Pig brains are sold as food, though not commonly.
Workers at Quality Pork Processing used compressed air hoses inserted into the pig's skull cavity to pulverize the brain matter and push it out the snout or the base of the skull.
Some industry experts say the high pressure hoses splattered brain tissue that may also have created a fine mist of brain matter. Workers weren't wearing face masks, so they could have inhaled the material.
The head of Mayo Clinic's Peripheral Nerve Section is Doctor P. James B. Dyck. He has been seeing workers from the Quality Pork Processing plant who have symptoms of a neurological illness.
Dyck says doctors aren't drawing any conclusions yet, but it's possible that contact with neural material could have led to an autoimmune response. That's when the body's immune system attacks itself.
"I think that's a very intriguing idea," he says. "That neural tissue could have perhaps set up some sort of immune response causing this type of neuritis."
Neuritis is the inflammation of the peripheral nervous system. That system controls much of voluntary and involuntary movement, like picking up a pencil or dilating pupils.
Dyck says researchers have been able to induce an allergic neuritis which eventually led to a nervous system disease. But Dyck says at this point there is no established link between contact with pig brains and an auto-immune response. Another potential cause could be a virus.
Dyck says two more workers are about to be tested at the Mayo Clinic. About half of the 11 workers who have already been tested showed signs of the disease Chronic Inflammatory Demyelinating Polyneuropathy (C.I.D.P.). The disease slowly weakens muscles and causes numbness.
However, after examining the workers, Dyck says it's unlikely these workers actually have the disease.
"These patients don't tend to have all the same symptoms and findings," he says. "There are some similarities on some testing, but there are also major differences."
Dyck says the difference is important because C.I.D.P. is a chronic disease. He says so far all they know is that this group of people have inflammatory nervous system diseases.
Mayo Clinic neurologist Doctor Daniel Lachance says it's too early to say whether these patients will recover.
"I can say that I've had one patient who I've followed without treatment who has improved enough to return to the same workplace, not to the same job. I made sure of that," he says.
Minnesota State Epidemiologist Ruth Lynfield says state health department officials are interviewing between 80 and 100 workers at Quality Pork Processing. Those include all of the people who worked at the table where pig heads were processed and a random sample of people from the production floor.
Lynfield says officials are questioning workers about exposures and symptoms they may have experienced.
"It's a long interview," she says. "Whenever you do this kind of investigation you are essentially shaking the bushes and looking for additional potential cases."
Quality Pork Processing volunteered to stop using the compressed air hoses, but Lynfield says she isn't sure what role the equipment played in this outbreak. She says she believes the hoses have been used for several years before the first worker showed signs of illness last year.
"But there may have been some changes perhaps associated with either how they were using it or the frequency. Not clear. That's part of the investigation," she says.
Lynfield says she expects it will be about two weeks before any significant developments in the case.