Sainsbury’s recalled pre-packed bagged salads containing watercress in September last year as part of an outbreak which sickened 19 people.
PHE said after restocking the watercress another case was reported with an illness onset date of 21 October 2013, having eaten bagged mixed salad containing watercress from the UK supermarket.
The watercress supplier is working with the Food Standards Agency to review their policies and procedures after the outbreaks.
Between 30 August and 19 September, 19 cases of VTEC O157 phage type 2 VT2 (14 in England, four in Wales and one in Scotland) were reported sharing the same Multi Locus Variable Number Tandem Repeat Analysis (MLVA) pattern (and single locus variants), not previously seen in the UK.
Another outbreak , different profile
Investigators found a second outbreak of six cases of VTEC O157 phage type 2 VT2 with a different MLVA profile.
Two cases reported eating watercress from the retailer prior to the recall, one consumed watercress prior to announcement but with no detail on where it was purchased, and one had mixed salad from the retailer during the product being withdrawn from sale.
Two further cases with onsets of 1 October 2013 were members of a family who had watercress as part of a meal at a pub.
Trace-back enquiries confirmed that the pub purchased unwashed watercress from the same supplier involved in the first outbreak.
Two cases with an identical MLVA profile were retrospectively identified with onset dates in February 2013.
One had consumed watercress and one pre-packaged salad, both from retailers representing a different supply chain, suggesting that the contamination is unlikely to be from the farms.
Contamination source unclear
Despite trace-back investigations, microbiological testing of watercress and environmental sampling at farms, the source of contamination remains unclear.
During sampling of the farms supplying watercress, VTEC O157 PT 2 VT2 identical on typing to isolates from the second outbreak was isolated from one of the watercress beds.
Environmental investigations said that this watercress bed was in close proximity to an adjacent field containing cattle – the primary reservoir for VTEC.
PHE said it seems likely that the cause of the second cluster of cases was transfer of VTEC from the field to the watercress bed either from wildlife entering the watercress farm or run-off water.