Part II

[B]Second part of New York Times story:[/B]

But why should exposure to hog brains cause illness? And why now, when the compressed air system had been in use in Minnesota since 1998?

At first, health officials thought perhaps the pigs had some new infection that was being transmitted to people by the brain tissue. Sometimes, infections can ignite an immune response in humans that flares out of control, like the condition in the workers. But so far, scores of tests for viruses, bacteria and parasites have found no signs of infection.

As a result, Dr. Lynfield said the investigators had begun leaning toward a seemingly bizarre theory: that exposure to the hog brain itself might have touched off an intense reaction by the immune system, something akin to a giant, out-of-control allergic reaction. Some people might be more susceptible than others, perhaps because of their genetic makeup or their past exposures to animal tissue. The aerosolized brain matter might have been inhaled or swallowed, or might have entered through the eyes, the mucous membranes of the nose or mouth, or breaks in the skin.

“It’s something no one would have anticipated or thought about,” said Dr. Michael Osterholm, an epidemiologist who is working as a consultant for Hormel and Quality Pork. Dr. Osterholm, a professor of public health at the University of Minnesota and the former state epidemiologist, said that no standard for this kind of workplace exposure had ever been set by the government.

But that would still not explain why the condition should suddenly develop now. Investigators are trying to find out whether something changed recently — the air pressure level, for instance — and also whether there actually were cases in the past that just went undetected.

“Clearly, all the answers aren’t in yet,” Dr. Osterholm said. “But it makes biologic sense that what you have here is an inhalation of brain material from these pigs that is eliciting an immunologic reaction.” What may be happening, he said, is “immune mimicry,” meaning that the immune system makes antibodies to fight a foreign substance — something in the hog brains — but the antibodies also attack the person’s nerve tissue because it is so similar to some molecule in hog brains.

“That’s the beauty and the beast of the immune system,” Dr. Osterholm said. “It’s so efficient at keeping foreign objects away, but anytime there’s a close match it turns against us, too.”

Anatomically, pigs are a lot like people. But it is not clear how close a biochemical match there is between pig brain and human nerve tissue.

To find out, the Minnesota health department has asked for help from Dr. Ian Lipkin, an expert at Columbia University on the role of the immune system in neurological diseases. Dr. Lipkin has begun testing blood serum from the Minnesota patients to look for signs of an immune reaction to components of pig brain. And he expects also to study the pig gene for myelin, to see how similar it is to the human one.

“It’s an interesting problem,” Dr. Lipkin said. “I think we can solve it.”

Susan Kruse, who lives in Austin, was stunned by news reports about the outbreak in early December. Ms. Kruse, 37, worked at Quality Pork for 15 years. But for the past year, she has been too sick to work. She had no idea that anyone else from the plant was ill. Nor did she know that her illness might be related to her job.

Her most recent job was “backing heads,” scraping meat from between the vertebrae. Three people per shift did that task, and together would process 9,500 heads in eight or nine hours. Ms. Kruse (pronounced KROO-zee) stood next to the person who used compressed air to blow out the brains. She was often splattered, especially when trainees were learning to operate the air hose.

“I always had brains on my arms,” she said.

She never had trouble with her health until November 2006, when she began having pains in her legs. By February 2007, she could not stand up long enough to do her job. She needed a walker to get around and was being treated at the Mayo Clinic.

“I had no strength to do anything I used to do,” she said. “I just felt like I was being drained out.”

Her immune system had gone haywire and attacked her nerves, primarily in two places: at the points where the nerves emerge from the spinal cord, and in the extremities. The same thing, to varying degrees, was happening to the other patients. Ms. Kruse and the index case — the man who extracted brains — probably had the most severe symptoms, Dr. Lachance said.

Steroids did nothing for Ms. Kruse, so doctors began to treat her every two weeks with IVIG, intravenous immunoglobulin, a blood product that contains antibodies. “It’s kind of like hitting the condition over the head with a sledgehammer,” Dr. Lachance said. “It overwhelms the immune system and neutralizes whatever it is that’s causing the injury.”

The treatments seem to help, Ms. Kruse said. She feels stronger after each one, but the effects wear off. Her doctors expect she will need the therapy at least until September.

Most of the other workers are recovering and some have returned to their jobs, but others, including the index case, are still unable to work. So far, there have been no new cases.

“I cannot say that anyone is completely back to normal,” Dr. Lachance said. “I expect it will take several more months to get a true sense of the course of this illness.”

Dr. Lynfield hopes to find the cause. But she said: “I don’t know that we will have the definitive answer. I suspect we will be able to rule some things out, and will have a sense of whether it seems like it may be due to an autoimmune response. I think we’ll learn a lot, but it may take us a while. It’s a great detective story.”