A Listeria outbreak involving Poland’s BK Salmon that has sickened four Danes, killing one, since May could have been worse.

The Dansk Supermarket Group with 1,500 stores in four countries, but only sold BK Salmon in Denmark. Danish health authorities say the surviving three victims are in very severe conditions and may be terminally ill.

According to Polish media, BK Salmon did not deliver any products to Dansk Supermarkets in Sweden, Germany or Poland. BK Salmon was one of the chain’s biggest suppliers in Denmark. Dansk removed all BK products from its shelves.

Dansk says it is both looking for other salmon suppliers and working with BK Salmon on issues related to the outbreak. The Danish Veterinary and Food Administration discovered the BK Salmon contaminated with Listeria.

A spokesman for BK Salmon said the company was taking all possible actions to cooperate and resolve the issues surrounding the outbreak. It does not yet know the source of the contamination.

While Denmark is bearing the burden of the Listeria outbreak, Polish media reports consumers in that nation are “overwhelmed by health concerns” over the poison salmon reports. Polish Netto stores want to help calm customers, so the chain is accepting returns of any salmon without the need for receipts.

Zelistrzewo, Poland, near the Baltic Sea, is the headquarters of BK Salmon. The company is known for making smoked slices using fresh Norwegian salmon.

It can take up to 70days after exposure to Listeria monocytogenes bacteria for symptoms to develop. The current outbreak began in May with one Dane sickened, and the three others followed August. That leaves open the possibility that more outbreak victims could be identified.

Food contaminated with Listeria monocytogenes causes listeriosis. The serious infection primarily affects older adults, people with weakened immune systems, and pregnant women and their newborns. Less commonly, people outside these risk groups develop serious cases of listeriosis.

Listeriosis can cause fever, muscle aches, headache, stiff neck, confusion, loss of balance and convulsions, sometimes preceded by diarrhea or other gastrointestinal symptoms.

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