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SIR, — Verocytotoxigenic Escherichia coli (vtec) O157 is recognised as an important cause of severe illness in humans. Cattle provide the main reservoir of infection but remain clinically unaffected. In a recent field study of 75 farms in England and Wales by Paiba and others (2003), the prevalence of excretion by individual cattle was 4·2 per cent. Although vtec o157 is the most important vtec serogroup from human infections in Britain, verocytotoxins (vts) can also be produced by other E coli serogroups, such as o26. Surveillance within the Veterinary Laboratories Agency (vla) for new and emerging strains of E coli in animals has recently been enhanced to include extended testing for virulence factors, serotype and antimicrobial resistance in isolates conforming to specified selection criteria.
We would like to report the findings from a recent farm investigation in which vtec o26 was recovered from clinically normal cattle following an initial isolation in faeces from a scouring calf. The isolate was submitted from a two-month-old calf in a 33-cow beef suckler and calf-rearing enterprise. The herd commenced calving indoors in a strawed yard in January 2006. Several of the suckler calves developed diarrhoea but only one was sampled and the sample submitted to the vla for limited diagnostic testing. This animal subsequently died.
Examination of the faecal sample using the standard procedures adopted within the vla did not identify any potential pathogens apart from E coli. The isolate was examined under the enhanced E coli surveillance programme and identified as E coli o26 with genes for vt1 and intimin (eae), which is associated with intestinal attachment and effacement.
At a follow-up visit to the farm, none of the cattle appeared to be scouring. Faecal samples were taken from the previously affected group of suckler calves housed with their dams and from two groups of heifers and steers that were kept in adjacent pens. The samples were screened for E coli o26 using an automatic immunomagnetic separation technique (DEFRA 2006). Eight of 10 samples from the calves and five of six from the cows were positive using this technique. Extended testing of three of the isolates revealed that they were also vt1 and eae positive by pcr. Four samples taken from cattle in unaffected groups were negative for E coli o26.
During the period May 2005 to May 2006, a total of 249 cattle isolates were included in the vla enhanced E coli surveillance survey. Of these, 14 (5·6 per cent) were vtec, of which serogroup o26 (six isolates [43 per cent]) was the most prevalent.
Pearce and others (2004) also reported that vtec o26 was the most frequently detected vtec serogroup in a longitudinal study on a cohort of beef cattle in Scotland. They found no association with housing or with diarrhoea. The results of our farm investigation have further confirmed a high prevalence of infection in clinically normal cattle. It is possible that vtec o26 contributed to the demise of the initial calf sampled from the farm. There are other documented reports of diarrhoea in cattle associated with E coli o26 (Gunning and others 2001).
Like vtec o157, vtec o26 carries the potential for human infection and severe illness (Brooks and others 2005). There was no associated human illness on this farm, and human cases of vtec o26 infection are rare in Great Britain (Willshaw and others 2001). However, other countries such as Austria, Germany and Italy report it as the second most prevalent enterohaemorrhagic E coli isolated from patients with haemolytic-uraemic syndrome and diarrhoea (Leomil and others 2005). All the vtec o26 isolates identified from the vla survey to date have contained the vt1 rather than the vt2 gene, which is consistent with many other surveys. However, strains of vtec possessing vt2 have emerged in recent years as having increased pathogenicity in man, in both the uk and North America (Jenkins and others 2003, Brooks and others 2005), and there is some evidence that E coli o26 is continuing to evolve by uptake of vt2 genes (Zhang and others 2000).
In summary, vtec o26 appears to be a frequent contributor to the gut flora of cattle and can be isolated from both scouring and clinically normal animals. There is clearly an occupational hazard associated with contact with farm animals and efforts should be made to reduce the risk by adhering to sensible hygiene precautions. Practitioners should also be aware that diarrhoeic calves may carry vtec infection either coincidentally or associated with clinical disease.
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