Dr. Daniel Lachance, a Mayo Clinic neurologist, said there are now 18 confirmed cases among people who have worked at the Quality Pork Processors plant in Austin, Minn. That's up from 13 cases reported as of February.
Lachance also said there are now about five cases among workers at a pork plant in Indiana, 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.
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, Lachance said. All the plants have discontinued the practice.
The working hypothesis, he told reporters, is still that some of the brain tissue was turned into a fine mist during the process, and that 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 in 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 that he evaluated a 22-year-old woman in Austin in 2004 with some similar symptoms, but that 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 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 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.
Sejvar said the CDC has been working with the World Health Organization to see whether the procedure has been used in any plants abroad. So far, they haven't heard of any.
Even if the illness turns out to be an isolated problem, Lachance said he hopes researchers will be able to apply what they've learned to other autoimmune illnesses. Scientists still don't know what triggers many of them, he said.
The Quality Pork Processors plant on Wednesday 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.