February 25, 2009 — More than a year after developing a novel neurological illness, apparently caused by exposure to aerosolized pig brains, a cluster of 24 workers from 2 pork-processing plants in the United States have all improved. However, none has complete resolution of symptoms, and many have ongoing pain.
Follow-up findings in confirmed cases of what investigators call immune polyradiculoneuropathy or progressive inflammatory neuropathy were released February 24 but will be formally presented at the upcoming American Academy of Neurology 61st Annual Meeting, in Seattle, which will take place April 25 to May 2.
In an interview with Medscape Neurology & Neurosurgery, study investigators Daniel H. Lachance, MD, and James B. Dyck, MD, both from the Mayo Clinic in Rochester, Minnesota, said most patients have experienced at least partial resolution of their presenting symptoms.
"The initial symptoms of pain, fatigue, difficulty walking, and feeling generally ill — those symptoms by and large are much better. The thing that tends to be an ongoing problem for many is pain — either in the sense of headache or, more often, pain in the hands and feet," said Dr. Dyck.
In fact, he added, on objective clinical measures that include the Neuropathy Impairment Score (NIS), the Neuropathy Symptoms and Change (NCS) questionnaire, nerve-conduction studies, serologic testing, and magnetic resonance imaging (MRI) all patients were "highly significantly improved," he added.
However, despite these objective indications of improvement, many individuals continue to report chronic pain, which Dr. Dyck characterized as "severe" in a minority of patients.
The reason for a discrepancy between objective findings that indicate significant improvement and ongoing pain in some patients is not clear.
Reports of the illness, which include symptoms ranging from a transverse myelitis syndrome (in a single patient) to pain, numbness, and tingling in the extremities, fatigue, and weakness, were first made public in December 2007 following an investigation by the Centers for Disease Control and Prevention (CDC).
All of the affected individuals had jobs harvesting pig brains, which involved the use of a compressed-air gun to remove the organs from the animals' skulls.
The hypothesis is that the air gun aerosolized the brain matter, turning it into a fine mist, which workers then inhaled, setting up an immune response and subsequent inflammation in the spinal cord and root nerves.
All identified cases underwent standardized clinical and laboratory testing at the Mayo Clinic's peripheral nerve center, including assessments of autonomic nervous system function, cerebral spinal fluid (CSF) analysis, MRI, and serologic testing.
Researchers found that 1 of the defining characteristics of the condition that was present in all patients was a novel profile of neuronal antibodies, including an IgG immunostaining pattern.
At that time, Dr. Lachance told Medscape Neurology & Neurosurgery that this novel antibody pattern was unique in the experience of the Mayo Clinic's neuroimmunology lab, which analyzes 30,000 serum samples annually.
"We had never seen this particular antibody pattern before," he said.
In addition, CSF and MRI findings revealed the majority of patients had elevated CSF protein levels and enlarged, sometimes thickened, spinal roots.
Cause and Effect
Patients were treated according to symptom severity. The mainstay of therapy was immunosuppression that included either high-dose intravenous immunoglobulin (IVIG) or intravenous methylprednisolone.
"The rationale [for therapy] was that this was an autoimmune process and therefore the patients' own immune systems were attacking their nervous systems, so we treated them with agents that would suppress the attack," said Dr. Dyck.
Following the first reports of the illness, the CDC and the Minnesota Department of Health conducted a joint investigation of 3 pork plants with suspected cases. These were located in Minnesota, Indiana, and Nebraska. However, only patients from the Minnesota and Indiana plants were seen at the Mayo Clinic.
According to Aaron DeVries, MD, a medical epidemiologist at the Minnesota Department of Health, all plants voluntarily stopped using compressed-air guns to process pig brains.
In addition, Dr. DeVries said the team that conducted the initial investigation in the Minnesota plant developed a number of additional safety recommendations for personal protective equipment that were also adopted by the other 2 plants in Nebraska and Indiana.
These related to additional personal safety equipment and included replacing protective goggles with a face shield to protect the nose and mouth from direct exposure and the addition of long-sleeved in place of short-sleeved coats to cover previously exposed skin on the arms. In addition, all employees continue to wear protective gloves and boots.
Since the compressed-air guns have been removed and these additional protective measures implemented, no new cases of the condition have emerged, suggesting cause and effect. "There is strong evidence to support that exposure to aerosolized brain tissue was the cause of the illness," Dr. DeVries told Medscape Neurology & Neurosurgery.
In addition, he said, an international investigation conducted by the CDC indicates there are no other locations employing this type of device in animal processing.
The Mayo Clinic clinicians will continue to follow these patients longitudinally. They also are conducting further research in a bid to better understand this unique condition. Among other issues, they are looking into potential differences between workers who developed the condition and those who were similarly exposed but who did not get sick.
In addition, said Dr. Lachance, they are hopeful that a better understanding of this unique autoimmune illness will provide insight into other autoimmune conditions with unknown triggers.
Specifically, Dr. Lachance said he is interested in looking at potential parallels between the immune response in this condition and those involved in paraneoplastic neurological diseases, which accompany various cancers and are triggered by an immune system response to malignancy.
"There are very few instances of this type of occurrence in the history of medicine, so it is almost unprecedented. This manmade experiment of nature will allow us to observe and document, with the many tools available to us, the pattern of injury to the nervous system in this type of circumstance," said Dr. Lachance.
The investigators report no relevant conflicts of interest.
American Academy of Neurology 61st Annual Meeting. Released February 24, 2009
Medscape Medical News © 2009 Medscape
Cite this: Caroline Cassels . AAN 2009: Pork Workers With Novel Neurological Illness Much Improved, but Pain an Ongoing Issue for Many - Medscape - Feb 25, 2009.