BUSINESS recall
Shiga toxin-producing
Escherichia coli contamination
OF LEAFY VEGETABLES
Risk management
Shiga toxin-producing Escherichia coli (STEC) are a sub-group of Escherichia coli (E. coli) bacteria that produce toxins known as Shiga toxins, which can cause severe foodborne illness in humans.
Words The following is a summary of a paper prepared by Sukhvinder Pal (SP) Singh, Senior Research Scientist, Central Coast Primary Industries Centre, Ourimbah (April 2025, Primefact PUB25/305, first edition)
STEC infections can cause a wide range of symptoms, from mild gastroenteritis to severe conditions like haemolytic uremic syndrome and life-threatening kidney disease.
The severity of illness depends on factors such as the strain of STEC involved, the number of bacteria ingested, and the individual’s health. Vulnerable populations, such as children, the elderly, and immunocompromised individuals, are at higher risk of severe disease outcomes.
STEC are transferred to humans via contaminated food and water, direct animal contact, environmental contamination and human-to-human transfer via the faecal-oral route.
The most common foodborne sources of STEC include undercooked meat, unpasteurised milk, ready-to-eat meat products, seed sprouts and leafy vegetables.
If workers or harvesting equipment come into contact with contaminated water, soil, or surfaces, they can spread STEC onto the leafy vegetables.
Although most cases of STEC infection in Australia are considered sporadic, major STEC outbreaks can occur, with the largest reported outbreak of serogroup O157 STEC in Australia occurring among 57 patients in Queensland in 2013, associated with exposure to an animal nursery at an agricultural show (Vasant et al. 2017).
The major sources and routes of STEC contamination in leafy vegetables are:
1. Irrigation water: one of the primary sources of STEC is the use of contaminated water for irrigation. Water contaminated with faecal matter from animals (livestock and wildlife) can carry pathogens like STEC, which then contaminate the surface of leafy vegetables. Surface water sources such as rivers, creeks and canals are highly prone to contamination with STEC from animal operations.
2. Organic soil amendments: raw or improperly composted manure contaminated with STEC can transfer these pathogens to leafy vegetables when used as fertilisers (Ongeng et al. 2015). STEC can be transferred to the leaves during production through direct contact with soil or indirect splashing with rain and overhead irrigation.
3. Growing site: the coexistence of fresh produce and livestock industries poses elevated risks of pathogen transfer from animal operations to the production field via contaminated irrigation water, dust storms and run-off. Compost yards and piles can be a significant risk for contaminating leafy vegetables in the field. They should be located as far away as possible and be screened or protected to avoid contamination via wind and run-off.
4. Harvesting: if workers or harvesting equipment come into contact with contaminated water, soil, or surfaces, they can spread the bacteria onto the leafy vegetables. Inadequate personal and equipment hygiene can also contribute to cross-contamination.
5. Cool chain management: pre-cooling leafy vegetables immediately after harvest reduces microbial risk. Maintaining a cool chain is critical to minimise microbial proliferation, particularly for businesses transporting produce long distances between harvest and postharvest processing.
6. Postharvest processing: after harvest, leafy vegetables are washed in water to remove dirt and residues, but this water can be contaminated if sanitisers are not maintained at appropriate concentrations to compensate for organic and microbial loads on harvested produce.
7. Extreme weather conditions: heavy rainfall that results in run-off or flooding, dust storms, bushfires, and heatwaves affect fresh produce safety (Singh 2023).
One of the primary sources of STEC is the use of contaminated water for irrigation.
STEC risk management strategies
Global Food Safety Initiative (GFSI) benchmarked quality assurance schemes provide guidance for microbial risk management. Having these schemes in place can help with managing microbial risk, including STEC.
Comprehensive food safety strategies must be implemented at various points along the leafy vegetable production and supply chain (Singh 2025).
Essential risk management practices include:
1. Preharvest water quality management — filtration and backflushing
2. Organic soil amendments — composts should be free from harmful pathogens
3. Coexistence with animal operations — appropriate buffer zones should be in place
4. Harvest and post-harvest management — hygiene, cool chain, water quality
5. Traceability — across all systems
6. Pre-harvest testing of leafy vegetables
7. Testing of soil
8. Testing of irrigation water
9. Post-harvest water testing and monitoring
10. Testing processed products.
Conclusions
The contamination of leafy vegetables with STEC poses a significant public health risk, particularly for raw and minimally processed products. The risks can be mitigated through a combination of preventative and detective control measures, both during production and postharvest processing. A comprehensive, multi-faceted approach to risk management is essential for reducing product recalls, preventing the incidence of foodborne illnesses linked to STEC contamination in leafy vegetables, and ensuring safe, high-quality produce for consumers.
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