E. coli O103, one of the more rare Shiga-toxin producing E. coli, sickened patients at two Victoria, British Columbia, hospitals last April. The Vancouver Island outbreak is now the subject of a study on food safety for high-risk products prepared for vulnerable populations.
Based on confirmed E. coli O103 cases reported to the Island Health infectious disease unit, whole-genome sequencing was used in the case study. The Canada Communicable Disease Report (CCDR) published the study in its January 2022 edition.
The Victoria outbreak identified six confirmed E.coli O103 cases, with 67 percent being female and a medium age of 61. All were inpatients or outpatients at the two hospitals. All consumed raw minced celery sandwiches prepared by the hospital food services.
There was little product testing of celery, and residual E. coli contamination occurred through the mincing process, and “temperature abuse” at the hospitals is suspected of contributing to the outbreak.
The April 2021 outbreak of E. coli O103 was an unusual increase. Previous E. Coli O103 outbreaks were linked to clover sprouts, bison meat, ground beef, cured mutton sausages, raw milk, and fermented sausages.
Celery previously was reported as a vehicle for Listeria monocytogenes, norovirus, and E. coli O157:H7, but not O103. The CCDR study objective “is to describe the first outbreak of non-O157 E coli associated with celery in Canada and to identify issues of good safety for high-risk products prepared for vulnerable populations, to reduce the likelihood of these outbreaks in the future.”
The study authors were from the Public Health Agency of Canada, Island Health, and the British Columbia Centres for Disease Control.
The report continued, saying:
“All cases had been admitted to or visited two Victoria-area hospitals during their exposure period. Of the six confirmed cases, four were admitted to Hospital A, one was admitted to Hospital B, and one case was not admitted to the hospital but visited the emergency room of Hospital.”
The study found no related cases identified within the same timeframe of the Victoria outbreak nationally in Canada or within the United States.
Discussion and finding
“This investigation resulted in several recommendations to improve food safety of this food item within the Island Health region,” according to the report. “Evidence from the epidemiological and food safety investigations support minced celery as the source of this outbreak. All six confirmed cases were exposed to the suspect source, and no other product was reported across all six confirmed cases, despite detailed menus for all inpatients. An outpatient who ate a tuna and celery sandwich only during their emergency room visit to Hospital A, the outlier case added further support to celery as the suspect source.
“This investigation also revealed strong traceback evidence — the minced celery served in Hospital A and Hospital B was provided by the same supplier; the investigation also revealed strong trace forward evidence — the supplier provided the minced celery product only to the two hospitals, and nowhere else. Because the contaminated product was no longer available by the time of the investigation, and due to the cleaning procedures at Processor A, neither product samples nor environmental samples were available for testing. Despite the lack of laboratory evidence, the authors believe the strong epidemiological, traceback, and trace forward evidence is sufficient to implicate minced celery in this outbreak.
“The outbreak highlights the risk of raw vegetables provided to vulnerable populations and draws particular attention to the risk of mincing during processing. While previous work has documented the potential food safety hazards of fresh-cut produce this outbreak serves to document the potential risks posed by mincing, which provides the opportunity for small amounts of bacteria remaining on the surface of a product, even after chlorination, to be spread throughout an entire batch. Attribution of the mincing step as problematic in this outbreak scenario is further supported as trace forward investigation revealed that more coarsely chopped celery from the same batch was supplied to a wide distribution network, exclusive of Hospital A and Hospital B, with no cases of the outbreak strain of E. coli O103 associated with this product.
“Despite providing food to a population of approximately 800 inpatients each day, identification of only six cases across Hospital A and Hospital B could potentially be explained by a low level of contamination, which may have caused illness only amongst those whose sandwiches were subjected to temperature abuse. Temperature abuse is a known vehicle for pathogen propagation and was reported by the hospitals during the investigation follow-up. It is hypothesized that any contamination present after the mincing step in Processor A was further propagated by these reports of temperature abuse, resulting in the illnesses reported. A recommendation was made at the two implicated hospitals to add a timestamp to all sandwiches to mark the time the product was taken out of the fridge, to reduce the risk of temperature abuse moving forward.
“There are several limitations to consider in the interpretation of these outbreak data. First, exposure data for celery was not available for the healthy population controls to directly compare with outbreak cases. However, given that 100% of confirmed cases had exposure to the suspect source, and this was the only common exposure across all six cases, the authors feel confident in the epidemiological evidence for this product. Second, the reporting delay for this outbreak was long, which in turn delayed the outbreak identification and investigation. Reporting delays are influenced by a multitude of factors, but comorbidities among the inpatient and outpatient cases in this outbreak may have delayed consideration of an enteric illness diagnosis and thus the requisition of a stool sample for testing. Third, several cases were missing onset dates as they could not be interviewed. For these individuals, their onset date likely predated their specimen collection date, which would also impact their exposure period. This was taken into account when interpreting the exposure data and analyzing hospital menus. Fourth, there were no food samples available to test for the presence of E. coli O103; therefore, there was no laboratory data to definitively confirm the source of this outbreak. However, despite the lack of laboratory confirmation, the authors believe the epidemiological evidence, the traceback data, and the trace forward data provided strong support of the suspect source. Lastly, it could not be determined where or how E. coli was introduced, as a further follow-up at the grower in the United States was outside the investigative jurisdiction of this outbreak.
“Raw vegetables, such as celery, are a known source of E. coli contamination and present a risk to vulnerable populations. Mincing during the processing of raw vegetables, and temperature abuse prior to consumption may provide additional layers of risk,” according to the report.
“This outbreak resulted in several recommendations to reduce the risk of minced celery served in hospitals, including more frequent testing at the processor, a review of the chlorination and mincing process, and a review of hospital food services practices to mitigate temperature abuse.”
(To sign up for a free subscription to Food Safety News, click here.)