Listeria in fresh produce. Are we any the wiser?
‘The role of food safety staff must go beyond compliance and keeping the regulatory guys happy,’ said Suresh DeCosta, Director of Food Safety, Lipman Family Farms (USA) and Technical Committee, Center for Produce Safety (USA), at the 5th Annual Fresh Produce Safety Conference in Sydney last week.
The number of deaths from the 2017 listeriosis outbreak in South Africa reached 216, making it the most lethal outbreak in history. The source was cured meats.
On a much smaller scale, Australia recently experienced lethal outbreaks traced to cheese and rockmelons. While the number of recorded hospitalised cases in Australia is low – around 70 a year – Listeria continues to be a major problem for the food industry, and a priority theme for the Fresh Produce Safety Centre Australia & New Zealand.
Microbiologist Dr Robert Premier explained there are 17 species of Listeria but most are harmless and only two are implicated in human infections.
The most dangerous is Listeria monocytogenes, yet even this has a virulent and non-virulent form.
Dr Premier identified three factors that made Listeria a key issue for fresh produce.
The first concerned the source of infections, which Dr Premier concedes there are more questions than answers. Sheep and cattle and humans are responsible for spreading both environmental (non-virulent) Listeria and infectious (virulent) Listeria, but what triggers the shedding of virulent L. monocytogenes in animals? Could it be drought-induced stress? Feedlots? Anti-biotic treatment? Nobody is sure.
Testing is another major factor. Rapid testing is prone to false positives, while the more rigorous Australian standard method test takes up to a week, a crucial delay during a crisis.
A third factor is the ability of Listeria to persist on the surface of fruit with inedible skins, such as rockmelons due to its natural unevenness and skin texture.
Proof of the risk of Listeria in rockmelons came with the 2011 Colorado outbreak that resulted in 33 deaths and earlier this year in Australia which saw the deaths of seven people.
The industry, Dr Premier said, needed to better understand the potential for growth in all horticultural product lines.
While Listeria is often found in the routine testing of fresh produce – with the exception of rockmelons – it not been linked to outbreaks in Australia. While the differences between virulent and non-virulent strains of L. monocytogenes are still not fully understood, what has been learnt from recent outbreaks is the fresh produce and meat industries require collaboration to solve this problem.
Dr Craig Shadbolt from the NSW Department of Primary Industries said that while listeriosis is a notifiable disease, its long incubation (typically two to four weeks but up to 70 days) made it very difficult to identify the food or environmental source.
The use of whole genome sequencing, however, had been a game-changer in outbreak surveillance and detection.
Dr Shadbolt highlights the need for better understanding of the importance of washing and sanitising. Among other things, the role of chemical sanitiser concentrations and problems with dust and hygiene in packing houses.
Mixing fungicide and chemical sanitiser Dr Shadbolt said, has the potential of one cancelling out the other. The investigation into the 2018 rockmelon Listeria outbreak found that the packing house in the Griffith was hygienic and well run. In Griffith, environmental factors were thought to have been responsible, with heavy rain and dust storms likely to have boosted the bacterial load before harvest and compromised the effectiveness of washing systems.
Extra washing to cope with soil loading and higher sanitiser concentration may help mitigate such factors in future.
‘The fresh produce industry needs to learn from past outbreaks to instil and adopt improved food safety practices to prevent future fatalities from listeriosis.’
At Lipman Family Farms, Mr DeCosta identified cleaning and sanitising as key issues. Too often, he said, the job of sanitising equipment was performed as overtime by ordinary workers. For the job to be done properly, it needed dedicated staff, properly trained and paid a premium – a change that could only be achieved with the full co-operation of management.
Effective training was the key. ‘Staff should be consulted in making investments and improvements,’ Mr DeCosta said. ‘The food safety team need to feel they are partners.’
Jessica Purbrick-Herbst, Executive Officer, Fresh Produce Safety Centre Australia & New Zealand
+61 439 014 188
About the FPSC A-NZ
The Fresh Produce Safety Centre Australia & New Zealand is purposed with disseminating relevant information and providing thought-leadership in fresh produce food safety that empowers organisations and companies to strive towards a reduction in food safety incidences.