The chicken was served in 45 of the 90 schools in Caerphilly it was supplied to. It was unclear if the entire batch of chicken was affected.
The samples in which the bacteria were found were taken from schools where the catering staff noticed an unusual smell and withdrew the chicken from service.
An investigation by the Foods Standards Agency (FSA), Public Health Wales and environmental health officers has yet to find a source for the contamination.
Manufacturer from England
However, the FSA said that a manufacturer from England that supplied the chicken and a distributor in Wales were being investigated.
Heather Lewis, consultant in health protection for Public Health Wales, said no cases of illness have been reported at this time.
“However, it can take anywhere from three to 70 days for the symptoms of listeriosis to show following consumption of a contaminated food product,” added Lewis.
Listeriosis is the illness caused by an infection of Listeria. Symptoms of listeriosis can be similar to flu and include high temperature, muscle ache or pain, chills, feeling or being sick and diarrhoea.
Being sick and diarrhoea
“There is no test that can be undertaken to confirm whether a person has the infection or likely to develop it, prior to the start of symptoms occurring,” explained Lewis.
“Based on the current assessment of the situation, the risk of getting listeriosis is very low for a vast majority of healthy children and staff.”
FSA director of Wales and Regulatory Delivery Nina Purcell said the organisation was probing the food chain, both in Wales and England, to ensure consumers are protected.
Meanwhile, listeria contamination has sparked a number of recalls this year, including smoked salmon and unpasteurised goat’s cheese.
Other food recalls in the past month
Undeclared allergens caused three of recalls in the past month, posing a health risk to consumers who may eat or drink the products.
Products recalled due to undeclared allergens were:
Pip & Nut coconut almond butter
Undeclared allergen: Peanuts
Pack size: 250g
‘Best Before’ date: June 28 2017
Batch code: 57956-28-5
ALDI Chinese chicken curry with egg fried rice
Undeclared allergen: Milk
Pack size: 450g
‘Use by’ date: All ‘Use by’ dates
Gower Brewing – four beers
Undeclared allergen: Wheat (gluten)
Product: Gower Gold (Golden Ale)
Pack size: 500ml bottles & cask
Batch Details: All dates up to and including Best Before End July 2017
Product: Gower Power (IPA)
Pack size: 500ml bottles & cask
Batch Details: All dates up to and including Best Before End July 2017
Product: Lighthouse (Lager)
Pack size: 500ml and 330ml bottles & cask
Batch Details: All dates up to and including Best Before End July 2017
Product: Black Diamond (Porter)
Pack size: 500ml bottles & cask
Batch Details: All dates up to and including Best Before End July 2017.