After months of research, doctors at the Mayo Clinic say they can show that the mysterious neurological condition discovered among pork-plant workers in Austin, Minn., is a unique and identifiable disease.

Mayo researchers presented their findings at the American Academy of Neurology conference in Chicago on Wednesday. It is the first time a detailed medical picture of the disease, called progressive inflammatory neuropathy (PIN), was provided at a national medical forum.

The discovery of a new disease or condition always generates excitement among medical researchers, especially when it can be clearly diagnosed. This one is also important because it could help shed light on the body's immune system and a family of similar autoimmune disorders that aren't well understood.

"Some of the greatest insights in medicine relate to things that are discovered incidentally," said Dr. Daniel Lachance, one of the Mayo clinic researchers studying the disease. "Maybe this is one."

The Mayo Clinic doctors are participating in a disease investigation that began in early December that now includes the Minnesota Department of Health, the federal Centers of Disease Control and Prevention, two other state health departments and various other entities. It was launched after 11 workers at the Quality Pork Processors plant in Austin, Minn., reported unusual and sometimes debilitating pain, weakness and numbness in their arms and legs.

Since then, the investigation has expanded to pork processing plants in Indiana and Nebraska. A total of 17 cases have met the precise definition investigators are using for the disease, said Ruth Lynfield, Minnesota's state epidemiologist, who is heading the investigation.

All of the employees worked in an area of the plants where pig brains were removed from skulls with a powerful air compression system that sprayed blood and brain tissue into the air.

After ruling out toxins and infectious agents, investigators are now focused on whether the workers' immune systems reacted to pig protein they inhaled from bits of brain tissue floating in the air around them.

Lachance, a neurologist who was among the first to recognize the unusual cluster of cases at the Austin plant, said that he and others at Mayo have found a unique antibody, a type of protective cell, that proves the workers' immune systems reacted to a specific trigger.

"This antibody is unique to that trigger," he said.

Whether or not pork protein is the trigger is under investigation at another lab at Columbia University in New York. The results of that will not be known for a few months, according to Lynfield.

Lachance said that all of the 18 workers used as a basis for the presentation on Wednesday shared the unique antibody. Antibodies, part of the body's defense system, attack foreign substances or infectious agents that are harmful.

But the immune system is not always discriminating. Sometimes it attacks the body's own tissue, resulting in lupus, arthritis, rheumatic fever, multiple sclerosis and dozens of other auto-immune diseases. Often what triggers those diseases is unknown.

Lachance said that the antibody that Mayo researchers identified in the pork-plant workers was affecting their nerves. It makes sense, he and others involved in the investigation said, because pig tissue is quite biologically similar to human tissue. It may be that the workers' immune systems created the antibodies to fight the foreign pig tissue that entered their bodies, but then also attacked their own nerve tissue, causing inflammation, tingling, weakness and pain.

But the second half of that theory -- that such a protein triggered the antibody -- has yet to be proven, Lachance said. If it is, it would provide an unusual chance to study an autoimmune disease caused by a known trigger from beginning to end, he said.

The conference is also the first time he and other Mayo neurologists were able to share the medical details of the PIN cases with other experts.

Initially, he said, the cases that showed up were quite severe. Some workers were unable to work for weeks or months, and a few are still unable to work. But as the investigation evolved, he began seeing patients with much milder symptoms as well. All three plants have stopped using the air compressor system, but new patients with suspect symptoms continue to come forward, he said.

The Minnesota health department is also reviewing old cases to find out whether workers in the past have reported symptoms that were not recognized as unique. Lachance, who worked as a neurologist at Mayo's Austin clinic, now recalls patients who came to see him with the same symptoms. Some have come back and are being treated. One is a patient who became ill after he left Quality Pork Processing and entered the military. His symptoms were so severe he was discharged, Lachance said.

It illustrates why recognizing the disease was difficult, he said. The meatpackers stand for long hours in an intensely physical job. When they complain that their legs and arms hurt, it's easy to assume that their discomfort is the result of fatigue or repetitive stress, he said.

Josephine Marcotty • 612-673-7394