A new human illness has been identified in pork packing plants, reportedly linked to employees who work in areas where pig heads are processed.

The disease, progressive inflammatory neuropathy (PIN), struck workers at two plants in Indiana and one in Minnesota, where a compressed-air device was used to extract pig brains.

The Centers for Disease Control is investigating that action, which is believed to have led to inhalation of aerosolized fragments of pig brains. During inhalation, a protein or other substance from the animal brains is suspected to trigger the workers’ immune systems into mistakenly attacking their own nerve tissue. Symptoms of PIN include numbness, tingling, difficulty walking and working. Some workers recover quickly, while others continue to have health issues.

The slaughterhouses in the two states were known to remove the brains of the swine, freeze and pack them for shipment to Korea and China as food.

A survey of slaughter plants in the United States found that only those plants identified in the two states, and a plant in Nebraska, were using compressed air to extract pig brains. As a result of the investigation, all plants have stopped this practice.

Discovery of the new illness at Quality Pork Processors (QPP) in Austin, MN, which slaughters pigs on one side of the building that it shares with Hormel Foods, which processes the pork, sparked the investigation. Minnesota state health officials have now broadened the probe at the QPP section of the plant to thousands of former meat packer workers going back a decade when the air-compression system was first installed.