Ventilation grilles as a potential source of methicillin-resistant Staphylococcus aureus causing an outbreak in an orthopaedic ward at a district general hospital

The spread of methicillin-resistant Staphylococcus aureus (MRSA) in a hospital is thought to be mainly by direct contact. Environmental sources such as exhaust ducting systems have been increasingly recognized as a source for MRSA outbreaks in intensive therapy units. We describe an outbreak of MRSA related to ventilation grilles in an orthopaedic ward. Six patients and one nurse were involved in an outbreak with EMRSA-15 during March 1996. The index case was transferred from a large university hospital in Leeds. One of the patients had shared the same bay with the index case. The rest of the patients were in another bay of the same ward and had no direct contact with the index patient. An environmental source was suspected and the ventilation grilles in bays 1 and 2 were found to be harbouring EMRSA-15. The ventilation system at that time was working on an intermittent cycle from 4 p.m.-8 a.m. Daily shut-down of the system temporarily created a negative pressure, sucking air in from the ward environment into the ventilation system and probably contaminating the outlet grilles. It is likely that contaminated air was blown back into the ward when the ventilation system was started. The system was thoroughly cleaned, appropriate infection control measures were instituted and the ventilation system was put back on a continuous running cycle and the outbreak terminated. Six months after the outbreak no isolates of EMRSA-15 had been made on the ward.