Investigation of Progressive Inflammatory Neuropathy Among Swine Slaughterhouse Workers --- Minnesota, 2007---2008

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On October 29, 2007, the Minnesota Department of Health (MDH) was notified by a tertiary-care provider of unexplained neurologic illnesses among workers in a swine slaughterhouse (plant A) in southeast Minnesota. As a result, MDH initiated a detailed investigation at plant A to characterize the outbreak. This report describes the ongoing investigation and outbreak-control measures undertaken by state public health officials and CDC.

Plant A, located in southeastern Minnesota, employs approximately 1,200 workers and processes 18,000 pigs per day. After being notified of the illnesses, MDH investigators initiated active case finding, interviewed workers at plant A, and reviewed the plant's occupational health and employment records. As of January 28, 2008, a total of 12 workers at plant A had been identified with confirmed (eight workers), probable (two), or possible (two) progressive inflammatory neuropathy (PIN) (Box). Illness onset ranged from November 2006 through November 2007. Median age of the 12 patients was 31 years (range: 21--51 years); six patients were female. All 12 patients reported being healthy before the onset of neurologic symptoms.

Symptoms ranged from acute paralysis to gradually progressive symmetric weakness over periods ranging from 8 to 213 days. Severity ranged from minor weakness and numbness to paralysis predominantly in the lower extremities affecting mobility. Eleven patients had evidence of axonal or demyelinating peripheral neuropathy by electrodiagnostic testing. Cerebrospinal fluid was obtained from seven patients. All seven had elevated protein levels (median: 125 mg/dL; range: 75--231 mg/dL [normal: 14--45 mg/dL]) with no or minimal pleocytosis (median: 1 cell/dL; range: 1--73 cells/dL in a nontraumatic tap); five patients had evidence of inflammation on spinal magnetic resonance imaging (four patients in peripheral nerves or roots and one patient in the anterior spinal cord).

All 12 patients reported either working at or having regular contact with an area where swine heads were processed (known as the head table), which was located within a larger processing area in plant A known as the warm room. A case-control study was conducted among plant A workers to identify specific risk factors associated with illness. The 10 patients with confirmed or probable cases were included in the study, along with two stratified control groups: 1) a random selection of 48 healthy warm-room workers and 2) all 65 healthy head-table workers. Statistically significant (p<0.05) differences were calculated by chi-square test. Blood samples and throat swabs were collected from all consenting case-patients and controls. As of January 30, laboratory investigations had not identified any infectious agent from the blood and throat-swab specimens that would explain the occurrence of PIN.

Results of the case-control study indicated that case-patients (seven of 10, 70%) were significantly more likely to have worked at the head table than the warm-room controls (12 of 48, 25%) (odds ratio [OR]: 7.0; 95% confidence interval [CI] = 1.3--42.2; p = 0.009). Case-patients also were more likely to have removed brains or remaining skeletal muscle from the pig head (a process known as backing heads) (four of 10, 40%) than controls (two of 46, 4%) (OR: 15.3; CI = 1.8--163.4; p = 0.006). Among head-table workers, case-patients were significantly more likely to have removed brains or skeletal muscle from the head (four of seven, 57%) than head-table controls (eight of 65, 12%) (OR: 9.50; CI = 1.40--70.2; p = 0.01). Illness was not determined to be associated with previous travel outside or within the United States; exposure to chemicals, fertilizers, or insecticides; use of medications; or receipt of previous vaccinations.

An environmental assessment of the plant was conducted on November 28, 2007. Standard personal protective equipment (PPE) used by workers at plant A included hard hats, laboratory coats (including some that were short-sleeved), boots, hearing protection, eye protection, and specialized gloves that varied with the particular task of the worker. A compressed air device was used in the plant to harvest brain tissue from pig heads at the head table. The device was placed into the skull of the pig through the foramen magnum, and the force of the air disrupted the brain material into a liquefied form that made it easier to remove (a technique known as "blowing brains"). This technique caused generation of small droplets and splatter, possibly including aerosolized brain material, to which workers operating the device and others nearby might have been exposed. In response to the investigation, plant A voluntarily suspended harvesting of brains and instituted additional mandatory PPE on November 28, 2007, including face shields and long sleeves, for workers stationed at the head table and other workers who chose to use additional PPE.

Results of Case-Finding Survey

A survey of the 25 federally inspected swine slaughterhouses with >500 employees in the United States indicated that only three plants (plant A in Minnesota and plants in Nebraska and Indiana) reported recent use of compressed air to extract pig brains. To date, no cases of PIN have been identified in association with workers at the Nebraska plant. However, several workers at the Indiana plant have been preliminarily identified with neurologic illnesses and similar histories of exposure to head-processing activities at that slaughterhouse. Further assessments of these patients, and additional measures to identify other workers with illness, are being conducted in Indiana. As a result of this investigation, all three plants have stopped using compressed air to extract brain material.

Reported by: D Lachance, Mayo Clinic, Rochester; S Goyal, PhD, Univ of Minnesota, St. Paul; R Danila, PhD, A DeVries, MD, R Lynfield, MD, Minnesota Dept of Health. J Howell, DVM, J Wyatt, MPH, Indiana State Dept of Health. T Safranek, MD, Nebraska Dept of Health and Human Svcs. E Belay, MD, J McQuiston, DVM, L Schonberger, MD, J Sejvar, MD, Div of Viral and Rickettsial Diseases; S Brueck, National Institute for Occupational Safety and Health; J Adjemian, PhD, B Buss, DVM, J Gibbins, DVM, S Holzbauer, DVM, EIS officers, CDC.

Editorial Note:

This report summarizes an ongoing investigation of PIN, a syndrome that appears to be associated with swine slaughterhouse workers who process pig heads. Several clinical and laboratory features of this illness and the distinctive epidemiology associated with patients appear unique. Pigs slaughtered at plant A have passed inspection by the U.S. Department of Agriculture Food Safety and Inspection Service, and the investigation has not identified any foodborne risk to the general population.

The investigation in Minnesota indicates that PIN appears associated with having worked at the head table, where a compressed-air device was used to extract pig brains. In the process of blowing compressed air into the pig skull, brain material might have been splattered or even aerosolized, and workers might have been exposed through inhalation or contact with mucous membranes. One hypothesis for development of PIN is that worker exposure to aerosolized pig neural protein might have induced an autoimmune-mediated peripheral neuropathy (1,2). Additional investigation of the characteristics and causes of PIN is under way.

Whether compressed-air devices are being used for pig-brain extraction in other slaughterhouses or processing facilities, in the United States or internationally, is unknown. Clinicians should provide CDC with information regarding swine slaughterhouse workers who might have illnesses similar to PIN, including patients with peripheral neuropathy, myelopathy, or features of both. Clinicians who identify such patients should report the cases to their state health department and contact CDC at 770-488-7100.

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