MINNEAPOLIS — The mysterious illnesses in 24 slaughtering facility workers in Minnesota and Indiana from 2006 to 2008 were caused by an auto-immune response to a mist of pig brain tissue, Mayo Clinic doctors and government public health experts confirmed, according to The Associated Press. The experts’ confirmation on this topic was published in the British medical journal Lancet Neurology on Nov. 30.

Dr. Daniel Lachance, a Mayo Clinic neurologist and the lead author, relays it was the first comprehensive account of the outbreak and response from Mayo, the state Health Department and the Centers for Disease Control and Prevention.

According to the experts, the immune response attacked the nervous systems of 21 workers in Minnesota and three in Indiana from November 2006 to May 2008, causing painful symptoms, ranging from weakness and fatigue to confusion and seizures.

Dr. Lachance said all affected plant workers are improving and most no longer have measurable symptoms, although two may have permanent damage. All of the stricken employees worked in or near areas where compressed air was used to extract pig brains. It was a rarely used process, he said, and he knows of no slaughterhouses that still use it.

The Minnesota patients told doctors the symptoms started to appear within weeks of a speed-up on the production line in 2006. Workers were able to use compressed air to blast the brains down out of the head and into a bucket under the table at slower speeds, he said. But as the line speed increased, the operator was apparently unable to handle the process properly — as a result, the material was being directed in all directions, Dr. Lachance estimates.

Blood taken by the Health Department from more than 85 other employees of the Minnesota slaughterhouse indicated 29 of them had antibodies indicating they were exposed to the brain mist, however, they didn't get sick. It's not clear why, Dr. Lachance said.

Researchers also discovered a strong connection between just how sick workers became and how closely they worked to the head table, he said. The exact biological mechanism of the disease wasn’t identified in the study and it might never been known, Dr. Lachance said.

Late in 2007, the slaughterhouses stopped removing brains with compressed air, and the Mayo Clinic hasn't seen a new case to study in more than a year.

In September 2007, the cluster of the unusual neurological disease was first identified by the Mayo Clinic among workers at the Quality Pork Processors plant in nearby Austin, Minn. Q.P.P. is a privately-owned supplier to Austin-based Hormel Foods Co. The clinic reviewed its records and discovered it saw its first patient with the symptoms in November 2006. The last patient was in May 2008.

The slaughterhouse in Delphi, Ind., was never identified by health officials, although Indiana Packers operates a large hog slaughtering facility there.