Outbreaks of Serratia marcescens have been previously reported in cardiothoracic centers. In 2008, there was an increase in S. marcescens cases at the University of Ottawa Heart Institute. This rise continued into 2009, with several clusters identified and a case count that doubled from 2008. An investigation into this increase was initiated.
The objectives of the investigation were to determine the characteristics and risk factors of patients who acquired Serratia and to identify possible environmental sources of the bacteria.
A retrospective case control analysis of Serratia cases occurring between January 1, 2008 and December 31, 2009 was performed. Each case was matched on date of surgery or admission to two controls. A chart review was conducted to collect data including, duration of mechanical ventilation, tube and parenteral feeding, and 30-day patient outcomes. Limited environmental sampling was performed. Process audits were also conducted.
Sixty (87%) of the 69 cases identified were surgical patients. The majority of cases (62%) had at least one isolate collected from the respiratory tract. Cases were seven times more likely to have been intubated for more than 72 hours compared to controls (p<0.001). Both parenteral nutrition and tube feeding were independently associated with increased risk of Serratia acquisition (p<0.001); however only duration of intubation remained associated with Serratia infection or colonization in multivariate analysis. Environmental samples collected from the operating room environment failed to grow Serratia. Process audits revealed several areas for improvement, but none that could be directly linked with the increase in Serratia cases.
Intubation exceeding 72 hours was associated with an increased likelihood of being colonized or infected with Serratia in the University of Ottawa Heart Institute. No definite environmental source of Serratia has yet been identified; acquisition of Serratia is likely multifactorial. Determining the source of endemic pathogens and terminating transmission can be extremely challenging.