Yersinia spp.

Associated infections

• Yersiniosis: acute enterocolitis and diarrhoea
• Mesenteric adenitis and terminal ileitis (pseudoappendicitis)
• Bacteraemia and sepsis, particularly with iron overload states or desferrioxamine therapy
• Reactive arthritis and erythema nodosum
• Pharyngitis
• Focal abscess, including hepatic and splenic abscess in iron-overloaded patients
• Transfusion-transmitted sepsis from contaminated red cell units

Transmission route

Yersiniosis is acquired predominantly by the faecal-oral route through contaminated food, notably raw or undercooked pork and unpasteurised milk, and through untreated water. Pigs are the principal reservoir of the serotypes that cause human disease, and consumption or handling of raw pork products is the dominant risk factor in most series. Direct contact with animals and, less commonly, person-to-person spread also occur. Iron availability is an unusual and clinically important modifier: the organism does not produce its own high-affinity siderophore in most pathogenic biotypes but exploits those of other organisms, so patients with iron overload or receiving desferrioxamine chelation are at markedly increased risk of invasive disease.

The organism's ability to multiply at refrigeration temperatures underlies its recognised role in post-transfusion sepsis, in which asymptomatically bacteraemic donor blood permits bacterial growth during cold storage, generating both high bacterial loads and endotoxin before the unit is transfused. This mechanism has no counterpart in endoscope reprocessing, and the analogy should not be drawn; it is noted here only because it is the context in which many clinicians will have encountered the organism.

Yersinia is not typical flora of an engineered healthcare water system and is not a routine target of endoscope final rinse water testing. It does not establish the oligotrophic biofilms characteristic of Pseudomonas, Serratia and other non-fermenters in reverse osmosis plant and distribution loops, and it is readily inactivated by validated high-level disinfection. A confirmed isolate from final rinse water or from a patient-ready endoscope is therefore an abnormal result requiring urgent investigation rather than routine repeat sampling. The realistic explanations are ingress of untreated surface or mains water into the treated water path, cross-connection or backflow, contamination introduced during sampling, or carry-over of gastrointestinal soil from inadequate manual cleaning. Affected reprocessors and endoscopes should be quarantined and infection prevention notified while the water path and cleaning process are reviewed.

Relevance in endoscopy and reprocessing

There is no established literature linking Yersinia to flexible endoscope reprocessing, and it should not be represented as an endoscopy-associated pathogen. It has no documented record of endoscopic transmission comparable to the historical Salmonella reports, and it plays no part in the contamination events described for Pseudomonas, Serratia, the non-tuberculous mycobacteria or the carbapenemase-producing Enterobacterales associated with duodenoscopes. Its presence in reprocessing reference material is a function of what a detection would signify, not of any documented reprocessing risk.

Where the organism is relevant to endoscopy is on the clinical side rather than the equipment side. Y. enterocolitica terminal ileitis produces endoscopic and histological appearances that overlap with Crohn's disease, and patients are not infrequently investigated by colonoscopy and ileoscopy for that reason. An endoscopy unit will therefore encounter the organism as a diagnosis arising from a procedure rather than as a contaminant recovered from one, and a Yersinia result reaching a reprocessing manager should first be checked against the possibility that it relates to a clinical specimen rather than an environmental one.

In the water system, Yersinia has no colonising role. It does not form the oligotrophic biofilms that characterise reprocessing plant contamination, it does not persist in terminal connecting tubes or reprocessor manifolds, and it is removed by reverse osmosis and by bacterial-retentive terminal filtration. Its psychrotrophy allows it to survive in cool untreated water for extended periods, which is precisely why it is associated with untreated surface water and wells, but survival in raw water is not the same as colonisation of a treated, circulated, filtered system. Searching for a Yersinia reservoir inside the treated water path is not a productive investigation; the productive question is how untreated water or faecal material entered that path.

Against validated high-level disinfection the organism has no relevant tolerance, and it is inactivated by peracetic acid, glutaraldehyde and ortho-phthalaldehyde at in-use conditions, by thermal disinfection and by chlorine at drinking water residuals. Survival in a reprocessing setting would require physical shielding within organic soil, an unbrushed channel, a damaged lumen or an incomplete cycle. Drying and storage are of less consequence for this organism than for the waterborne non-fermenters, though its ability to grow at low temperatures means that, unusually, a cool damp storage environment would not restrain it in the way it restrains most Enterobacterales.

Interpreting a detection

A Yersinia isolate from a water, final rinse or endoscope sample is an abnormal result and should not be managed by routine resampling. The first step, however, is confirmation and characterisation, because this organism carries an interpretive complication that Salmonella and Shigella do not: many environmental isolates of Y. enterocolitica, particularly biotype 1A, lack the virulence determinants required to cause invasive human disease. Ask the laboratory to confirm the species and, where the result will drive an incident response, to biotype the isolate or refer it for virulence gene detection. A non-pathogenic environmental biotype recovered from water still indicates that untreated water or environmental material has entered the treated path, and remains a water integrity finding requiring investigation, but the clinical implications and the threshold for patient look-back differ substantially from those attaching to a pathogenic biotype.

Sampling and handling artefact should be considered on the same basis as for other enteric organisms, by looking for a specific mechanism rather than assuming one. Establish who took the sample and what they had handled, whether the outlet was disinfected and adequately pre-flushed, whether a sterile neutralising container was used, the transport interval and temperature, and whether the sample could have contacted a drain, sink or soiled item. Because Yersinia grows at refrigeration temperatures, an extended transport or storage interval at 4 degrees Celsius will not suppress its growth as it would for most Enterobacterales, so a delayed sample may show a higher count than was present at the outlet. Ask the laboratory whether cold enrichment was used, since that technique is designed to selectively recover Yersinia from mixed populations and will amplify a very small number of organisms into a positive result, which changes how the count should be read.

The substantive investigation follows the same two tracks as for other enteric findings. On the water side, review backflow prevention and any cross-connection with mains, waste, drain, rainwater or non-potable services; check whether any untreated or partially treated water can reach the treated path through a bypass, a temporary hose or recent plumbing work; verify terminal filter integrity and change history and whether the housing has been disturbed; and confirm reverse osmosis plant performance. Untreated surface or bore water deserves particular attention with this organism given its ecology. On the process side, audit manual cleaning by direct observation, covering brush condition and access, channel perfusion, detergent dilution and contact time, leak testing and the delay between procedure and cleaning.

A single confirmed isolate warrants a full investigation rather than a repeat sample, and the affected reprocessor and the endoscopes processed on it since the last satisfactory result should be quarantined pending the outcome. Escalation to infection prevention should occur at first confirmed detection, and should extend to the facility executive and the state or territory health authority where a pathogenic biotype is confirmed and a patient exposure pathway cannot be excluded, non-pestis yersiniosis being notifiable in Australian jurisdictions. Isolates should be retained. Repeated detection, or detection alongside other faecal indicator organisms such as E. coli, points to a continuing structural breach of the treated water path and should move the response from the reprocessing service to facilities engineering.

Antimicrobial resistance

Y. enterocolitica produces chromosomal beta-lactamases, typically an AmpC-type cephalosporinase and a class A penicillinase depending on biotype, and is consequently resistant to ampicillin, amoxicillin and first-generation cephalosporins. The pattern varies between biotypes, with biotype 1A commonly showing a different profile from the pathogenic biotypes, which is one further reason biotyping has practical value.

Isolates generally remain susceptible to trimethoprim-sulfamethoxazole, fluoroquinolones, third-generation cephalosporins, aminoglycosides and tetracyclines, though susceptibility varies by biotype and region and testing is advisable. Acquired resistance is less prominent in this species than in Salmonella or Shigella, and extensively drug-resistant Yersinia is not an established clinical problem, but resistance surveillance for the genus is comparatively sparse and the absence of reports should not be read as strong evidence of absence.

Uncomplicated enterocolitis in an immunocompetent patient is usually self-limiting and does not require antimicrobial therapy; treatment is reserved for severe, systemic or bacteraemic disease and for compromised hosts, including those with iron overload, cirrhosis, diabetes or immunosuppression, in whom invasive disease is both more likely and more severe. Where treatment is required for bacteraemia, combination therapy with a third-generation cephalosporin and an aminoglycoside, or a fluoroquinolone, is commonly used.

Disinfectant and biocide resistance is not a feature of this organism. It is inactivated by chlorine at drinking water residuals, by peracetic acid, glutaraldehyde and ortho-phthalaldehyde at high-level disinfection concentrations, by thermal disinfection at washer-disinfector parameters and by steam sterilisation. Its one environmentally distinctive property, growth at refrigeration temperatures, confers no protection against any of these processes; it affects only how long the organism persists and how readily it multiplies in cool untreated water, in cold-stored blood products and in refrigerated foods. In a reprocessing context, survival would require physical shielding within organic soil or a location the disinfectant did not reach, not any intrinsic tolerance of the chemistry.