Clostridium difficile infection: the next big thing!

Clostridium difficile is an anaerobic Gram positive spore-forming bacterium and the leading cause of infectious diarrhoea in hospitalised humans, usually after disruption of colonic flora by antimicrobials. The major virulence factors are two toxins, TcdA (an enterotoxin) and TcdB (a cytotoxin), while the role of a third “binary” toxin is unknown. Contamination of the environment with C. difficile spores plays a critical role in transmissibility. Spores are resistant to many commonly-used disinfectants and may remain in the environment for months¹. There has been a suggestion that C. difficile is a threat to Australia’s biosecurity². Now there is evidence that this warning has gone unheeded as rates of human CDI increase dramatically³ and C. difficile is recovered from Australian production animals^4,5.

The emergence of strains causing C. difficile infection in the community (CA-CDI) is becoming more commonplace⁶. There is concern in Australia about the recent emergence of a novel highly virulent strain of C. difficile of community origin, the source of which has not been identified^7-9[REMOVED HYPERLINK FIELD]. Unlike healthcare infections CA-CDI is associated with younger healthy people (particularly females), often without prior exposure to antimicrobials or contact with hospitalized patients^10-13. Community strains differ from predominant hospital strains and are more heterogeneous, with many previously unidentified PCR ribotypes¹⁴. This suggests that other reservoirs of infection contribute to CA-CDI, possibly animals.

C. difficile is a recognized enteric pathogen in a variety of animals including companion animals (cats, dogs, horses) and food animals (cattle, sheep, goats, pigs)^15,16. In Australia C. difficile has been isolated from piglets, sheep, lambs, horses, cats, dogs, and cattle, with the highest prevalence in neonatal animals due to a lack of established gut flora at birth. For this reason predisposing antibiotics may not be required for development of CDI in young animals although there is anecdotal evidence in Australia of routine use of extended-spectrum cephalosporins in production animals. This is particularly concerning in the pork industry where gross contamination of facilities with C. difficile spores is commonplace. C. difficile can be isolated from the faeces of piglets 1 hour after birth, presumably ingested from their environment. Within 48 hours 100% of piglets had acquired C. difficile of the same molecular type that was found in the piggery environment¹⁷. A 2011 Australian study showed contamination with toxigenic C. difficile increased from 0%-61% of sites within a swine facility only one month after occupation with pigs¹⁸[REMOVED HYPERLINK FIELD]. Airborne C. difficile spores can be found up to 20 metres from the pig facility¹⁹. The predominant genotype isolated from food production animals outside Australia is PCR ribotype 078, toxinotype V, NAP 7/8, REA group BK²⁰. This ribotype is now the third most common European human ribotype²¹.

Overseas, meat products, seafood, ready-to-eat salads, salad leaves and vegetables are also contaminated with C. difficile, predominantly ribotype 078-like strains^16,22-24. Contamination may occur through spillage of gut contents at slaughter or direct contamination by food handlers during processing or retailing. Environmental contamination may also play a role. C. difficile spores survive in treated piggery effluent, the by-products of which are then applied to agricultural land, used in retail compost manufacture, or recycled within the swine facility²⁵.

Outside Australia, the increasing prevalence of PCR ribotype 078 in humans, food production animals and food products suggest potential zoonotic transmission. In the Netherlands, where infections with ribotype 078 increased more than four-fold from 2005 to 2008, patients infected with this ribotype were younger and acquired C. difficile in the community more frequently, particularly if they lived in rural pig producing areas²⁶. In the USA, the prevalence of ribotype 078 infections in humans has increased from 0.02% to 1.3% (pre-2001 to 2006) and ribotype 078 is increasingly associated with CA-CDI. These strains are indistinguishable or very closely related to animal ribotype 078 strains by PFGE analysis²⁷. Ribotype 078 strains from Dutch humans and pigs are indistinguishable by MLVA subtyping²⁸. Transmission from humans to animals may also occur. C. difficile can be isolated from the faeces of hospital pet therapy dogs that had prior negative bacteriological cultures for C. difficile. These dogs were >2 times more likely to be colonised with C. difficile than dogs not visiting hospitals²⁹.

The Australian situation is less clear. Investigations are currently underway to identify the major strains of C. difficile causing community-onset infection and their origin. Interestingly, ribotype 078 has never been isolated from companion or production animals in Australia, and from very few human cases of CDI. Instead, we have found a variety of novel ribotypes and sequence types (STs) in animals, with some overlap of human strains. Many of these strains fall into clade 5, the clade of C. difficile that contains predominantly animal isolates. One, a pig strain, is genetically unique but MLST analysis shows it is similar to ribotype 078 (ST 11, clade 5)³⁰. Much more work needs to be done, however it is likely that the use of certain antimicrobials in production animals is resulting in amplification of C. difficile and subsequent spillover into humans by an as yet undetermined mechanism. While the organism will be new to many proponents of “One Health” the concept will not and it will require a multi-disciplinary approach to prevent animal strains of C. difficile infiltrating human health systems.