Background: Nasal carriage of Staphylococcus aureus is common among health care workers, but outbreaks caused by such carriers are relatively uncommon. We previously reported outbreaks of S. aureus skin infections that affected newborn infants and were attributed to an S. aureus nasal carrier who had had an associated upper respiratory tract infection (URI) during the outbreak period.
Objective: To investigate the contribution of a nasal methicillin-resistant S. aureus (MRSA) carrier (physician 4) who contracted a URI to an outbreak of MRSA infections that involved 8 of 43 patients in a surgical intensive care unit during a 3-week period.
Design: An epidemiologic study of an outbreak of MRSA infections and a quantitative investigation of airborne dispersal of S. aureus associated with an experimentally induced rhinoviral infection.
Setting: A university hospital.
Participants: 43 patients in a surgical intensive care unit and 1 physician.
Measurements: Molecular typing was done, and risk factors for MRSA colonization were analyzed. Agar settle plates and volumetric air cultures were used to evaluate the airborne dispersal of S. aureus by physician 4 before and after a rhinoviral infection and with or without a surgical mask.
Results: A search for nasal carriers of MRSA identified a single physician (physician 4); molecular typing showed that the MRSA strain from physician 4 and those from the patients were identical. Multivariate logistic regression analysis identified exposure to physician 4 and duration of ventilation as independent risk factors for colonization with MRSA (P < equals 0.008). Air cultures showed that physician 4 dispersed little S. aureus in the absence of a URI. After experimental induction of a rhinovirus URI, physician 4's airborne dispersal of S. aureus without a surgical mask increased 40-fold; dispersal was significantly reduced when physician 4 wore a mask (P < equals 0.015).
Conclusions: Physician 4 became a “cloud adult,” analogous to the “cloud babies” described by Eichenwald and coworkers who shed S. aureus into the air in association with viral URIs. Airborne dispersal of S. aureus in association with a URI may be an important mechanism of transmission of S. aureus.