ARTICLE IN BRIEF
Most pork plant workers who had developed an inflammatory neuropathy after being exposed to aerosolized pig brain tissue returned to work, but they continue to have headaches and pain in their necks, lower backs, and feet.
Workers at two pork processing plants who developed a mysterious neuropathy about a year and a half ago are doing significantly better, but many of them still have some symptoms and ongoing pain, according to Mayo Clinic physicians who are investigating the cases.
Jennifer A. Tracy, MD, a neuromuscular fellow at the Mayo Clinic in Rochester, MN, will report their findings at the AAN Annual meeting in late April in Seattle.
The Mayo doctors are tracking about two dozen workers who developed neuromuscular symptoms, including pain, fatigue, and weakness and tingling in their extremities. They have identified the condition as an inflammatory neuropathy, which they labeled specifically as an autoimmune sensory-predominant polyradiculoneuropathy.
The workers' illness has been blamed on exposure to aerosolized pig brain tissue in the processing plants. Investigators believe that the workers breathed in tiny bits of pig brain that set off an immune response that attacked their nervous system, particularly the nerve roots exiting the spinal cord and nerve endings in the hands and feet. Stereotypical abnormalities that confirmed those regions of involvement were found on neurophysiological testing.
Reports of the illness emerged in late 2007, and the cases also have been investigated by the Minnesota Department of Health and the federal Centers for Disease Control and Prevention.
“The patients are doing much better, but they have not fully recovered. They have some ongoing symptoms,” P. James Dyck, MD, associate professor of neurology and head of the Peripheral Nerve Section and Laboratory at the Mayo Clinic, told Neurology Today in a telephone interview. Most commonly, they have ongoing headaches, and pain in their necks, lower backs, and feet.
Dr. Dyck said that much of the lingering pain can be blamed on the continued involvement of the nerve roots and nerve terminals, since stretching nerve roots can cause neck and back pain and irritation of damaged nerve terminals can cause burning pain in the feet.
In an abstract released in advance of the conference, his team described the latest findings on 24 workers who range in age from 20 to 54. According to the abstract, 17 workers were given immunotherapy, most typically intravenous methylprednisolone or IV immunoglobulin, and improved with the treatment. Six other workers were not treated and showed spontaneous improvement; another worker was lost to medical follow-up. Most of the treatments were given for three months.
The workers, most of whom came from the Quality Pork Processors plant in Austin, MN, and a few from an Indiana facility, had jobs that involved the use of air compression equipment to clean brain tissue out of pig heads.
“This is unprecedented in terms of any kind of illness that has been associated with occupational exposure,” said Daniel Lachance, MD, assistant professor of neurology and a member of the Mayo Clinic's neuro-oncology section, who is caring for many of the workers and who co-authored the research report .
Dr. Lachance told Neurology Today that some workers report having pain that is not consistent with clinical testing, a fact that he said he could not explain. The majority of the workers have gone back to work, though they're not in the processing area anymore, he said.
The Mayo Clinic researchers plan to continue periodic testing of the workers. They also want to look more closely at why some plant workers got sick and others didn't. The researchers said there was no evidence that the illness had been overlooked for a long time. Rather, the workers got sick during a stretch when the processing was accelerated, apparently increasing their exposure to the aerosolized brain tissue, Dr. Lachance said.
“Understanding this disease and that it can be induced by breathing in aerosolized pig brain will help us understand other immune-mediated diseases,” Dr. Dyck said.
Commenting on the report, Herbert Schaumburg, MD, professor of neurology and a neurotoxicologist at Albert Einstein Medical Center in New York, said the Mayo researchers' investigation has been groundbreaking.
“The idea that you can inhale something like brain tissue and get inflammatory neuropathy is a revolutionary idea,” Dr. Schaumburg said. The fact that the workers' symptoms improved after receiving immunotherapy supports the researchers' belief that an inflammatory response triggered the problems, he added.
Arthur K. Asbury, MD, Van Meter Professor of Neurology Emeritus at the University of Pennsylvania, said: “It was pretty good fortune that the outbreak occurred in Minnesota” because the Mayo researchers were called in early to evaluate the cases. “They characterized the distinctive clinical and laboratory features of the sickened workers and recognized that the likely cause was an allergic response to aerosolized pig brain.”
After the cases of the sickened workers surfaced at the Minnesota and Indiana plants and at another facility in Nebraska, plant owners agreed to change their processing techniques, according to Doug Schultz, a spokesperson for the Minnesota Department of Health. He said the air-compression technique is no longer used and workers now wear better protective clothing, including plastic face shields instead of just eye goggles, longer rubber gloves and long-sleeved work coats.
“We have not seen any additional cases since those measures were taken,” Schultz said. He said his department's investigation is complete and the findings will be published in a medical journal, probably appearing with a full report by the Mayo researchers.
NERVE CONDUCTION STUDIES REVEAL PROBLEMS
According to the Mayo Clinic abstract, nerve conduction studies showed abnormalities in most of the 22 workers, including proximal or distal slowing in 19, prolonged distal latencies in 15, F-wave latencies in 16, and trigeminal blink latencies in 12. Abnormalities also were found on quantitative autonomic testing (sweat abnormality).
A neuroantibody, which has not been seen before, was identified in all of the pork workers studied, said P. James Dyck, MD, associate professor of neurology and head of the Peripheral Nerve Section and Laboratory at the Mayo Clinic. He added that MRI scans of nerve roots and peripheral nerves showed increased signal and nerve enlargement in many of the patients. The CSF protein was also elevated in many patients.
“The sensory-predominant polyradiculoneuropathy occurring in plant workers exposed to aerosolized pig brain produced characteristic demyelinating nerve conduction abnormalities involving very distal and very proximal nerves,” the abstract said. “Mild cases need specialized autonomic and sensory tests to detect abnormality.”