Eleven workers at an Austin, Minn., pork processing plant mysteriously fell ill between last December and July with a neurological disorder whose cause remains unknown, state health officials said Monday.

The condition afflicting five of the workers at Quality Pork Processors Inc. has been identified as a rare disease called chronic inflammatory demyelinating polyneuropathy or CIDP, which normally strikes fewer than two people per 100,000. In this instance, it may have struck 11 out of about 100 people in a particular part of the plant, state officials said. It is most often a chronic disease that results in nerve damage and can lead to disability.

Never before have so many cases of this type occurred in a particular locale, specific type of work, or in association with a particular animal, experts said.

State health officials said there is no evidence to date that the public faces an increased risk or that the food supply has been affected.

The 1,300-employee Quality Pork Processors is a hog slaughtering and processing operation that was spun off from Hormel's Austin plant in 1989. In 1995, CEO Kelly Wadding bought the firm, which makes meat products for Hormel and other food companies. The privately owned company had an estimated $280 million in revenue in 2005.

With CIDP, something -- perhaps a vaccine, a virus or a bacteria, or something else altogether -- triggers the immune system to attack the protective sheath that surrounds nerves, said Dr. Suraj Muley, an associate professor of neurology at the University of the Minnesota and an expert on the disease.

In the case of the affected workers, "the question is whether the animal might harbor bacteria or a virus that triggered it," Muley said.

Doctors' observations

Dr. P. James B. Dyck , an expert on the disease at the Mayo Clinic in Rochester who is familiar with some of the recent cases, said that an infectious agent is less likely than some other unknown factor among the workers or the plant.

"Everyone worries about infection," he said. "But more likely there is something there that is triggering an immune response," he said. Dyck's father, also named P. James Dyck, was the first to identify the disease in 1975, and still practices at the Mayo Clinic.

Muley, who is not involved in the medical investigation, suggested that the Austin plant be shut down until investigation is complete because if it is an infectious agent, then other workers might be at risk.

But Dyck and state health officials said there is no cause for alarm.

"All of the information we have to date indicates that the general public is not at increased risk for developing this type of illness," said State Health Commissioner Sanne Magnan. "There is no evidence that the food supply has been affected."

The disease is an inflammation of the nervous system that can cause muscle weakness, tingling sensations in the arms and legs and pain over several months, Magnan said. Two of the workers were hospitalized but have since been released, and all are recovering. But some workers may experience "residual numbness or weakness after treatment," she said.

Pattern of symptoms emerged

The state investigation began at the end of October after plant health officials reported to the Health Department an unusual pattern of symptoms that was restricted to a group of employees working in a single area of the hog butchering process.

That part of the production line uses compressed air to clear away unwanted brain tissue so that meat in the head of the pig can be removed, Wadding said.

A study of the affected pork plant workers, who are of different ages, genders and ethnic groups, showed that their work area was the only thing they all had in common, said state epidemiologist Dr. Ruth Lynfield. Existing diseases that can make people predisposed to the illness, such as lymphoma or diabetes, were ruled out as causes, she said.

"This is a very unusual occurrence," Lynfield said. "We are working very hard with Quality Pork Processors and many partners in public health, environmental health, medicine, veterinary medicine, agriculture and the swine industry to determine the cause."

During the investigation, the plant has continued to operate but it has stopped using compressed air in the processing of hog heads and has issued safety goggles to workers, Wadding said. Workers also are being provided with sleeves to wear over their arms (they already wore gloves) and towels for showers after work.

No lawsuits have been filed in connection with the illnesses, Wadding said. Most of the 11 affected workers either have returned to the plant or never left work, he said.

Although the investigation is at an early stage, the cluster of illnesses was revealed Monday in the interests of full disclosure and because the state will be widening its probe by checking to see whether the illness has occurred at other processing plants around the country, Magnan said.

Wadding said his company is anxious to know what happened. "If there is found to be a cause here, it will be a novel one because we've been told this disease has never been tied to any animals or workplace before," he said. "But there are some many variables and possible causes out there that the investigation has not begun to scratch all the possibilities yet."