P. Andrew Sleigh

Aerial Dissemination of Clostridium difficile spores

BackgroundClostridium difficile-associated diarrhoea (CDAD) is a frequently occurring healthcare-associated infection, which is responsible for significant morbidity and mortality amongst elderly patients in healthcare facilities. Environmental contamination is known to play an important contributory role in the spread of CDAD and it is suspected that contamination might be occurring as a result of aerial dissemination of C. difficile spores. However previous studies have failed to isolate C. difficile from air in hospitals. In an attempt to clarify this issue we undertook a short controlled pilot study in an elderly care ward with the aim of culturing C. difficile from the air.MethodsIn a survey undertaken during February (two days) 2006 and March (two days) 2007, air samples were collected using a portable cyclone sampler and surface samples collected using contact plates in a UK hospital. Sampling took place in a six bedded elderly care bay (Study) during February 2006 and in March 2007 both the study bay and a four bedded orthopaedic bay (Control). Particulate material from the air was collected in Ringers solution, alcohol shocked and plated out in triplicate onto Braziers CCEY agar without egg yolk, but supplemented with 5 mg/L of lysozyme. After incubation, the identity of isolates was confirmed by standard techniques. Ribotyping and REP-PCR fingerprinting were used to further characterise isolates.ResultsOn both days in February 2006, C. difficile was cultured from the air with 23 samples yielding the bacterium (mean counts 53 – 426 cfu/m3 of air). One representative isolate from each of these was characterized further. Of the 23 isolates, 22 were ribotype 001 and were indistinguishable on REP-PCR typing. C. difficile was not cultured from the air or surfaces of either hospital bay during the two days in March 2007.ConclusionThis pilot study produced clear evidence of sporadic aerial dissemination of spores of a clone of C. difficile, a finding which may help to explain why CDAD is so persistent within hospitals and difficult to eradicate. Although preliminary, the findings reinforce concerns that current C. difficile control measures may be inadequate and suggest that improved ward ventilation may help to reduce the spread of CDAD in healthcare facilities.

The ventilation of multiple-bed hospital wards: review and analysis.

BACKGROUND Although the merits of ventilating operating theatres and isolation rooms are well known, the clinical benefits derived from ventilating hospital wards and patient rooms are unclear. This is because relatively little research work has been done in the ventilation of these areas compared with that done in operating theatres and isolation rooms. Consequently, there is a paucity of good quality data from which to make important decisions regarding hospital infrastructure. This review evaluates the role of general ward ventilation to assess whether or not it affects the transmission of infection. METHODS A critical review was undertaken of guidelines in the United Kingdom and United States governing the design of ventilation systems for hospital wards and other multibed rooms. In addition, an analytical computational fluid dynamics (CFD) study was performed to evaluate the effectiveness of various ventilation strategies in removing airborne pathogens from ward spaces. RESULTS The CFD simulation showed the bioaerosol concentration in the study room to be substantially lower (2467 cfu/m(3)) when air was supplied and extracted through the ceiling compared with other simulated ventilations strategies, which achieved bioaerosol concentrations of 12487 and 10601 cfu/m(3), respectively. CONCLUSIONS There is a growing body of evidence that the aerial dispersion of some nosocomial pathogens can seed widespread environmental contamination, and that this may be contributing to the spread infection in hospital wards. Acinetobacter spp in particular appear to conform to this model, with numerous outbreaks attributed to aerial dissemination. This suggests that the clinical role of general ward ventilation may have been underestimated and that through improved ward ventilation, it may be possible to reduce environmental contamination and thus reduce nosocomial infection rates.

Bactericidal action of positive and negative ions in air

BackgroundIn recent years there has been renewed interest in the use of air ionisers to control of the spread of airborne infection. One characteristic of air ions which has been widely reported is their apparent biocidal action. However, whilst the body of evidence suggests a biocidal effect in the presence of air ions the physical and biological mechanisms involved remain unclear. In particular, it is not clear which of several possible mechanisms of electrical origin (i.e. the action of the ions, the production of ozone, or the action of the electric field) are responsible for cell death. A study was therefore undertaken to clarify this issue and to determine the physical mechanisms associated with microbial cell death.ResultsIn the study seven bacterial species (Staphylococcus aureus, Mycobacterium parafortuitum, Pseudomonas aeruginosa, Acinetobacter baumanii, Burkholderia cenocepacia, Bacillus subtilis and Serratia marcescens) were exposed to both positive and negative ions in the presence of air. In order to distinguish between effects arising from: (i) the action of the air ions; (ii) the action of the electric field, and (iii) the action of ozone, two interventions were made. The first intervention involved placing a thin mica sheet between the ionisation source and the bacteria, directly over the agar plates. This intervention, while leaving the electric field unaltered, prevented the air ions from reaching the microbial samples. In addition, the mica plate prevented ozone produced from reaching the bacteria. The second intervention involved placing an earthed wire mesh directly above the agar plates. This prevented both the electric field and the air ions from impacting on the bacteria, while allowing any ozone present to reach the agar plate. With the exception of Mycobacterium parafortuitum, the principal cause of cell death amongst the bacteria studied was exposure to ozone, with electroporation playing a secondary role. However in the case of Mycobacterium parafortuitum, electroporation resulting from exposure to the electric field appears to have been the principal cause of cell inactivation.ConclusionThe results of the study suggest that the bactericidal action attributed to negative air ions by previous researchers may have been overestimated.

Mathematical models for assessing the role of airflow on the risk of airborne infection in hospital wards

Understanding the risk of airborne transmission can provide important information for designing safe healthcare environments with an appropriate level of environmental control for mitigating risks. The most common approach for assessing risk is to use the Wells–Riley equation to relate infectious cases to human and environmental parameters. While it is a simple model that can yield valuable information, the model used as in its original presentation has a number of limitations. This paper reviews recent developments addressing some of the limitations including coupling with epidemic models to evaluate the wider impact of control measures on disease progression, linking with zonal ventilation or computational fluid dynamics simulations to deal with imperfect mixing in real environments and recent work on dose–response modelling to simulate the interaction between pathogens and the host. A stochastic version of the Wells–Riley model is presented that allows consideration of the effects of small populations relevant in healthcare settings and it is demonstrated how this can be linked to a simple zonal ventilation model to simulate the influence of proximity to an infector. The results show how neglecting the stochastic effects present in a real situation could underestimate the risk by 15 per cent or more and that the number and rate of new infections between connected spaces is strongly dependent on the airflow. Results also indicate the potential danger of using fully mixed models for future risk assessments, with quanta values derived from such cases less than half the actual source value.

2D Process-Based Morphodynamic Model for Flooding by Noncohesive Dyke Breach

AbstractInundation models based on the shallow water equations (SWE) have been shown to perform well for a wide variety of situations even at the limit of their theoretical applicability and, arguably, somewhat beyond. One of these situations is the catastrophic event of floods induced by dyke breach and consequent dyke erosion. The dyke collapse is often not sudden—as assumed by many flood simulations in which the dyke boundary is treated as a “dam-break.” The dyke erosion is a gradual and complex process that delays the onset of the flood, affecting the hydrograph of the flow. To simulate correct temporal passage of a flood, it is important to understand the rate at which these dykes collapse. In this paper, an overtopping flood event combined with dyke erosion is simulated. The model is built upon the two-dimensional (2D) shallow water equations together with sediment-flow interactions and incorporates a sediment transport equation. The model is solved using a second-order Godunov-type finite volume me...

Multimode Morphodynamic Model for Sediment-Laden Flows and Geomorphic Impacts

AbstractSediment-laden flows are a complex solid-fluid interaction process. This study presents a multimode morphodynamic model system combined with shallow water theory and a nonequilibrium assumption for sediment transport. The model system aims to simulate the morphological change caused by sediment-laden flows with various sediment transport modes. It involves three modules: a hydrodynamic module, a sediment transport module, and a morphological evolution module. The hydrodynamic model is governed by modified shallow water equations considering the interaction effects of flow and sediment. A flexible sediment transport model is presented that incorporates a weight coefficient. The model can adaptively choose an appropriate transport mode according to local, real-time flow conditions. Bedload, suspended load, and total mixed sediment load are all involved. The model is solved by a second-order Godunov-type finite-volume method that is robust and accurate. Validation is demonstrated through a series of ...

Effect of negative air ions on the potential for bacterial contamination of plastic medical equipment

BackgroundIn recent years there has been renewed interest in the use of air ionizers to control the spread of infection in hospitals and a number of researchers have investigated the biocidal action of ions in both air and nitrogen. By comparison, the physical action of air ions on bacterial dissemination and deposition has largely been ignored. However, there is clinical evidence that air ions might play an important role in preventing the transmission of Acinetobacter infection. Although the reasons for this are unclear, it is hypothesized that a physical effect may be responsible: the production of air ions may negatively charge items of plastic medical equipment so that they repel, rather than attract, airborne bacteria. By negatively charging both particles in the air and items of plastic equipment, the ionizers minimize electrostatic deposition on these items. In so doing they may help to interrupt the transmission of Acinetobacter infection in certain healthcare settings such as intensive care units.MethodsA study was undertaken in a mechanically ventilated room under ambient conditions to accurately measure changes in surface potential exhibited by items of plastic medical equipment in the presence of negative air ions. Plastic items were suspended on nylon threads, either in free space or in contact with a table surface, and exposed to negative ions produced by an air ionizer. The charge build-up on the specimens was measured using an electric field mill while the ion concentration in the room air was recorded using a portable ion counter.ResultsThe results of the study demonstrated that common items of equipment such as ventilator tubes rapidly developed a large negative charge (i.e. generally >-100V) in the presence of a negative air ionizer. While most items of equipment tested behaved in a similar manner to this, one item, a box from a urological collection and monitoring system (the only item made from styrene acrylonitrile), did however develop a positive charge in the presence of the ionizer.ConclusionThe findings of the study suggest that the action of negative air ionizers significantly alters the electrostatic landscape of the clinical environment, and that this has the potential to cause any Acinetobacter-bearing particles in the air to be strongly repelled from some plastic surfaces and attracted to others. In so doing, this may prevent critical items of equipment from becoming contaminated with the bacterium.

Appraising healthcare ventilation design from combined infection control and energy perspectives

This article considers an approach for assessing the balance between energy use and infection control in hospital ward ventilation by combining a stochastic disease outbreak model with a cost evaluation. Disease dynamics are simulated using a susceptible-exposed-infector-removed infection modeling approach, with the contact rate due to airborne transmission incorporated through coupling with the Wells-Riley model. Results presented for a hypothetical ward scenario demonstrate that stochastic effects in a small population, such as a hospital, are a controlling factor in the risk of an outbreak and that conventional deterministic models may give misleading results. Cost appraisals clearly show that the trade-off between ventilation provision and infection risk depends on many factors, including disease characteristics, the people concerned, ventilation system design, and the rate and costs of both providing ventilation and treating infections. Although limitations in the input data currently reduce the robustness of the outputs, the approach is shown to be a useful framework for a tool that can quantitatively assess ventilation design from different perspectives for healthcare environments. This article also highlights some of the knowledge required from further research to enable better quantification of the behavior of pathogens and the transmission processes for hospital infections.

Computational fluid dynamics analysis to assess performance variability of in-duct UV-C systems

UV-C is becoming a mainstream air sterilization technology and is marketed in the form of energy-saving and infection-reduction devices. An accurate rating of device performance is essential to ensure appropriate microbial reduction yet avoid wastage of energy due to over performance. This article demonstrates the potential benefits from using computational fluid dynamics to assess performance. A computational fluid dynamics model was developed using discrete ordinate irradiation modeling and Lagrangian particle tracking to model airborne microorganisms. The study calculates the UV dose received by airborne particles in an in-duct UV system based on published EPA experimental tests for single-, four-, and eight-lamp devices. Whereas the EPA tests back calculated UV dose from measured microorganism inactivation data, the computational fluid dynamics model directly computes UV dose, then determines inactivation of microorganisms. Microorganism inactivation values compared well between the computational fluid dynamics model and the EPA tests, but differences between UV dosages were found due to uncertainty in microorganism UV susceptibility data. The study highlighted the need for careful consideration of test microorganisms and a reliable dataset of UV susceptibility values in air to assess performance. Evaluation of the dose distribution demonstrated the importance of creating an even UV field to minimize the risk of ineffective sterilization of some particles while not delivering excessive energy to others.