Doctors at the Mayo Clinic and government public health experts have confirmed the mysterious illnesses in 24 slaughterhouse workers in Minnesota and Indiana from 2006 to 2008 was caused by an autoimmune response to a mist of pig brain tissue.
Their article was published Monday in the British medical journal Lancet Neurology. Mayo Clinic neurologist Dr. Daniel Lachance, the lead author, said 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.
"This was really a kind of unique experiment of nature where an unusual form of harvesting a part of an animal was utilized and inadvertently exposed individuals through their respiratory tract or their eyes or mouth and ended up triggering an autoimmune response in their own bodies," he said.
The immune response attacked the nervous systems of the 21 workers in Minnesota and three in Indiana from November 2006 to May 2008, causing painful symptoms that included weakness and fatigue to confusion and seizures.
All are improving and most no longer have measurable symptoms, Lachance said, although two may have permanent damage.
All the patients worked in or near areas where compressed air was used to extract pig brains, which are considered a delicacy in some Asian countries. It was a rarely used process then, he said, and he knows of no slaughterhouses that still use it.
Lachance said the Minnesota patients told them the symptoms started to appear within weeks of a speed up on the production line in 2006. "The line speed, the line speed, that's what we heard over and over again," he said.
At slower speeds, he said, workers were able to use compressed air to blast the brains down out of the head and into a bucket under the table.
"But it sounds as if as the line speed increased, the operator was not able to handle the process properly and as a consequence this material was being directed in all directions," Lachance said. "That's our best estimate of what was really going on."
Lachance said it's interesting that blood drawn by the Health Department from more than 85 other employees of the Minnesota slaughterhouse showed that 29 of them had antibodies indicating they were exposed to the brain mist, but didn't get sick. It's not clear why.
"There are actually many, many examples of this in medicine," he said. "There are people who have antibodies to HIV, yet don't have AIDS. There are many instances where the antibodies are the markers for a disease, but aren't a main factor in the appearance of the disease."
The researchers also found a strong connection between how sick workers got and how closely they worked to the head table, he said.
The paper doesn't report on the exact biological mechanism of the disease and Lachance said it might never been known. The slaughterhouses stopped removing brains with compressed air in late 2007, and the Mayo Clinic hasn't seen a new case to study in more than a year.
"Essentially, this is something that has come and passed," he said.
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 in September 2007. 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.
QPP is a privately owned supplier to Hormel Foods Co. The name of the slaughterhouse in Delphi, Ind., has not been made public.
Minnesota Health Department epidemiologist Dr. Aaron DeVries said his department hopes to have its own article published soon. It will focus more on what changed in the slaughterhouses in 2006 to prompt the outbreak and the work done nationally by public health authorities to identify other victims.