No more than 25% of cases of Clostridium difficile in hospitals are actually acquired in the hospital, a three-hospital study suggested.
The finding comes from a mathematical analysis of a large set of C. difficile samples, collected from the hospitals that provide service to the city of Oxford, England, according to Sarah Walker, PhD, of John Radcliffe Hospital in Oxford, and colleagues.
The study, published online in PLoS Medicine, calls into question the assumption that most hospital cases of C. difficile infection are nosocomial and suggests that many are imported, Walker and colleagues argued.
But that conclusion, while "tantalizing," is not yet borne out by the evidence, commented Stephan Harbarth, MD, of the University of Geneva in Switzerland and Matthew Samore, MD, of the University of Utah School of Medicine in Salt Lake City.
In an accompanying perspective article, Harbarth and Samore note that the study only looked as the possibility of transmission within wards; transmission between wards, such as when patients moved around the hospital for x-rays or other procedures, was not assessed.
The researchers also didn't examine asymptomatic C. difficile carriage when patients entered or left the hospitals, so they can say nothing about the possibility of imported disease.
Proving that most hospital cases of C. difficile disease are imported would be "revolutionary," they wrote, but the evidence from the study "is not yet sufficient to prove this hypothesis."
Walker and colleagues looked at data collected over a 2.5-year period in the three hospitals in the Oxford Radcliffe Hospitals NHS Trust, which serve the approximately 600,000 people in the area.
As a matter of routine, the researchers noted, stool samples from any patient with diarrhea were sent for enzyme immunoassay testing and positive samples were cultured.
Over the period from Sept. 1, 2007 to March 31, 2010, some 1,282 samples (or 4.4% of those tested) were positive for C. difficile on both tests; of those, 1,276 from 927 patients were genotyped.
Analysis of the genotypes found 69 distinct sequence types and the researchers used those data to construct possible networks and transmission events. They found that, on average, there were 102 tests and 9.4 cases of disease per 10,000 overnight stays among inpatients and 238 tests and 15.7 cases per month in outpatients and primary care patients.
In addition, only one in four cases could be linked to a potential ward-based inpatient source, ranging from a high of 37% in renal or transplant patients to 6% in those undergoing specialist surgery.
Most of the apparent transmissions -- some 65% -- occurred no more than a week after the onset of symptoms in the index patient, and only 10% occurred after eight weeks.
The clinical implication of the findings is that current infection control measures may be falling short, the researchers said.
"A better understanding of other routes of transmission and reservoirs is needed to determine what other types of control interventions are required to reduce the spread of C. difficile," they concluded.
Primary source: PLoS Medicine
Walker AS, et al "Characterisation of Clostridium difficile hospital ward–based transmission using extensive epidemiological data and molecular typing" PLoS Med 2012; 9: e1001172.
Additional source: PLoS Medicine
Harbarth S, Samore MH "Clostridium: transmission difficile?" PLoS Med 2012; 9: e1001171.