The US Centers for Disease Control and Prevention issued an update last week to its investigation of an outbreak of a paralysing condition that is affecting certain meat processing plant workers who use compressed air to remove the brains from the heads of pig carcases.

The illness is called Progressive Inflammatory Neuropathy (PIN) and its symptoms range from acute paralysis to gradual increase of weakness on both sides of the body, which in some cases happens over 8 days and in others over 213 days. The symptoms vary in severity from slight weakness and numbness to paralysis that affects mobility, mostly in the lower extremities.

The current thinking, which is yet to be proved, is that the meat workers are being exposed to splatter and aerosol droplets of pig brain tissue created by the compressed air blast, which liquefies the tissue before expelling it from the pig skull. Once inhaled, small particles of pig brain tissue are then is attacked by the worker’s immune system which uses antibodies that also attack the body’s own almost identical human nerve tissue.

Following the PIN outbreak in a Minnesota meat processing plant, the CDC launched a nationwide survey of large slaughterhouses and found two more meat plants that had used the compressed air system recently. One of the plants was also reporting cases of a neurologic illness among its meat processing workers.

The outbreak came to the attention of the Minnesota Department of Health (MDH) in October last year, when it started getting reports of an unexplained neurologic illness among meat processing workers in a swine slaughterhouse in Southeast Minnesota, and which the CDC calls “plant A”.

The MDH conducted a full and detailed investigation of the plant, which employs some 1,200 workers and processes 18,000 pigs a day. They interviewed workers and looked at health records, and found a total of 12 employees who have either been confirmed as having PIN (8 so far), probably have it (2), or possibly have it (2).

The 12 workers, six of whom are female, started having symptoms between November 2006 and November 2007 and reported being healthy beforehand. They ranged in age from 21 to 51 years.

According to the MDH, 11 of the 12 workers affected showed evidence of axonal or demyelinating peripheral neuropathy (damage to nerve fibres and surrounding protective tissue). Spinal fluid taken from 7 of the workers showed they had high protein levels with little or no rise in pleocytosis or white blood cell count. A raised white blood cell count is usually evidence of inflammation.

However, five patients showed evidence of inflammation when examined by spinal MRI (magnetic resonance imaging), four of them in peripheral nerves or roots and one in the anteorior spinal cord.

All 12 affected workers said they either worked at or had contact with the area of the plant where pig heads were processed (called the “head table”). The head table was in an area of the plant known as the “warm room”.

The MDH conducted a case-controlled study that included 10 of the 12 workers (the 8 confirmed and the 2 probable cases of PIN), and two control groups: a random selection of 48 healthy workers from the warm room, and all 65 healthy workers from the head table. After examining throat swabs and blood samples from the consenting participants, the MDH to date has not found any infectious agent that could explain the PIN symptoms.

Futher results from the case-control study showed that 7 of the 10 workers with PIN (70 per cent of the case patients) were seven times more likely to have worked at the head table than the controls from the warm room (25 per cent, or 12 of 48). Case patients were also more likely to have handled brains and muscle from pigs’ heads than members of either control group.

Travel outside the US or inside the US, exposure to chemicals, including fertilizers and insecticides, or having had vaccinations, were also not found to be causes of the PIN illness.

The health authorities then investigated the processing methods and safety equipment in the warm room and the head table in particular, and concluded, for now, that the compressed air method used to liquefy and remove the brain tissue from the pigs’ heads could be causing the PIN through creating aerosol droplets of brain tissue that is then inhaled by the workers.

The plant operator has suspended use of the compressed air equipment voluntarily and stepped up use of personal protective equipment (PPE), including face shields and long sleeves for the head table workers and any other workers who want to use additional PPE, said the CDC report.

Following the Minnesota plant A outbreak report, the CDC surveyed all 25 federally inspected swine slaughterhouses in the US with more than 500 employees. They found three plants, including Minnesota plant A, using the compressed air method to remove pigs’ brains. The other two are in Nebraska and Indiana. Of those, only the Indiana plant has reported cases of suspected PIN, which are currently being further assessed. In the meantime, all three plants have stopped using the compressed air method to remove brain tissue, said the CDC.

The CDC said that:

“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.”

If doctors se any patients who work in slaughterhouses with these symptoms, they should report them to their state health department and also let the CDC know on 770-488-7100.

A puzzling feature of this outbreak was raised by the plant owner at one of the affected plants, who started as a floor walker in 1970. He told the Washington Post that they had been “harvesting pig brains since 1998, using the same method and the same 70-pound pressure air hose.” So why did the outbreak not take place ten years ago, when they started using the method? The plant owner said that was the “million-dollar question”.

“Investigation of Progressive Inflammatory Neuropathy Among Swine Slaughterhouse Workers — Minnesota, 2007—2008:.”

Morbidity and Mortality Weekly Report, US Centers for Disease Control and Prevention (CDC).

MMWR January 31, 2008 / 57 (Early Release);1-3.

Full report.

Sources: CDC MMWR, Washington Post.

Written by: Catharine Paddock, PhD