MINNEAPOLIS (AP) -- The number of mysterious neurological illnesses among workers who processed pig brains at pork plants in Minnesota, Indiana and Nebraska has grown to up to 24, and other possible cases are being evaluated, researchers said Wednesday.
Dr. Daniel Lachance, a Mayo Clinic neurologist, said there are now 18 confirmed cases among people who have worked at the Quality Pork Processors Inc. plant in Austin. That's up from 13 cases that were reported as of February.
Lachance also said there are now around five cases among workers at a pork plant in Indiana -- a number he called a ''guesstimate'' -- compared with two confirmed earlier, and one recently identified case at a plant in Nebraska. Officials have not publicly named the Indiana and Nebraska plants.
Lachance spoke in a teleconference from Chicago, where he and other researchers presented details about the ongoing investigation at a neurology conference hosted by the St. Paul-based American Academy of Neurology. He said the common thread among the affected workers is that they all worked in a part of the plants that used compressed air to blow pig brains out of skulls.
The working hypothesis, he told reporters, is still that some of the brain tissue was turned into a fine mist during the process, the workers became exposed to it and somehow developed an autoimmune response that caused nerve damage.
''The precise mechanism by which that is occurring, we do not yet understand,'' Lachance said.
Common symptoms include pain, weakness, fatigue and numbness. A unique pattern of antibodies has been found in all the patients, Lachance said.
A Spanish-language interpreter at an Austin clinic and plant nurses realized last year they were seeing a pattern of similar illnesses among the workers. The Mayo Clinic reported 12 cases to the state Health Department last November. In January, the Centers for Disease Control and Prevention gave the condition a name -- progressive inflammatory neuropathy, or PIN.
The 18 patients identified by the Mayo Clinic developed their symptoms from the end of November 2006 through the first week of this month.
But the illness might have appeared earlier. Lachance said he evaluated a 22-year-old woman in Austin in 2004 with some similar symptoms, but she refused to have a spinal tap, has since returned home to Mexico and is not being studied. He said another patient he first saw in November 2005 is still being evaluated.
Researchers still don't think the general public is at risk.
''It doesn't appear that the slaughtered pigs have been ill,'' said Dr. James Sejvar, a neurologist and epidemiologist at the CDC. ''It doesn't appear that this is in any way a foodborne illness. And it doesn't appear as if this particular illness can be transmitted person to person.''
Lachance said none of the patients have recovered completely, though all have either improved or stabilized to a degree. He also said some have had relapses. Some of the patients have required only pain medication, while the most seriously ill have undergone drug treatments to suppress their immune systems.
The three plants are the only ones investigators have found in the U.S. that used compressed air to harvest pig brains, which are considered a delicacy in some Asian countries, and all have discontinued the practice. Sejvar said the CDC has been working with the World Health Organization to see if the procedure has been used in any plants abroad but so far they haven't heard of any.
Even if the illness turns out to be an isolated problem, Lachance said he's hopeful that researchers will be able to apply what they've learned from it to other autoimmune illnesses. Scientists still don't know what triggers many of them, he said.
In another development Wednesday, the Quality Pork Processors plant won a workplace safety award from the American Meat Institute. It was one of 41 winners of the Award of Honor, the highest category among the institute's three levels of worker safety recognition awards. The industry group honored 121 plants in all.
The awards are based on a point system developed by the National Safety Council, which reviews each plant's award application. Institute spokesman Dave Ray said the awards measure the effectiveness of a plant's total health and safety program, not its response to a single situation.
Other plants with Minnesota connections winning awards were several Cargill and Hormel plants across the country, as well as three other plants in Minnesota.
Quality Pork and union officials did not immediately return phone calls seeking comment on the award.
Sejvar said Quality Pork officials have cooperated with the investigating authorities.
''I would have to say that the collaboration and cooperation has been excellent,'' he said.