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Food Safety Focus (121st Issue, August 2016) – Incident in Focus

Shiga toxin-producing Escherichia coli Contamination in Prepackaged Flour

Reported by Dr. Fiona FONG, Research Officer,,
Risk Assessment Section,
Centre for Food Safety

Since the initial recall on 31 May 2016, several varieties of flour products have been recalled in the US due to possible contamination with Shiga toxin-producing Escherichia coli (STEC). Some people, who had eaten or handled raw dough, got sick. The US Centers for Disease Control and Prevention (USCDC) has reported that as of 25 July 2016, 46 people were infected with the outbreak strains of STEC O121 (45 people) or O26 (1 person). The USCDC and the US Food and Drug Administration (USFDA) are still investigating the outbreak.

Certain batches of the affected products had been imported into Hong Kong. Upon noting of the occurrence the incident in the US, the Centre for Food Safety (CFS) has immediately instructed the trade to stop selling the affected products and initiate recalls. This article discusses the risk of STEC and the follow-up actions taken by the CFS regarding the incident.

A kid licking raw cookie dough
A kid licking raw cookie dough

What is STEC?

E. coli normally live in the intestines of people and animals. Most E. coli are harmless and actually are an important part of healthy human intestinal tract, but some strains of E. coli can cause diseases through the production of a toxin called Shiga toxin. These bacteria are called STEC but may also be called Verocytotoxin-producing E. coli (VTEC) or enterohaemorrhagic E. coli (EHEC). STEC is an important group of E. coli that can cause severe diarrhoea and has been implicated in many foodborne outbreaks worldwide.

The serotyping of STEC is based on their O antigen and H antigen. E. coli O157:H7 is the most common serotype while others E. coli serogroups, including O121 and O26, are called “non-O157 STECs”. STEC can grow in temperatures ranging from 7 °C to 50 °C, with an optimum temperature of 37 °C. Some can grow in acidic foods, down to a pH of 4.4. It can be destroyed by thorough cooking until the core temperature of the food reaches 70 °C or above.

The incubation period for STEC infection is two to 10 days, usually three to four days. Symptoms of STEC infection include abdominal pain and watery diarrhoea that may in some cases progress to bloody diarrhoea. Fever and vomiting may also occur. A serious complication, haemolytic uraemic syndrome (HUS), occurs in two to 15% of STEC infection cases. HUS, a condition characterised by abnormal destruction of red blood cells, kidney damage and low platelet count, is more common in children under five years of age. According to the Centre for Health Protection of the Department of Health, 12 cases of STEC infection were recorded from 2012 to 2015.

Sources of Infection

Ruminants, especially cattle, are the natural reservoir for STEC. STEC may reside asymptomatically in the intestines of ruminants and may be shed intermittently in faeces. According to USFDA, flour is derived from grains that come from fields directly and is not treated to kill bacteria in general. If animals defecate or urinate in the field, their waste may contaminate the grains, which are subsequently harvested and milled into flour. People should therefore be aware of the risks associated with the consumption and handling of flour and raw dough.

STEC transmission occurs through consumption of contaminated food and water, or direct contact with STEC carrying animals. Person-to-person transmission of STEC is also possible. Raw or undercooked ground meat products, unpasteurised milk and contaminated raw vegetables and sprouts are of particular concern. Consumption of contaminated food may pose a high risk of illness as a low dose of STEC can cause illness in susceptible populations.

Actions Taken by the CFS

Certain batches of the affected products, including prepackaged flour and cake mix, had been imported into Hong Kong. Upon learning the incident occurred in the US, the CFS has immediately advised the public not to consume the affected products, liaised with the US authorities and alerted the trade to stop selling the affected products and initiated recalls. No local food poisoning cases related to the consumption of the affected products have been reported. The CFS will remain vigilant, continue to monitor the situation and take appropriate follow-up actions in a bid to safeguard food safety and public health.

Key Points to Notes:

  1. STEC infection is transmitted by faecal-oral route through contaminated food and water, or direct contact with STEC carrying animals.
  2. E. coli cannot survive under high temperature and can be killed by thorough cooking.
  3. People should maintain good personal hygiene and bake items made with raw flour dough or batter before eating them.

Advice to the Trade and Consumers

  • Wash hands, work surfaces, and utensils thoroughly before and after handling raw flour dough.
  • Be aware of the potential of cross-contamination in food preparation areas.

Advice to Consumers

  • Do not eat or taste raw flour dough or batter.

Advice to the Trade

  • Ensure that the flour and flour products are fit for human consumption.
  • As a precaution, do not serve raw dough to customers or allow children and other guests to handle raw dough.
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Last Revision Date : 17-08-2016