All Shiga toxin-producing E. coli strains are pathogenic and potentially associated with severe illness, according to an EFSA scientific opinion.

While serotype is important in epidemiological tracking, including incidence, emergence of new clones, and detection and investigation of outbreaks, it is not possible to exclude pathogenicity or possibility of severe illness based on this information, according to the European agency.

The opinion found all STEC subtypes may be associated with severe illness such as hemolytic uremic syndrome (HUS), bloody diarrhea and hospitalization. Although stx2a had the highest rates of these, all other Shiga toxin (stx) subtypes, for which there was sufficient data, were associated with at least one of these severe illness outcomes. There are four stx1 and 12 stx2 subtypes.

Presence of intimin (eae gene) was an aggravating factor, but this virulence marker was not always essential for severe illness. The minimum combination of genes required to cause severe illness is unknown.

Serogroups and outbreak sources
A European Food Safety Authority (EFSA) opinion on STEC pathogenicity in 2013 advocated a molecular approach in which presence of eae or aaic and aggR were associated with a “high” risk for serogroups O157, O26, O103, O145, O111 and O104 or ‘unknown’ risk of severe illness.

From 2012 to 2017, 330 STEC outbreaks were reported in 18 countries around Europe, involving 2,841 cases, 463 hospitalizations and five fatalities. The food vehicle was identified in 164 outbreaks.

Source attribution analysis, based on “strong evidence” outbreak data in this period, suggests that “bovine meat and products thereof, milk and dairy products, tap water including well water and vegetables, fruit and products thereof are the main sources of STEC infections but a ranking could not be made due to insufficient data. Other foods are also potentially associated with STEC infections but rank lower.

In at least six of 14 outbreaks linked to milk and dairy products, the actual source was raw milk. Most ‘tap water including well water’ outbreaks occurred in one country.

In the EU, the top five serogroups in human STEC infections in 2012 to 2017 were O157, O26, O103, O91 and O145. Serogroups most frequently associated with severe infections (HUS, hospitalization or BD) were O157 and O26. Serogroups O111, O80 and O145 were among the five-most-commonly reported in HUS cases, O145, O103 and O111 in the hospitalized cases, and O103, O145 and O91 in BD cases. In total, 49, 88 and 95 different O-serogroups were reported in HUS, hospitalized and BD cases, respectively.

In the United States, the STEC serogroups associated with human illness are O157, O26, O45, O103, O111, O121 and O145, according to the CDC.

Surveillance, detecting and reporting
ISO/TS 13136:2012 is under revision. The revised standard will be divided into two parts, one on the detection and isolation of STEC from food and feed while part two will contain specifications for the characterization of isolated STEC strains.

Most current enrichment methods were developed for STEC O157 and may inhibit other STEC.

Molecular methods for screening, detecting, confirming and/or characterizing STEC include polymerase chain reaction (PCR), real-time PCR, other PCR-based genetic methods and metagenomic sequencing but all have limitations including a lack of sensitivity and selectivity and reliability which could be overcome by using whole genome sequencing.

Surveillance systems for STEC infections have national coverage in all countries except France, Italy and Spain. The opinion states STEC surveillance should ensure all member states collect data on all STEC infections and not just HUS cases.

Experts said a major overhaul of current STEC testing and reporting for animal, food, feed and human isolates is required in the EU.

“A microbiological criterion has been defined for sprouts only, while the reporting of STEC presence in the remaining food commodities as well as in animal samples are only generically described in the Directive 2003/99/EC, a situation which should change to facilitate a better understanding of sources, pathogenicity and the emergence of novel strains.

“The STEC testing framework within the EU should be harmonized including sampling strategies, sampling methods and reporting. This will require all member states using the same case definition and outbreak investigation systems. Moreover, it should be a mandatory requirement to report all data (animal, food, feed and human) to EFSA/ECDC and this should be enforced by all member states.”

The regulatory limit for sprouts is STEC O157, O26, O111, O103, O145 and O104:H4 must be “absent in 25 grams” for sprouts placed on the market during their shelf-life.

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