with MRSA identical to three patients’ isolates was taken off duty for one week. Along with other colonized staff, he received nasal mupirocin ointment and chlorhexidine baths for five days. In addition to gown and gloves, a mask was required when ventilator tubing was opened.

"I think the therapists paid attention to careful hand washing," Mermel says. "They weren’t touching secretions or anything; they were just opening the system. They had gloves and gowns on because they were in MRSA [isolation] rooms, but they didn’t have a mask on routinely."

The outbreak subsided after the measures were taken, and now the hospital is considering implementing ventilator systems that do not have to be opened to dispense medication.

"We all cringe in infection control when you have to break any system, be it a Foley catheter or a ventilator or whatever," says Mermel, who presented the study recently in Toronto at the annual conference of the Society for Healthcare Epidemi-ology of American (SHEA).1

Two other studies were presented at SHEA involving unusual airborne routes of transmission of staph.

In one, MRSA was isolated from five patients on a head and neck surgical ward at Vrije University Medical Center in Am-sterdam in May 2000.2 Investigators found that dust filters on two ultrasonic nebulizers used in patient rooms cultured positive for MRSA. Phage typing and amplified fragment length polymorphism typing showed that all strains belonged to the same clone.

The nurses indicated that they changed the tubing, pot, and sterile water of the nebulizer twice a week. However, they did not wash the dust filter weekly, as prescribed in the maintenance protocol.

Another SHEA study investigated the so-called "cloud phenomenon," which hypothesizes that workers with cold symptoms may spread the S. aureus colonized in their nares.3 In a study that used volunteers and a surrogate organism, researchers determined that persistent nasal carriers of staph can disperse the organism into the air under such conditions. Wearing of gowns and masks significantly reduces airborne dispersal, they noted.


1. Mermel L, Dempsey J, Parenteau S. An MRSA outbreak in a surgical intensive care unit — Possible role of aerosol transmission from opened ventilator tubing. Abstract 16. Presented at the Society for Healthcare Epidemiology of America. Toronto; April 2001.

2. Meester M, Schultsz C, Boeijen-donkers L, et al. Evidence for airborne transmission of methicillin-resistant S. aureus. Abstract 36. Presented at the Society for Healthcare Epidemiology of America. Toronto; April 2001.

3. Bischoff W, Bassetti S, Bassettii-wyss B, et al. The cloud phenomenon’: Predictors of Staphylococcus aureus airborne dispersal associated with rhinovirus infection. Abstract 100. Presented at the Society for Healthcare Epidemiology of America. Toronto; April 2001.