By Katherine Dillinger and Neha Mukherjee, CNN

(CNN) — Twelve deaths have been reported as part of an outbreak of Listeria monocytogenes infections linked to frozen supplemental shakes, the US Food and Drug Administration said Monday. At least 38 people in 21 states had been infected, with 37 of them needing to be hospitalized.

The Sysco Imperial and Lyons ReadyCare shakes are manufactured by Prairie Farms Dairy Inc. in Fort Wayne, Indiana, and were mainly distributed to hospitals and long-term care facilities. Prairie Farms and distributor Lyons Magnus have recalled the shakes.

The FDA is conducting an onsite investigation.

The US Centers for Disease Control and Prevention began investigating these illnesses in 2018, it said Monday, but there was not enough information to identify a specific cause of Listeria infections in long-term care facilities and nursing homes. The agency reopened the investigation in October, after six new cases were reported, and the outbreak strain of bacteria was discovered in Prairie Farms samples this month.

There are probably more cases than what has been reported, according to the CDC.

Recalled products should not be consumed. Additionally, the FDA recommends that people clean areas that may have been in contact with the products such as refrigerators, containers and surfaces.

Sysco said in a statement Friday that it has recalled the supplemental shakes, notified customers and halted purchases of other products supplied by Lyons Magnus from the Prairie Farms Dairy facility in Fort Wayne.

“Sysco expresses our most sincere condolences with those affected by this outbreak and their families,” the company said. “Food safety is and will continue to be our top priority, and our focus will always be on improving these programs that protect our customers and our communities from foodborne illnesses.”

Listeria, a type of bacteria, is the third leading cause of death from foodborne illness in the US, killing about 260 people a year.

Symptoms may start days or weeks after consuming the bacteria and can include fever, muscle aches and fatigue. An infection can also cause a stiff neck, a headache, confusion or seizures. Those most at risk of severe outcomes are older adults, pregnant people and those with weakened immune systems.

Anyone who develops symptoms of Listeria infection should get medical care right away. Most people won’t need treatment, but antibiotics may be needed in severe cases.

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