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Emergence of a new and more virulent strain of VTEC – E. coli O104
Kevin Pollock
Health Protection Scotland
The spectre of verotoxin-producing E. coli (of which E. coli O157 is the most wellknown serotype) has emerged once again. Between the 1st May and 20th July of this
year, an outbreak of E. coli O104:H4 in Germany and continental Europe resulted in
3,039 cases of infection with an additional 727 cases of the haemolytic uraemic
syndrome (HUS). More than 50 people died due to the infection making this the
largest and most deadly VTEC outbreak ever recorded.
Aside from the magnitude and seriousness of the outbreak, it was notable for several
other reasons. The infectious organism is not a typical VTEC strain. It appears to be a
hybrid strain which has characteristics of VTEC such as the Shiga toxin 2 but it has
also acquired genes (notably aggA) from the less pathogenic enteroaggregative E.
coli, which facilitated enhanced binding of the pathogen to colonic epithelial cells. It
is the property of having both these proteins, which makes this strain so virulent i.e.
prolonged binding of the pathogen to cells permits the Shiga toxin to cross from the
gut into the bloodstream and cause injury to the kidneys. This was evidenced in the
progression rate from bloody diarrhoea and abdominal pain to the more serious HUS.
E. coli O157 infections tend to progress to HUS in about 10% of cases but with this
outbreak strain, the number of cases progressing to HUS was approximately 20%.
Epidemiologists were also surprised at the demographic of those being infected and
hospitalised. E. coli O157 infections generally affect the young and elderly but in this
outbreak, it was females in their thirties and forties who were disproportionately
affected. This, coupled with the preliminary epidemiological investigations led
experts to believe that women were more likely to consume salad vegetables such as
cucumbers than their male counterparts. It was only when further investigations were
performed such as case-control and restaurant-cohort studies, that contaminated
sprouts were considered to be the likely source of infection. Trace back studies carried
out by the German E. coli Task Force and the local authorities of Lower Saxony
identified a single sprout-producing farm in Lower Saxony as being the most likely
source of sprouts contaminated with E. coli O104. Furthermore, subsequent actions
resulted in forward tracing establishing that all 41 case clusters identified at that time
in Germany were linked to consumption of sprouts originating from the sproutproducing farm.
A second outbreak in France with the same strain appeared to confuse matters since
nobody had consumed sprouts from the facility in Germany. All French cases
reported eating raw sprouts however these were fenugreek, rocket and mustard
sprouts, which had been procured from a local garden centre, supplied by a UK-based
company. Therefore, the European Food Safety Authority was asked to perform a
detailed trace back exercise to elucidate whether the two outbreaks shared a common
source. Investigations found that an importer from Egypt was considered the most
likely common link and as such, a ban was placed on further imports as well as
national public health messages to thoroughly cook the aforementioned sprouts. It
should be noted that microbiological confirmation of the pathogen from the suspect
sprouts has yet to be proven however, this cannot be interpreted as proof that batches
were not contaminated since results are limited by the analytic and diagnostic
performance characteristics as well as the sampling plan. It is most likely that the
contamination of fenugreek seeds happened at farm level in the country of origin i.e.
Egypt.
Although the outbreak affected many countries in Europe, Scotland was largely
unscathed with only one imported case who developed uncomplicated infection.
However, in recent years the Scottish E. coli O157/VTEC reference laboratory and
Health Protection Scotland have detected a number of VTEC strains, which are not
attributable to E. coli O157. These include the sorbitol-fermenting VTEC strain,
which resulted in a nursery outbreak in 2006 1 and the E. coli O26 strain 2, both of
which resulted in serious forms of HUS in Scottish children. Elucidating the source of
infection is only possible by thorough environmental health investigations into cases
and in Scotland, we are blessed in having an excellent network of Environmental
Health and Food Safety Officers who understand the unique epidemiology of these
pathogens. Salient public health messages on simple precautionary behaviour also
need to be regularly reinforced as prevention of VTEC infection prevents HUS.
1
http://www.cdc.gov/eid/content/16/5/881.htm
Pollock et al., (2011) Emergence of highly virulent E. coli O26, Scotland. Emerging Infectious
Diseases, vol. 17, issue 9 (in press)
2