Not all food poisoning can be tracked to a source.

The source of many food poisoning outbreaks may never be found, regardless of the work done by authorities. In 2016 there was a Listeria outbreak in Denmark that could not be tracked to a specific source.

Although the actual source could not be confirmed, it was suspected after the investigation that it was  only in Jutland, Denmark.

There were six cases, five women and one man, all aged between 30 and 91 years old between 2016 and 2019.

Typical Listeriosis symptoms may include sudden onset of fever, headache, backache, nausea, vomiting and neck stiffness. The onset time varies from a few to 70 days with an average of three weeks. It can take up to three months depending upon the person and situation. Newborn babies, the elderly, immune suppressed people and pregnant women are more susceptible and their infection more serious.

The 2016 outbreak had been linked to hummus and investigations by Statens Serum Institut (SSI), Fødevarestyrelsen (Danish Veterinary and Food Administration) and the National Food Institute, Technical University of Denmark  this year identified that the infections were likely to be sourced to a specific local shop in Jutland.

This happened after both interviews with those with illness in 2019 and in 2016, as well as with whole genome sequencing of Listeria isolates , which allowed SSI to identify the outbreak and compare sequences from Listeria found in patients with those discovered in the food and production environment.

Testing of hummus samples from the store  this year confirmed that the Listeria was the same isolate.

Production of foods was halted in September and there have been no new cases. The shop is being allowed to sell packaged products and can make foods but only under strict conditions. but consumers were advised to throw out any high water activity foods, dates, hummus and olives in oil from the shop.

Identifying the source of an outbreak is occasionally easy but mostly it is a difficult task with many steps, and in quite a few cases the source simply can’t be confirmed.