Is there a causal relationship between contamination burden and transmission risk?

contamination v transmission There’s an age-old problem in science: how do you prove a causal relationship between variables that correlate? Proving that the variables are correlated is the easy part; it’s more difficult to disentangle cause from effect. This can be seen in several studies that identify a correlation between environmental burden and the number of patients that are infected or colonized with pathogens.DentonFigure 1. Correlation between the number of patients infected with Acinetobacter spp. and the number of positive Acinetobacter spp. environmental cultures per calendar month during an outbreak on a neurosurgical ICU.1

SalgadoFigure 2. Correlation between microbial burden and the number of patients who acquired an HAI in ICUs.2

WhiteFigure 3. Correlation between the number of hygiene failures and the number of patients who acquired an infection on a surgical intensive care unit each week.3

So can we conclude that the higher burden of contamination resulted in an increased risk of acquisition? Or is it that more patients were infected or colonized with pathogens, which resulted in more environmental shedding? From these studies, you can’t be sure.

If you were seeking to prove the role of a gene in a process, you’d knock out the gene and demonstrate that the process stopped or changed. So, the only way to disentangle cause and effect in contamination and transmission is to perform an intervention to reduce environmental contamination and show that this correlates with reduced transmission. While the Salgado study evaluated an intervention, the data correlating contamination burden with HAIs was not stratified by the intervention, which would have been one way to assess likely causation.2

There is some further in vitro and epidemiological data supporting that the degree of transmission may be proportional to the environmental burden. An in vitro mouse model established a ‘dose-response’ relationship between the degree of contamination with C. difficile spores and the development of CDI.4 Furthermore, this model showed that disinfectants that achieved a greater log reduction of C. difficile spores were more able to interrupt transmission.

Also, one of the studies demonstrating that admission to a room previously occupied by a patient with VRE increases the chances of VRE acquisition identified something amounting to a ‘dose response’.5 The greatest increased risk was for patients admitted to a room with an environmental culture positive for VRE, and being admitted to a room where the immediate prior room occupant was colonized with VRE carried a greater increased risk than being admitted to a room where any patient in the 2 weeks prior to admission was VRE colonized (Figure 4).

DreesFigure 4. How the increased risk of acquiring VRE from the prior room occupant changes due to patient and environmental factors.5

Is there a causal relationship between contamination burden and transmission risk? On balance, the answer seems to be yes, though it would be useful to have a solid intervention study to prove that an increasing environmental burden causes an incrementally increase in transmission risk.

Article citations:

  1. Denton M, Wilcox MH, Parnell P et al. Role of environmental cleaning in controlling an outbreak of Acinetobacter baumannii on a neurosurgical intensive care unit. J Hosp Infect 2004; 56: 106-110.
  2. Salgado CD, Sepkowitz KA, John JF et al. Copper surfaces reduce the rate of healthcare-acquired infections in the intensive care unit. Infect Control Hosp Epidemiol 2013; 34: 479-486.
  3. White LF, Dancer SJ, Robertson C, McDonald J. Are hygiene standards useful in assessing infection risk? Am J Infect Control 2008; 36: 381-384.
  4. Lawley TD, Clare S, Deakin LJ et al. Use of purified Clostridium difficile spores to facilitate evaluation of health care disinfection regimens. Appl Environ Microbiol 2010; 76: 6895-6900.
  5. Drees M, Snydman D, Schmid C et al. Prior environmental contamination increases the risk of acquisition of vancomycin-resistant enterococci. Clin Infect Dis 2008; 46: 678-685.

2 thoughts on “Is there a causal relationship between contamination burden and transmission risk?

  1. I would agree that there is a direct correlation between the level of contamination found within the environment and propensity for cross contamination to healthcare workers hands. Patient environments such as a neurosurgical ICU with long term stay patients present a significant challenge to domestic / environmental services to deliver adequate decontamination both in daily and discharge cleaning practices. Open plan ICU facilities often preclude the use of area decontamination methods to supplement manual cleaning.
    Outbreak situations require a substantial change to standard cleaning practice, including increasing frequency of cleaning and time allocated to allow diligent cleaning of the patient environment and equipment. A confounding question is one of responsibility, when items of patient equipment are not cleaned by the cleaning staff and is the responsibility of nursing staff.
    One study evaluated the impact of training cleaning staff, to a technically competent standard, to undertake the cleaning of the total patient ‘zone’ including clinical equipment, (Wilson APR et al. The Impact of enhanced cleaning within the intensive care unit on contamination of the near-patient environment with hospital pathogens: A randomized crossover study in critical care units in two hospitals. Crit Care Med 2011 Vol 39 No 4). This study included outbreak events which incorporated a ‘barrier cleaning’ protocol to eliminate the risk of cross contamination by restricting the dedicated barrier cleaning staff’s movement from colonised / infected patient zones into other ward areas.
    A significant result was that during the enhanced cleaning phase the overall bioburden found within both in the patient environment and on the hands of health care workers was substantially reduced. An indicator that an investment in the competency training of front line ‘Environmental Hygienists’ can have a significant impact on risk reduction to patient safety, particularly in acute, long term care facilities.


    • Michael, a useful comment thanks. Interesting that you mention a direct correlation between contamination of surface and of hands. One of the articles that I considered for the blog identified exactly this association.

      I wonder whether, over time, the enhanced cleaning that, as you mention, often occurs during outbreaks will become closer to the standard of care? Put another way, will the gap between the quality of “enhanced” cleaning and disinfection and “normal” cleaning and disinfection narrow? I certainly hope so!

      The Wilson study that you cite is useful, demonstrating reduced microbial burden on hands and surfaces. However, this study did not find a significant reduction in the transmission of MRSA associated with the enhanced cleaning approach, which suggests that other factors were at play.


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