A recall issued in December for bags of fresh broccoli sold at Walmart stores for listeria concerns has been escalated by the Food and Drug Administration.

Upgrading the recall alert to a class one means the agency determined there's a higher risk of serious health consequences or even death associated with consuming the product.

The bags of ready-to-eat, Marketside-branded broccoli florets were sold at Walmart locations in at least 20 states with a "best if used by" date of Dec. 10. Officials said customers may have frozen the product and said they should throw it away immediately, if so.

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The recall was voluntarily issued by Braga Fresh after health officials in Texas discovered the presence of listeria in product samples from a Walmart store.

So far, there have not been any reported illnesses or deaths associated with the recall.

Listeria infections typically cause fever, muscle aches and tiredness and may cause stiff neck, confusion, loss of balance and convulsions. Symptoms can occur quickly or up to 10 weeks after eating contaminated food, according to the Centers for Disease Prevention and Control.

The infections are especially dangerous for older people, those with weakened immune systems or those who are pregnant.