Emergence of Clostridium difficile-associated disease in North America and Europe

Authors

  • E. J. Kuijper,

    1. Leiden University, Leiden, The Netherlands
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  • B. Coignard,

    1. Institut de Veille Sanitaire, Saint-Maurice, France
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  • P. Tüll,

    1. ECDC, Stockholm, Sweden
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  • the ESCMID Study Group for Clostridium difficile (ESGCD),

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    • *

      E. J. Kuijper, Leiden University, Leiden, The Netherlands; I. Poxton, University of Edinburgh, Edinburgh, UK; J. Brazier, University Hospital of Wales, Cardiff, UK; B. Duerden, Department of Health, London, UK; M. Delmée, Université Catholique de Louvain, Bruxelles, Belgium; P. Mastrantonio, Istituto Superiore di Sanita, Rome, Italy; P. Gastmeier, Institute for Medical Microbiology and Hospital Epidemiology, Hannover, Germany; F. Barbut, Hôpital-Saint-Antoine, Paris, France; M. Rupnik, University of Maribor, Maribor, Slovenia; B. Coignard, Institut de Veille Sanitaire, Saint-Maurice, France; C. Suetens, Scientific Institute of Public Health, Brussels, Belgium; M. Baldari, ECDC, Stockholm, Sweden; P. Tüll, ECDC, Stockholm, Sweden; A. Collignon, Université Paris XI, Paris, France.

  • EU Member States and the European Centre for Disease Prevention and Control (ECDC)

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    • C. McDonald, CDC, Atlanta, Georgia, USA; D. N. Gerding, Hines Veterans Affairs Hospital, Hines; I. Tjallie van der Kooi, Centre for Infectious Disease Control, National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands; S. van den Hof, Centre for Infectious Disease Control, National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands; D. W. Notermans, Centre for Infectious Disease Control, National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands; A. Pearson, Health Protection Agency, Centre for Infection, London, UK; E. Nagy, Anaerobe Reference Laboratory of Hungary, Szeged, Hungary; A. Colville, Royal Devon and Exeter Hospital NHS Foundation Trust, Exeter, UK; M. Wilcox, University of Leeds, Leeds, UK; P. Borriello, HPA, Centre for Infection, London, UK; H. Pituch, Medical University of Warsaw, Warsaw, Poland; N. Minton, University of Nottingham, UK.


Corresponding author and reprint requests: E. J. Kuijper, Reference Laboratory for Clostridium difficile at Leiden University Medical Centre and the Centre for Infectious Disease Control, National Institute for Public Health and the Environment (RIVM), PO Box 9600, 2300 RC Leiden, The Netherlands
E-mail: ejkuijper@gmail.com

Abstract

The clinical spectrum of Clostridium difficile-associated disease (CDAD) ranges from diarrhoea to severe life-threatening pseudomembranous colitis. Although not always associated with previous antibiotic exposure, it is in the majority of cases. CDAD is recognised increasingly in a variety of animal species and in individuals previously not considered to be predisposed. C. difficile can be transmitted via personal contact or environmentally. The role of patients and healthcare workers who are symptom-free but colonised with C. difficile in the intestinal tract is unclear. C. difficile, with more than 150 PCR ribotypes and 24 toxinotypes, has a pathogenicity locus (PaLoc) with genes encoding enterotoxin A (tcdA) and cytotoxin B (tcdB). Genes for the binary toxin are located outside the PaLoc, but the role of this toxin is unclear. The recently completed genome sequence of C. difficile 630 revealed a large proportion of 11% of mobile genetic elements, mainly in the form of conjugative transposons. Diagnostic assays include tests for the detection of C. difficile products or genes and culture methods for isolation of a toxin-producing bacterium. Enzyme immunoassays to detect toxin in faeces are widely available, with varying sensitivities and specificities. Despite practical drawbacks and sensitivity less than 100%, the cell cytototoxicity assay is still considered to be the standard. Rapid diagnostic assays are available on a limited scale and require much improvement. Molecular tests enable the detection of carriers of toxigenic and non-toxigenic strains, as does culture. It is highly recommended to culture C. difficile from toxin-positive faeces samples and to store isolates for future characterisation and typing. The financial impact of CDAD on the healthcare system is substantial (€5–15 000/case in England and $1.1 billion/year in the USA). Assuming a European Union population of 457 million, the potential cost of CDAD can be estimated to be €3000 million/year, and is expected to almost double over the next four decades. In North America, increasing rates of CDAD have been reported in Canada and the USA since March 2003, involving a more severe course, higher mortality, increased risk of relapse and more complications. This increased virulence is presumably associated with higher levels of toxin production by fluoroquinolone-resistant strains belonging to PCR ribotype 027, pulsed-field gel electrophoresis (PFGE) type NAP1, REA (restriction endonuclease analysis) type BI and toxinotype III. In Europe, outbreaks of CDAD due to the new, highly virulent strain of C. difficile PCR ribotype 027, toxinotype III have been recognised in 75 hospitals in England, 16 hospitals in The Netherlands, 13 healthcare facilities in Belgium and nine healthcare facilities in France. These outbreaks are very difficult to control, and preliminary results from case-control studies indicate a correlation with fluoroquinolones and cephalosporins. Information concerning community-acquired cases of ribotype 027 is lacking, and data concerning its incidence in nursing homes are limited. European countries should first develop early-warning and response capabilities at a national level. Depending on the nature of the notifications received, countries should implement laboratory-based or patient-based surveillance systems in specific, targeted populations.

Introduction

The European Centre for Disease Prevention and Control (ECDC) was established in 2005 by the European Parliament and Council, with a mission ‘to identify, assess and communicate current and emerging threats to human health from communicable diseases’. The ECDC works through networks established among experts within the member states of the EU. Since 2003, outbreaks of severe nosocomial diarrhoea, caused by the new hypervirulent Clostridium difficile PCR ribotype 027 and toxinotype III, have been recognisd in Canada and the USA. Soon thereafter, three European member states reported hospital outbreaks due to an identical C. difficile strain, which spread rapidly and involved more than 100 hospitals in total. At the initial stage, the strain did not seem to have spread to other member states in Europe, but recently a fourth member state also reported an outbreak. The ECDC subsequently organised meetings with experts in the field of C. difficile infections, including experts from the US CDC, in order to achieve a consensus concerning this new emerging pathogen and to discuss methods for preparedness in all European member states. The current review represents a summary of these findings and proposals to date.

C. Difficile-associated disease, an increasing healthcare threat

Clinical spectrum

C. difficile is an anaerobic bacterium, widely distributed in soil and in the intestinal tracts of animals. Its vegetative cells are capable of forming spores, which confer resistance to heating, drying and chemical agents, including disinfectants. C. difficile was identified as the cause of pseudomembranous colitis and its milder form, C. difficile-associated diarrhoea, in the 1970s. The spectrum of disease ranges from asymptomatic carriage to a fulminant, relapsing and potentially fatal colitis [1]. The disease is mediated by the production of toxins of C. difficile, but there is no correlation between the severity of the disease and faecal toxin levels [2]. The rate of fatality associated with C. difficile-associated disease (CDAD) ranges from 6% to 30% when pseudomembranous colitis is present, and is substantial even in the absence of colitis [1,3]. C. difficile also appears to be an important cause of enteric disease in a variety of animal species, including horses, dogs, cats, birds, rodents, and especially neonatal pigs [4], suggesting that animals may serve as a reservoir for human pathogens.

The typical manifestations of CDAD are abdominal cramps, profuse diarrhoea (mucoid, greenish, foul-smelling and watery stools), low-grade fever and leukocytosis [1], which may manifest several days after antibiotic therapy is initiated, or up to 8–10 weeks after its discontinuation [5]. Although colitis can occur throughout the colon, it is usually more severe in the distal colon and rectum. However, when patients develop colitis of the caecum and right side of the colon, they may experience little or no diarrhoea. The clinical presentation in this case involves fever, marked right-sided abdominal pain, marked leukocytosis and decreased intestinal motility.

Traditionally, CDAD has been considered to be an antibiotic-associated nosocomial infection, but the role of antibiotics as predisposing factors for CDAD can be overestimated [6,7]. Patients can develop CDAD following numerous conditions that affect the colonic flora, also outside the hospital setting. The CDC have reported an increase in severe community-acquired CDAD in populations previously considered to be at low risk in Philadelphia and four surrounding counties [6]. However, this observation must be evaluated further before recommendations for testing of community-acquired CDAD are proposed. Interestingly, of the 33 patients who developed CDAD, eight (24%) reported no exposure to antimicrobial agents within 3 months prior to the onset of CDAD. The minimum annual incidence of community-associated disease was estimated to be 7.6 cases per 100 000 population or one case per 5000 prescriptions of antimicrobial agents for outpatients. A recent report from the UK indicated that the incidence of C. difficile in patients diagnosed by their general practitioners had increased from fewer than one case per 100 000 in 1994 to 22 cases per 100 000 in 2004, but these preliminary data require verification in this patient population, with appropriate diagnostic tests for CDAD and collection of clinical data [7]. Similar to experience elsewhere [6], although antibiotic use was the most important drug-related risk-factor for CDAD, only 37% of the cases had received an antibiotic in the 90 days prior to diagnosis [7].

Epidemiological characteristics

C. difficile can be cultured from the stool of 3% of healthy adults and up to 80% of healthy newborns and infants [1,8]. The assumption that C. difficile is not pathogenic for neonates and children, based mainly on anecdotal evidence, should be reconsidered. Stool carriage of C. difficile reaches 16–35% among hospital inpatients, with the percentage being proportional to the duration of hospital stay and increasing with exposure to antibiotics [3]. C. difficile persists in the stools of 10–40% of patients with CDAD, irrespective of the antibiotic used for treatment [9]. Contaminated environmental surfaces, other patients with CDAD and hand carriage on the part of healthcare personnel are considered to be potentially important means for C. difficile transmission in hospitals [10]. Patients with CDAD are cared for separately to minimise the spread of C. difficile. Diarrhoea can often be unexpected and explosive, and results in increased shedding of C. difficile spores. Consequently, spores have been found in far greater quantities in the environment of CDAD patients in comparison with that of patients who are not infected with C. difficile[9,10]. C. difficile spores are highly resistant to many commonly used disinfectants and may persist for months in hospital environments [10]. It is not known whether healthcare workers and patients with symptom-free colonisation by C. difficile in the intestinal tract spread the bacterium. The frequency of C. difficile-positive hand culture of healthcare personnel has been shown to correlate significantly with the intensity of environmental contamination [11]. Clearly, good hand hygiene practice with soap and water (rather than alcohol hand gels) is essential in reducing the incidence of hand carriage. The true significance of the environment as a potential reservoir for C. difficile and its role in subsequent patient infection remains unclear, primarily because it has been difficult to determine whether environmental contamination is a cause or a consequence of diarrhoea. Environmental contamination involves mainly floors, including toilet floors, commodes and bed frames [12,13]. The bed frame was the most common site from which C. difficile was recovered, while the floor was the site most contaminated in terms of total numbers of colonies [12].

There is a general lack of evidence concerning the use of detergents or disinfectants for routine cleaning of patient areas. Routine cleaning with detergent is often unsuccessful in eliminating C. difficile from the environment [13]. For example, Kaatz et al. recovered C. difficile from 31% of ward environmental samples [14]. There is evidence that contamination with C. difficile may persist after environmental cleaning with hypochlorite; disinfection with unbuffered hypochlorite (500 p.p.m. available chlorine, too low to kill spores reliably) resulted in a 21% decrease in surface contamination levels, and this coincided with the resolution of an outbreak of CDAD. Phosphate-buffered hypochlorite (1600 p.p.m. available chlorine, pH 7.6) was found to be more effective in reducing environmental C. difficile levels (98% reduction in surface contamination). The results of another study found that unbuffered 1:10 hypochlorite solution (5000 p.p.m.) was effective in decreasing patients' risk of developing CDAD [15]. Furthermore, in a crossover study involving two wards for elderly patients, hypochlorite-based cleaning was associated with a reduction in both C. difficile environmental prevalence and the incidence of C. difficile infection on one of the two wards [16]. However, long-term environmental use of hypochlorite is questionable, given its corrosive nature. The lack of sporicidal, but environmentally friendly, disinfectants is a problem. Preliminary findings of a prospective study on the effect of hydrogen peroxide vapour (Bioquell, Andover, UK) demonstrated efficacy in the eradication of C. difficile in the environment in four intensive care wards (J. C. Boyce et al., ICAAC 2005). However, use of hydrogen peroxide vapour is expensive, and rooms must be vacated and sealed before use, limiting its practicality.

Pathogenicity

Pathogenic C. difficile organisms release two potent toxins that ultimately mediate diarrhoea and colitis [1]. These large exotoxins, toxin A (TcdA), a 308-kDa enterotoxin, and toxin B (TcdB), a 270-kDa cytotoxin, exhibit an overall homology of approximately 63% at the amino acid level [17] (Fig. 1). Most enteropathogenic strains produce both toxins simultaneously. It is suggested that TcdA and TcdB work synergistically, based on the fact that a TcdB effect is dependent on tissue damage brought about by TcdA. TcdA has been regarded as the most important factor in diarrhoeal disease, but an increasing number of reports show disease caused by TcdA-negative strains, thereby implying a more important and TcdA-independent role of TcdB in pathogenesis. In recent years, epidemics due to TcdA-negative strains have been described [18,19]. Additionally, a binary toxin of C. difficile is currently being studied as a possible new virulence marker [20–22]. This binary toxin, an actin-specific ADP-ribosyltransferase, can be present in up to 10% of C. difficile strains, but its prevalence is influenced by the selection of strains [20–22]. The binary toxin is encoded by the cdtA gene (the enzymic component) and the cdtB gene (the binding component) [20,21]. The extent to which this toxin contributes to the pathogenicity of C. difficile is unknown; however, the C. difficile strain in which the binary toxin was first detected caused severe pseudomembranous colitis. Recently, Geric et al. reported that TcdA- and TcdB-negative, but binary toxin-positive, C. difficile strains caused fluid accumulation in the rabbit ileal loop assay but did not lead to diarrhoea or death in the widely used hamster model [23]. The effect of binary toxins on the intestinal tract of other animals, e.g., horses and young calves, is unknown.

Figure 1.

 Schematic presentation of three toxins produced by Clostridium difficile and their coding regions [73]. (a) Toxin A (TcdA) and toxin B (TcdB) are encoded on the large chromosomal region PaLoc, which encompasses two toxin genes (tcdA and tcdB) and three additional genes coding for regulatory and putative transport functions (tcdR,E,C). In non-toxigenic strains, PaLoc is replaced by a 115-bp sequence. On a protein level, TcdA and TcdB sequences are 63% homologous, and both are large, single-chain proteins with three functional regions involved in toxic effects, translocation and cell binding. (b) Binary toxin CDT is coded by a separate region with two genes (cdtA and cdtB). On a protein level, binary toxin is composed of two protein chains that are not linked but are both involved in toxic effects.

The complete genome sequence of the multidrug-resistant and virulent C. difficile strain 630 was published recently [24]. The genome of 630 consists of a circular chromosome of 4 290 252 bp and a plasmid of 7881 bp. The chromosome encodes 3776 predicted coding sequences. The plasmid encodes 11 predicted coding sequences, but without obvious function. A large proportion of the genome (11%) consists of mobile elements, putatively responsible for antimicrobial resistance, virulence, host interactions and surface structures. Interestingly, genes involved in spore germination (GerA family of germinant receptors) in other clostridia and Bacillus species were not found in C. difficile, suggesting that the germination process in C. difficile is rather unique.

Diagnostics

The diagnosis of CDAD requires the detection of C. difficile toxins or toxin-producing C. difficile in a diarrhoeal stool specimen. Diagnostic assays can be divided into tests for the detection of C. difficile products (e.g., glutamate dehydrogenase, volatile fatty acids, toxins), tests for the detection of C. difficile genes (16S rRNA, toxin genes) and culture methods for the isolation of a toxin-producing bacterium [25]. A European survey of diagnostic methods for C. difficile revealed marked discrepancies among laboratories and among countries in the methods and strategies for the diagnosis of C. difficile[26]. Culture of toxigenic C. difficile requires at least 4 days before results are available and has therefore no immediate diagnostic value. As non-toxigenic strains exist, cultured C. difficile must be tested for toxin production. Cultured isolates are necessary for typing, and culturing C. difficile from faecal samples is easy to implement in the routine setting of diagnostic laboratories. Results of antimicrobial susceptibility testing of the isolates are indicative for certain PCR ribotypes, such as 027. It is therefore highly recommended to culture C. difficile from toxin-positive faecal samples. Isolates should be stored in the local laboratory for future characterisation and typing studies.

Toxins of C. difficile can be detected either by virtue of their biological properties (cell cytotoxicity assay) or by immunological methods (latex agglutination, immunoassay). The cell cytotoxicity test remains the standard by which other tests are measured [1,16,25,27–30] but suffers from some drawbacks. The laboratory requires a supply of cultured cell monolayers, and the results are known to vary according to the cell line, dilution factors, reagents used and storage conditions. Additionally, the turnaround time is very slow, typically 24 h to demonstrate cytotoxicity and a further 24 h to neutralise this cytotoxicity. Enzyme immunoassays (EIAs) are easier to perform and provide rapid results. Two types of immunoassay have been developed: conventional EIAs and a membrane immunochromatography test. Numerous publications have compared the performance of different kits for EIAs, but no meta-analysis has been performed in an attempt to demonstrate the superiority of a particular test. The National C. difficile Standard Group in the UK recommend the use of EIAs that detect both TcdA and TcdB, because of an increasing awareness of TcdA-negative/TcdB-positive strains [18,19,28]. A second generation of tests has been put on the market recently. The principal advantage of the membrane immunochromatography assay is speed, since results can usually be obtained within 15–30 min. Another advantage is the simplicity of the assay, which does not require a high level of technical skills. Recently, a new rapid immunoassay (Immunocard toxins A and B, Meridian Bioscience Europe, Boxtel, The Netherlands) has been compared with an in-house real-time PCR in a prospective multicentre study using the cytotoxicity test as the standard [29]. It was concluded that the new rapid immunoassay is a quick and easy-to-perform test for the diagnosis of CDAD, but that the performance could be improved. Some unpublished data suggest that the presence of blood in the stools may result in a false-positive result. The detection of C. difficile gene sequences in stool samples has focused on 16S rRNA and toxin genes [31– 33]. An important disadvantage of the 16S rRNA approach is that non-toxigenic, as well as toxigenic, strains are detected. Therefore, more attention was given to the tcdA and tcdB genes of C. difficile and a successful approach was published in 1993 [32, 33]. Real-time PCR results can be obtained within a working day, which is shorter than the time required for the cytotoxicity assay [29,31]. Also, the hands-on time is markedly less than that required for other methods, negating the need for post-PCR analysis. Other advantages of real-time PCR are the low risk of carryover contamination and the fact that it will also detect asymptomatic carriers.

Impact on healthcare

CDAD is primarily a potentially severe nosocomial infectious disease that can be prevented by robust infection control practice. CDAD is currently the most frequently occurring nosocomial infection in some European hospitals, e.g., in the UK, and has the potential to become the most frequent in others if appropriate surveillance, prevention and control measures are not implemented. In the UK, for instance, twice as many deaths were attributed to C. difficile in 2003 as were attributed to methicillin-ressitant Staphylococcus aureus. An increase in death-associated CDAD has also been reported recently. The number of cases in which C. difficile either contributed to the death or was the underlying cause of death was 2.3 times higher in 2004 than in 1999 [34]. The organism is resistant to various antibiotics, and capitalises on the ensuing disruption of the normal intestinal flora to colonise and cause disease. Several factors have contributed to the worrying escalation in the incidence of CDAD. The elderly and immunocompromised are particularly at risk, with 80% of cases occurring in individuals over 65 years of age. The proportion of the population in these high-risk groups is increasing with the general ageing of the population in Europe. Publications from Spain over the past several years that have reported clones apparently resistant to metronidazole or vancomycin are of concern [35,36], and very recently, metronidazole resistance has also been observed in Israel [37].

The impact of CDAD in healthcare settings is considerable. Patients require isolation, revised supportive therapy for underlying disease as well as for CDAD, specific therapy to eliminate C. difficile, scrupulous hygiene in patient care, environmental decontamination, and, in the case of outbreaks, cohort isolation and ward closure. Reports indicate that such patients spend from 1 to 3 additional weeks in hospital. In terms of cost, this translates into €5–15 000 per case in England and $1.1 billion per year in the USA [38–41]. Taking the latest incidence figures from the mandatory surveillance programme in England (44 488 in 2004) and allowing for 3% annual inflation since 1996, the cost of management of CDAD in England can be estimated at €340 million per annum. Assuming a population for the EU of 457 million, the potential cost of CDAD can be estimated at €3000 million per annum. Moreover, as CDAD is a disease occurring predominantly among the elderly, these costs are expected to rise as the proportion of the European population over 65 years of age increases. The latest figures from Eurostat (the statistical bureau for the EU) indicate that the proportion of individuals over 65 years of age is expected to almost double over the next four decades, from 75.3 million in 2004 to 134.5 million in 2050.

Emergence of C. difficile PCR ribotype 027, pulsed-field gel electrophoresis (PFGE) type NAP1, REA type BI, toxinotype III in Canada and the USA

The rate and severity of CDAD is increasing in both the USA and Canada, and may be associated with a new strain of C. difficile marked by increased virulence and/or resistance. Since March 2003, outbreaks of severe cases of CDAD have been reported in hospitals in Montreal and southern Quebec, with an increased risk of relapses [42–44]. In 2004, institutions in the region of Quebec experienced a sharp rise in the incidence of CDAD, involving more than 14 000 patients [39,44,45]. In January 2005, 30 hospitals in Quebec reported rates of nosocomial CDAD five-fold greater than the historical average. A new, more virulent variant (ribotype 027) strain was associated with this increase, which prompted the government of Quebec to reserve €13.7 million to combat CDAD in hospitals and nursing homes [46]. Guidelines for surveillance studies and for prevention and control of CDAD were also formulated [47,48]. A study conducted in 2004 at 12 hospitals in Quebec included a total of 1703 patients with 1719 episodes of nosocomial CDAD [49]. The estimated incidence was 22.5 cases per 1000 admissions, with a 30-day attributable mortality rate of 6.9%. Both incidence and mortality increased with increasing age. Compared with matched controls, patients with CDAD seem more likely to have received fluoroquinolones (OR 3.9; 95% CI 2.3–6.6) or cephalosporins (OR 3.8; 95% CI 2.2–6.6). However, in this study, patients with CDAD received antibiotics more frequently and 46% more antibiotics per case (1.9 vs. 1.3) than did controls (both p < 0.0001), and duration of antimicrobial treatment was not examined [50]. The most common strain, which was resistant to fluoroquinolones, was identified in 82.2% (129/157) of patients. In addition, 84.1% (132/157) of isolates had binary toxin genes as well as partial deletions of the tcdC gene [49]. As of June 2006, the strain has spread to seven provinces in Canada, with the highest incidence in Quebec at 13 per 1000 admissions. Quebec also has the highest case fatality rate at 7.9%[51].

The CDC reported a steady increase in the incidence of CDAD, from 2.7 per 10 000 hospital admissions in 1987 to 4.2 in 2001, and have subsequently proposed standardisation of definitions and notification of CDAD [52,53]. The number of US hospital discharges for which CDAD was listed among the diagnoses doubled between 1996 and 2003 [53]. A study of isolates from the USA revealed that a previously uncommon strain of C. difficile, resistant to fluoroquinolones and also manifesting genetic variation, was responsible for geographically dispersed outbreaks [54,55]. The US study collected 187 C. difficile isolates from eight healthcare facilities in six states in which CDAD outbreaks had occurred between 2000 and 2003 [54]. At least half of the specimens from five of the eight facilities belonged to REA group BI and PFGE type NAP1. BI/NAP1 isolates were positive for the CDT binary toxin, had a deletion in the tcdC locus and produced higher amounts of TcdA and TcdB [56]. Resistance to moxifloxacin and gatifloxacin was more common among BI/NAP1 C. difficile isolates than among other types (100% vs. 42%, p < 0.001). None of the historic BI/NAP1 isolates was resistant to moxifloxacin and gatifloxacin. The CDC also reported the strain to be associated with high rates of morbidity and mortality during outbreaks in hospitals in at least 11 states [54] (http://www.cdc.gov/ncidod/dhqp/index.html). Only two isolates were recovered from the patients involved in the CDC report of an increase in severe community-acquired CDAD in populations previously considered to be at low risk in Philadelphia and four surrounding counties [6] and, although toxin variant, these two isolates were not BI/NAP1. The minimum annual incidence of community-associated disease was estimated as 7.6 cases per 100 000 population or one case per 5000 outpatient antimicrobial prescriptions.

Emergence of C. difficile PCR ribotype 027, toxinotype III in Europe (UK, Belgium, France and the Netherlands)

The Communicable Diseases Surveillance Centre (CDSC) for England and Wales noticed that the number of CDAD notifications had risen from 1000 in 1990 to 15 000 in 2000 and 35 500 in 2003, with PCR ribotype 027 being very rare. A recent health statistic report on the deaths involving C. difficile in England and Wales also revealed an increase from 975 in 1999 to 2247 in 2004 [34]. This report examined trends in those deaths that involved C. difficile as a contributory factor using the specific ICD-10 code A04.7. Among deaths that occurred in National Health Service (NHS) general hospitals and nursing homes, CDAD accounted for up to 0.52% and 0.45%, respectively. Most of the deaths involved individuals aged 65 years or older, and usually those who were already very ill as a consequence of underlying disease. In the period between October 2003 and June 2004, the PCR ribotype 027 strain was recognised in the UK at the Stoke Mandeville Hospital (Buckinghamshire Hospitals NHS Trust) in an outbreak involving 174 cases and 19 (11%) deaths that were definitely or probably due to C. difficile. A second outbreak occurred between October 2004 and June 2005 in Stoke Mandeville hospital involving 160 new cases and 19 (12%) further deaths [57,58]. The Healthcare Commission investigation concluded that the outbreaks were a consequence of a poor environment for patient care, poor practice in the control of infection, lack of facilities to isolate patients and insufficient priority being given to the control of infection by senior managers (report available at http://www.healthcarecommission.org.uk/). Shortly thereafter, the Royal Devon and Exeter Hospital submitted 18 representative isolates of C. difficile with a history of an outbreak that coincided with a change of antibiotic regimen to moxifloxacin for the treatment of patients with community-acquired pneumonia; almost all were type 027. As of April 2006, 450 isolates of type 027 had been referred to the Anaerobe Reference Laboratory in Cardiff from 75 hospitals. Some were from clinically recognised outbreaks, and others were routinely submitted as part of the mandatory surveillance programme for CDAD in England.

In July 2005, the medical microbiological laboratory at the Leiden University Medical Centre was requested to type C. difficile strains from an outbreak in a hospital in Harderwijk [59–61]. The incidence of CDAD in the hospital had increased from four cases per 10 000 patient admissions in 2004 to 83 per 10 000 in the months April–July 2005. Cultured isolates were subsequently identified as toxinotype III and PCR ribotype 027. Measures taken by the hospital included isolation of all patients with diarrhoea, cohorting of all C. difficile-infected patients on a separate ward, banning of all fluoroquinolone use and limitation on the use of cephalosporins and clindamycin. In January 2006, the situation appeared to be under control, as the number of positive patients per month had decreased. CDAD in all nine patients diagnosed between September 2005 and January 2006 was caused by non-027 ribotypes. However, a resurgence of CDAD caused by type 027 was noticed in February 2006, when six new patients were diagnosed. A second outbreak occurred in another hospital 30 km from the first and was probably related to the first outbreak through a transferred patient with CDAD [59,60]. In this second outbreak, 85 CDAD patients were identified by December 2005, 19 (22%) patients died and 16 (19%) relapses were observed. In response to the outbreaks in The Netherlands, the Centre for Infectious Disease Control (CIb) at the National Institute for Public Health and the Environment (RIVM) in Bilthoven organised a meeting with experts in the fields of microbiology, infectious diseases, infection control and epidemiology. The team agreed to make use of those parts of existing national hospital guidelines that were relevant for infection control in the context of CDAD, and to use national and international experience in drawing up specific CDAD guidelines for infection control and treatment, with separate guidelines for hospitals and nursing homes [62]. Furthermore, diagnostic facilities were increased and made accessible to all microbiology laboratories in The Netherlands. Relevant professionals were informed through several communication channels, including the various scientific societies, and plans were made to register and monitor new outbreaks. Subsequently, three hospitals in the western part of the country also reported an increase in the incidence of severe CDAD. A nursing home in the same region was found to harbour patients with CDAD caused by PCR ribotype 027, with evidence of spread within the facility. A cluster of 12 patients with CDAD caused by PCR ribotype 027, toxinotype III was reported in July and August 2005 in a large teaching hospital in Amsterdam. One patient died as a result of the consequences of CDAD and two other patients developed severe complications [63]. Another hospital in Amsterdam also reported an increase in severe cases of CDAD in July 2005 among patients who were cared for in the department of geriatrics. Two hospitals in the centre of The Netherlands did not notice an increase in the incidence of patients with CDAD, but submitted strains to the reference laboratory for typing. Type 027 was found in six of 17 (35%) and one of four (25%) isolates tested, respectively [59,60]. As of April 2006, type 027 was found to be the cause of an outbreak in 11 hospitals and was isolated from sporadic cases in five hospitals [60,64].

In September 2005, the PCR ribotype 027 strain was isolated from four patients with CDAD in Ieper, Belgium [65]. The incidence had increased from 10/10 000 admissions to 33/10 000 admissions in September 2005. Subsequently, the same pattern was identified among strains from three outbreaks that had occurred in Brussels in 2003–2004, involving 17, six and five patients, respectively. Another outbreak took place in Ostende, Belgium, involving four patients. More recently, 14 strains were identified from a severe outbreak in the Hospital of Libramont, Belgium. As of July 2006, type 027 had been identified as the causative agent in outbreaks in 11 hospitals in Belgium, and sporadic isolates of type 027 were found in two other healthcare facilities.

On 27 March 2006, a cluster of CDAD cases was reported to the Institut de Veille Sanitaire (InVS) by a hospital in northern France [66]. Of 33 cases, 16 (48%) occurred in the geriatrics ward and four were diagnosed as pseudomembranous colitis; nine (27%) patients died within 30 days but CDAD was not found to be the primary cause of these deaths. On the geriatrics ward, the incidence of CDAD rose between January and March 2006 from 13 to 116 CDAD cases per 10 000 patient-days. Of 14 strains sent for typing, 11 were characterised as PCR ribotype 027, toxinotype III. The origin of this outbreak remains unknown, although transfers of patients between French hospitals and Belgian nursing homes are frequent and are under investigation [66]. The InVS informed all French regional infection control coordinating centres and healthcare facilities and disseminated recommendations for reporting, investigation, surveillance and control of CDAD. As of August 2006, the InVS and regional infection control coordinating centres had investigated 15 clusters of CDAD, encompassing 222 patients. Nine episodes (194 patients) were due to type 027, all in Northern France; among these patients, 12 (6%) deaths were attributable to CDAD; updates about this outbreak will be posted on the InVS web site: http://www.invs.sante.fr/raisin/

Characteristics of C. difficile PCR ribotype 027, PFGE type NAP1, REA type BI and toxinotype III

C. difficile can be divided into more than 150 ribotypes and 24 toxinotypes [55,67] (http://www.mf.uni-mb.si/mikro/tox). Toxinotyping involves detection of polymorphisms in the tcdA and tcdB and surrounding regulatory genes, an area of the genome known collectively as the pathogenicity locus or PaLoc [68]. PCR ribotyping is based on differences in profiles generated by PCR amplification of the intergenic spacer regions between the 23S and 16S rRNA genes [67,69].

The epidemic strain isolated in Canada, the USA, the UK, Belgium, France and The Netherlands was characterised as PCR ribotype 027 and toxinotype III. Strains from North America were further characterised as PFGE type 1 and restriction endonuclease analysis group type BI. Some strains from Europe have also been typed as PFGE type 1 and REA type BI, and it is very likely that all PCR ribotype 027 and toxinotype III strains belong to this REA and PFGE type. The strain carried the binary toxin gene cdtB and had an 18-bp deletion in tcdC. In the PCR ribotype 027 strains isolated in Canada, an additional single-base-pair deletion was detected in the tcdC sequence at position 117 [70].

Strains of toxinotype III (belonging to ribotypes 027, 034, 075, 080) have been found sporadically and represent 2–3% of isolates from large collections [55,71,72]. Strains belonging to toxinotype III produce binary toxin in vitro, but the importance of binary toxin CDT as a virulence factor in C. difficile has not been established [21]. The binary toxin, an actin specific ADP-ribosyltransferase, is encoded by the cdtA gene (the enzymic component) and the cdtB gene (the binding component), which are not located within the pathogenicity locus [20,21]. Non-pathogenic strains containing cdtA and cdtB, but lacking the pathogenicity locus, are also capable of producing binary toxin. The binary toxin is present in up to 10% of all C. difficile isolates, and is mostly present in variant toxinotypes [20–22].

The application of restriction analysis as a typing technique for C. difficile was reported in 1987 and subsequently standardised in 1993 [73,74]. The REA patterns are visually compared to the restriction patterns of the reference REA types. At least 100 groups have been recognised by REA, and the eight most common toxigenic REA groups are Y, B, G, L, E, J, R, N and BD. REA group type BI, which was first identified in 1984, was uncommon (n = 18) among isolates from the historic database (1984–1992) of 6000 isolates [54].

PCR ribotype 027 was first assigned in 1988 from a culture collection of M. Popoff (Paris, France) and originated from a 28-year-old woman with severe pseudomembranous colitis. Until March 2004, it was considered to be an unimportant and very rare PCR ribotype. The clonality of PCR 027 is currently a topic of research. PCR ribotype 027 exhibits two PFGE patterns with 94% similarity [54]: North American PFGE types 1a and 1b (NAP1a and NAP1b). As demonstrated for PCR ribotype 001, other typing techniques (DNA fingerprinting, REA, AP-PCR) may reveal additional subgroups. Preliminary data from D. Gerding (Chicago, USA) indicate that REA is able to discriminate further at least 24 subtypes of BI within PCR ribotype 027. Currently, various typing techniques are being applied to a large collection of strains from Canada, the UK, the USA and The Netherlands.

The importance of the 18-bp deletion in tcdC of the PCR ribotype 027, toxinotype III strains is also unknown. The tcdC gene is considered to be a negative regulator of the production of TcdA and TcdB, but it is not known whether this 18-bp deletion results in a non-functional product. A recent report indicates that toxinotype III isolates produce TcdA and TcdB in considerably greater quantities in vitro than do toxinotype 0 isolates [56]. The significance of these in-vitro findings to the understanding of in-vivo virulence remains uncertain, since an association between faecal toxin levels and clinical severity of disease has not been established [2]. Also, deletions in tcdC are frequently present in toxigenic isolates. Of 32 toxigenic strains studied in 2002, eight belonged to toxinotypes 0, V and VI and contained deletions in tcdC of 18 bp or 39 bp, without an association with clinical severity of disease [75]. The recently discovered 117 deletion in tcdC represents a frameshift and premature stop in the early portion of the gene, and the downstream effects on the functional capacity of any resulting tcdC transcripts would constitute a major disruption of tcdC function [70].

Little information is available concerning the sporulation characteristics and spread of NAP/027 into the environment, in comparison with other types. It has been reported that sporulation levels of outbreak type 027 strains and outbreak type 001 exceed those of other strains (LB-28-2005: S. Underwood et al., ICAAC 2005). The highest sporulation levels were achieved when strains were exposed to non-chlorine-based cleaning agents, suggesting that the use of such cleaning agents in hospitals may enhance the spread of CDAD.

Implications for Europe

Discrepancy among methods and strategies for diagnosing CDAD

A European surveillance study of diagnostic methods and testing protocols for C. difficile among 212 hospitals in eight countries in 2002 revealed marked differences among laboratories concerning the methods and the strategies used for diagnosing CDAD [26]. In 58% of cases, laboratories undertook investigations for CDAD only when specifically requested by the physician, and only 55% of the laboratories were capable of culturing for C. difficile. These results are also in agreement with a survey performed in the UK by the Healthcare Commission and Health Protection Agency among 118 NHS Trusts in 2005, which was conducted in order to make best use of the reported incidence of CDAD, the reported approaches to prevention, management and control of outbreaks, and the views of professionals concerning prevention. Also, the survey was undertaken to obtain information regarding diagnostic testing and processing of samples from suspected cases of CDAD. In 2004, guidelines and recommendations were published by the National Clostridium difficile Standards Group and mandatory surveillance was introduced in 2004, according to which infections in patients aged 65 years and older were required to be reported [28,76]. All laboratories were using a recommended test for diagnosing CDAD, but only 25% performed C. difficile culture. There was considerable variation in the use of culture and typing among different laboratories. As part of the mandatory surveillance programme, as of January 2005 all laboratories were required to submit isolates of C. difficile for typing in a structured programme. During 1 week each year, in a consecutive programme, a particular laboratory is asked to send toxin-positive stool samples (up to a maximum of ten) to the Regional Laboratory of the Health Protection Agency. Culture of the samples is performed at the Regional Laboratory and the isolates of C. difficile are sent to the Anaerobe Reference Laboratory (Cardiff, UK) for typing.

In The Netherlands, a survey conducted among 12 laboratories also revealed marked discrepancies in the methods and the strategies for diagnostic testing. During a 3-month pilot study, using an optimal test algorithm at four university laboratories, a nearly 20% increase in the number of CDAD patients diagnosed was found (unpublished results). This algorithm enabled the microbiological laboratories to test all faecal specimens of patients hospitalised for more than 3 days who developed diarrhoea, irrespective of a physician's request. This algorithm has now been introduced in 15 laboratories.

Incidence of CDAD in the member states of the EU

Limited information on the incidence of CDAD is available from a European survey involving 212 hospitals that was performed by the European Study Group of C. difficile (ESGCD) in the UK, France, Belgium, Denmark, Germany, Italy and Spain in 2002 [26]. The incidence varied considerably, depending on the testing strategies and the tests applied. The incidence was 11 per 10 000 admissions. In contrast, data from studies in the USA showed that the incidence among hospitalised patients is much higher, ranging from 10 to 200 per 10 000 admissions [77]. A second European surveillance study was performed in 2005.

The reporting of cases in all EU member states takes place on a voluntary basis, except in England, where mandatory reporting was introduced in 2004. The first set of results from the mandatory surveillance scheme revealed 44 488 cases of C. difficile in those over 65 years of age in England during 2004 [76]. Two-thirds of the Trusts reported an increase in CDAD cases during the past 3 years, and 25% have dedicated a ward during the past 12 months for CDAD care. It is very likely that these data underestimate the real incidence of CDAD, since the surveillance scheme was restricted to patients over 65 years of age and to nosocomially acquired CDAD. Information concerning the extent of CDAD in patients in nursing or residential homes, in other healthcare facilities and in individuals under the care of a general practitioner is lacking.

The role of medical microbiologists, infection control practitioners, infectious disease specialists and epidemiologists in combating CDAD

To reduce severe outcomes of CDAD, early diagnosis and initiation of specific antimicrobial treatment are important. Prevention of outbreaks of CDAD in hospitals can only be accomplished by early recognition, adequate isolation measures and prompt treatment. Based on recent experience with epidemics of C. difficile type 027, most microbiological laboratories are currently implementing a new rapid immunoassay test as part of routine diagnostics. Some microbiological laboratories have developed rapid molecular tests (real-time PCR) to diagnose CDAD. It is expected that in the next few years microbiological laboratories will introduce other rapid diagnostic tests. Development of these new tests should lead to assays with a better performance than those currently available. The medical microbiology laboratories have a central role in surveillance of CDAD in hospitals and other healthcare institutions, as well as the community, but testing strategies and algorithms must be well-defined.

The hospital hygiene/infection control department plays a central role in the prevention of nosocomial infections and thus a major role in the prevention of CDAD by ensuring adequate isolation of affected patients, institution of precautions and formulation of practice guidelines.

The hospital hygiene team develops strategies for hand washing, environmental hygiene and outbreak control; however, contamination with C. difficile spores has been demonstrated in 30–60% of sites in hospital wards. Thus, appropriate and adequate cleaning of the hospital environment is an important part of the programme for prevention of CDAD. Infection control is of special importance in an outbreak situation in order to control transmission. Ideally, patients with suspected or proven CDAD will be isolated, and will be cohorted if sufficient isolation facilities are not available.

Medical microbiologists and infectious disease professionals can play a major role in the prevention of CDAD by reducing antibiotic prescriptions; up to 50% of all antibiotic usage in hospitals is inappropriate. Hospital infections caused by antibiotic-resistant bacteria are associated with higher rates of mortality and morbidity and prolonged hospital stay compared with infections caused by antibiotic-susceptible bacteria. An analysis by the Cochrane Institute (19 October 2005; 4:CD003543) showed that interventions to improve and reduce antibiotic prescribing to hospital inpatients are successful and can significantly reduce antimicrobial resistance and the incidence of hospital-acquired infections such as CDAD.

Currently, PCR ribotyping is considered to be the standard method for typing of C. difficile in Europe [67,69]. Other typing methods have also been developed and applied, but standardisation of these methods and exchange of data among laboratories has never been achieved. The results of PCR ribotyping can be stored as TIFF files and, for further analysis, imported into the BioNumerics software (Applied Maths, Kortrijk, Belgium). The Anaerobe Reference Laboratory, University Hospital of Wales, Cardiff, UK holds isolates from all the PCR ribotypes in its database, which will allow future epidemiological investigations at an international level. It is possible to make this database available on a remote access server so that other reference laboratories will be able to identify not just type 027 but other PCR ribotypes. Further characterisation of the strains for the presence of virulence markers, including genes for TcdA and TcdB, genes for the binary toxin, and deletions in a toxin regulator gene (tcdC), should be performed by the reference laboratories. Finally, there is a need for reference laboratories to develop new typing techniques with discriminative capacities better than those currently available.

Antibiotic susceptibility testing of C. difficile is not routinely performed at every microbiological laboratory; however, surveillance of the antibiotic sensitivity of C. difficile is of the utmost importance. The antibiotic of choice for treatment of infections associated with C. difficile is metronidazole, followed by vancomycin as a second choice. Unfortunately, reports from some laboratories mention the occurrence of metronidazole resistance and vancomycin resistance in C. difficile, although the exact mechanism is unknown and confirmation of these findings by reference laboratories is urgently needed [35–37,78].

Conclusions of the working group

Increased awareness of CDAD should be a priority in all European member states

CDAD has a broad clinical spectrum and is not always associated with previous antibiotic use. Although CDAD presents most frequently as a hospital-acquired/nosocomial infection, recent reports suggest an increase in community-acquired CDAD in populations not considered at risk. Therefore, knowledge of CDAD is of importance for all healthcare workers.

Clear methods and strategies for diagnosing CDAD

Guidelines for CDAD diagnostic strategies should be formulated according to regional incidence rates of CDAD and local laboratory capacities. There is an urgent need for rapid diagnostic tests with a better performance than the currently available assays.

Interim recommendations for CDAD case definitions are necessary

For surveillance purposes, interim case definitions are proposed that focus on disease and do not refer to a particular strain. These definitions are based on past experiences in the USA, Canada, the UK and The Netherlands and on current discussions within European and US working groups; they may evolve in the near future.

CDAD case

This is a patient to whom one or more of the following criteria applies:

  • 1diarrhoeal stools or toxic megacolon, and a positive laboratory assay for C. difficile TcdA and/or TcdB in stools or a toxin-producing C. difficile organism detected in stool via culture or other means;
  • 2pseudomembranous colitis revealed by lower gastrointestinal endoscopy;
  • 3colonic histopathology characteristic of C. difficile infection (with or without diarrhoea) on a specimen obtained during endoscopy, colectomy or autopsy.

This definition may be focused on criterion no. 1 in laboratory-based surveillance systems performing tests for C. difficile only on unformed stools (i.e., stools that take the shape of their container). All three criteria can be used in patient-based surveillance systems targeting diarrhoeal symptoms (i.e., at least three liquid or unformed stools for at least 24 h).

This definition excludes diarrhoea with other known aetiology (as diagnosed by the attending physician), and asymptomatic patients with a stool culture positive for toxin-producing C. difficile or an assay positive for C. difficile TcdA and/or TcdB.

Recurrent CDAD case

This is a patient with an episode of CDAD that occurs within 8 weeks following the onset of a previous episode. A recurrence can correspond to a relapse involving the same strain or to a re-infection with a different strain [79–82]. The simultaneous occurrence of multiple PCR ribotypes in faecal samples may also result in isolation of a different strain in a recurrent episode [83]. In clinical practice, it is not possible to differentiate between relapse and re-infection; the term recurrence is therefore used as a designation for both. The risk of complications in the case of a recurrence due to the new emerging strain may be higher than previously thought [43].

Severe CDAD case

This is a CDAD patient to whom any of the following criteria apply:

  • 1admission to a healthcare facility for treatment of community-associated CDAD;
  • 2admission to an intensive care unit for treatment of CDAD or its complication (e.g., for shock requiring vasopressor therapy);
  • 3surgery (colectomy) for toxic megacolon, perforation or refractory colitis;
  • 4death within 30 days after diagnosis if CDAD is either the primary or a contributive cause.

Outbreak of CDAD

An outbreak can be defined as the occurrence of two or more related cases of CDAD over a defined period agreed locally, taking account of the background rate [28].

Origin (Fig. 2)

Figure 2.

 Relationship among epidemiological definitions.

The proposed categories are based on information concerning the origin of CDAD (healthcare-associated or community-associated) and the onset of symptoms (within the context of healthcare or within the community).

Healthcare-associated case

This is a CDAD case patient with onset of symptoms at least 48 h (>48 h) following admission to a healthcare facility (healthcare-onset, healthcare-associated) or with onset of symptoms in the community within 4 weeks following discharge from a healthcare facility (community-onset, healthcare-assocciated).

Community-associated case

This is a CDAD case patient with onset of symptoms while outside a healthcare facility, and without discharge from a healthcare facility within the previous 12 weeks (community-onset, community-associated) or with onset of symptoms within 48 h following admission to a healthcare facility without residence in a healthcare facility within the previous 12 weeks (healthcare-onset, community-associated).

Unknown case

This is a CDAD case patient who was discharged from a healthcare facility 4–12 weeks before the onset of symptoms.

Onset (Fig. 2)

Healthcare onset

Symptoms start during a stay in a healthcare facility.

Community onset

Symptoms start in a community setting, outside healthcare facilities.

Necessity for investigation and reporting of outbreaks on both national and european levels

The definitions proposed above may be used in implementing CDAD surveillance schemes in specific populations. Depending upon the populations and the reasons for surveillance and reporting, all or some of these definitions may be appropriate. For example, in the UK (http://www.hpa.org.uk/cdr/archives/2005/cdr3405.pdf), the population can be restricted to patients over 65 years of age, regardless of the presence or absence of specific risk-factors (e.g., prior antimicrobial therapy).

Since the implementation of comprehensive and systematic surveillance systems at the national level in each of the European member states will require some time, countries should first develop early-warning and response capabilities in order to detect and notify to regional or national public health authorities severe cases of CDAD related to the PCR ribotype 027 strain of C. difficile.

When analysing data concerning cases that are ‘borderline’, i.e., those that could be either community- or healthcare-associated, the notification report should indicate the most probable origin and justify the conclusion.

The nature of the notifications received will determine how individual countries will subsequently implement laboratory-based or patient-based surveillance systems in specific, targeted populations (e.g., patients in hospitals, patients in nursing homes). For feedback and benchmarking purposes, healthcare-associated case rates should be expressed as cases per reporting time period (e.g., month or quarter) per 1000 patient admissions and per 10 000 patient-days, as the average length of patient stay may vary from one facility to another. Data that are separated according to the epidemiological markers, i.e., the time period between hospital admission and discharge, as well as overall data will be most valuable. Community-associated case rates should be expressed as cases per 100000 population over the reporting period (usually person-years). When calculating healthcare- or community-associated case rates, recurrence rates should be separated from other cases.

Investigation of the incidence of CDAD 027 on the part of EU member states

In order to gain insight into the incidence of CDAD due to C. difficile 027 in the 12 European member states, the ongoing surveillance study undertaken by the ESGCD, which includes assessment of the distribution of the most frequently occurring PCR ribotypes and characterisation of the strains for specific virulence markers and antimicrobial susceptibility patterns, should be completed as soon as possible. A second surveillance study should be developed in which all European member states participate and in which the incidence of CDAD in hospitals, long-term care facilities and the community will be determined.

Because of the epidemic potential, as well as the severity of the burden that CDAD imposes on the healthcare system, each EU member state should consider instituting a national working group for C. difficile. Such a group would encompass the respective national institutes of health and the epidemiologists and experts in prevention, diagnosis and treatment of CDAD. National reference laboratories should be established, with which the working group would collaborate closely.

Research priorities concerning new emerging C. difficile

The new emerging 027 strain offers a good opportunity to develop models for in-depth research into the pathogenesis of CDAD. C. difficile, however, has proven to be particularly difficult to manipulate genetically. As a consequence, our understanding of the pathogenesis of infections caused by C. difficile lags well behind that of other bacterial infections. For instance, several putative surface adhesins and surface layer proteins have been recognised, but their role in pathogenesis has not been established. The major impediment has been an inability to insertionally inactivate chromosomal genes. Promising new developments for creating insertion mutants, as well as information concerning the crystal structures of the two toxins, were reported at the International ClostPath Meeting (June 2006, Nottingham, UK). These developments pave the way for functional genomic studies in which the role of putative genes in pathogenesis may be assigned.

Although the mechanisms of action of TcdA and TcdB are known, their individual importance in the disease process is unclear. The receptors are still unknown but could be identified in the near future by structural studies of newly published partial crystal structures for TcdA and TcdB [84,85]. An important area for research is the application of new animal models, e.g., zebrafish embryos, to investigations of the systemic action of C. difficile toxins. Additionally, the exact role of all genes encompassing the PaLoc is not known and more information concerning the molecular mechanism of toxin regulation is needed. The full genome of two C. difficile strains has been sequenced, including the C. difficile 630 strain and the type 027 strain. An understanding of these genomes and the future development of microarray techniques in the context of C. difficile have the potential to open a new era of C. difficile research. Production of spores may play an important role in the spread of CDAD, and an understanding of its regulatory systems and intracelluar communication systems (quorum-sensing) may provide opportunities to inhibit this process. Although much attention has been given to virulence factors in the context of C. difficile, innate and acquired immunity in humans appear to play important roles in protection. The fact that CDAD occurs mainly in the elderly suggests that some form of immunodeficiency may predispose, which opens another possibility for research. The preliminary results of studies on vaccines containing formalin-inactivated TcdA and TcdB also demonstrate the importance of adequate humoral immunity and call for further studies.

Conclusion

In summary, CDAD due to the new emerging C. difficile type 027 has resulted in the recognition of CDAD as a major nosocomial infection, creating emerging threats to human health and the community. Understanding of the pathogenicity of the disease could lead to better prevention of the disease in humans and animals.

Ancillary