Disease at the head table: follow-up

I'm an environmental epidemiologist with a special interest in surveillance. So it would be nice to say that epidemiological investigations and surveillance systems were responsible for discovering most of the workplace diseases we see nowadays. But the simple truth seems to be that most environmental and occupational diseases are still discovered the old fashioned way: by astute clinicians, workers or family members. Such was the case for the recent cluster of cases of a progressive neurologic syndrome among slaughterhouse workers we posted on a while back. The Minnesota Department of Health (MDH) was first alerted something was going on by a medical specialist who had seen unexplained neurologic illnesses in several of his/her patients. CDC and MDH have now come out with an official outbreak investigation, filling in the details:

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.

[snip]

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. (CDC, Morbidity and Mortality Weekly Reports [MMWR])

A small study that compared 10 patients with the nerve condition to a random sample of 48 healthy warm-room workers and all 65 healthy head-table workers (this kind of study is called a case-control design). The comparison showed that the patients were much more likely than the general warm room workers to have worked at the head table (70% vs. 25%; appropriate tests to rule out likely sample error were done). The comparison of patients and healthy workers who worked at the head-table shoed that the cases were more likely to have removed brains (57% vs. 12%). Inquiry into other factors that might have explained the condition (previous travel outside or within the United States; exposure to chemicals, fertilizers, or insecticides; use of medications; or receipt of previous vaccinations) didn't show any associations with illness.

So what did the workplace look like?

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.

CDC then surveyed 25 of the larger (more than 500 employees) plants that undergo federal inspection and =found only three (the Minnesota plant and one eac in Nebrask and Indiana) that use compressed air to extract pig brains. No cases were found at the Nebraska plant but several at the Indiana plant are suspect cases. The use of compressed air has been voluntarily stopped at all plants.

CDC is calling this workplace neurologic syndrome Progressive Inflammatory Neuropathy, or PIN. So far it is an occupational disease of swine slaughterhouse workers who process pig heads. No infectious agent has been identified and the pigs had all passed inspection. At this point the most probable mechanism would seem to be an autoimmune one, that is, a condition where the worker is exposed to pig brain tissue which provokes an immune reaction that also targets human tissue. Whether this can occur in other ways is unknown, as is the true prevalence of working conditions that can cause this new occupational disease in the US (say in smaller slaughterhouses) or outside the US.

While at this point the scope of the health hazard to the population is confined to a particular kind of worker, this episode is a classic example of how new occupational diseases are discovered, some of which later turn out to be of more general significance. Even if this is not the case in this example, however, the alertness of a health car provider may yet save dozens or more US workers and an unknown number of workers outside the US from severe disability or even death.

Something to remember if you see something that doesn't seem right.