TSEs: Dealing with Uncertainty

Transmissible spongiform encephalopathies (TSEs) returned to the front burner with the announcement on May 20, 2003, by Canadian authorities of a case of bovine spongiform encephalopathy (BSE)—or mad cow disease—in a cow in Alberta.

The U.S. Dept. of Agriculture responded by banning any imports from Canada of products from any ruminant. With the U.S. ban still in place in early July, journalists reported that Canadian officials had begun to question the viability of the highly integrated market that had previously encouraged the flow of cattle and beef between the two countries.

BSE has been shaking and reshaping markets since researchers first identified the disease in cattle in the United Kingdom in 1986. The disease was transmissible, slow to develop, and fatal. By 1989, researchers had come to believe the cause to be a prion protein that spread through cattle feed derived from rendered livestock. To stop the spread of BSE, governments in the UK and around the world imposed bans on the feeding of ruminant-derived feed to ruminants. Such feed bans have proved effective in cutting the rate of new infections and in slowing the spread of the disease into other regions.

The UK had long experience with a TSE in livestock. Scrapie, a wasting and fatal disease of sheep, was first described in the late 1700s. Scrapie had not been known to affect humans, even though it was likely that affected sheep had been shorn, worn, and eaten by humans for generations. But with a new disease came a new question: Since it appeared that scrapie moved from sheep to cattle to cause BSE, might the BSE pathogen spread to humans?

In March 1996, the UK government announced that several new cases of a variant of a human TSE disease called Creutzfeld-Jakob disease were most likely caused by exposure to beef from BSE-affected cattle. This announcement sundered confidence in the food supply and shook the public’s confidence in science as a way of dealing with uncertainty.

Here was a classic example of the difference between statements about nature and statements about our best understanding of nature. In the period before March 1996, did the public hear a concise and confident four-word message that “British beef is safe,” or did the public hear a longer, tentative, 14-word message that “We have no evidence yet that British beef is less safe than other beef”? The shorter version was a statement about nature; the longer was a statement about our best understanding of nature. The shorter version was clear, certain, and unsound. The longer version was sound but also sounded like waffling, wobbling, and wiggling.

TSEs earn our attention because of the double-edged threat—spread from livestock to livestock, and spread from live-stock to people—and the diceyness of dealing with uncertainty.

Both the threats and the uncertainties are justifications for USDA’s surveillance program that tests 19,900 cattle in the U.S. each year, a program with a sensitivity designed to detect one case in a million. So far, BSE has not been detected in the U.S.

Yet many people in the U.S. have another TSE to ponder. In 1967, researchers observed a wasting disease of deer in Colorado; in 1978, this disease was named chronic wasting disease (CWD). For many years, CWD was limited in range to west of the Mississippi River. But on February 28, 2002, Wisconsin officials announced the discovery of CWD in deer killed near Mt. Horeb, about 30 miles southwest of Madison. The news sent a shudder through a state of 5.3 million people with 688,000 licensed deer hunters who harvest 336,000 deer every year, filling freezers with venison and refrigerators with summer sausage.

When CWD is detected for the first time in a region, at least two questions come to mind, both echoing off the BSE experience:
(1) Might CWD be transmitted to humans? and (2) How might we best manage CWD in deer?

Based on epidemiology of human TSEs, there is no evidence yet that CWD transfers to humans. That will always be a tentative conclusion. It also is a baffling issue for food safety specialists: What advice should they give to hunters about the risks of and precautions for butchering and eating deer? Since CWD is not known to be transmitted to humans, can the deer disease even be considered a food safety issue? Extension specialists offer guidance and precautions: Know the CWD status of your hunting ground; butcher your deer yourself rather than taking the carcass to a butcher; and do not eat brains, spinal cord, eyeballs, spleen, or other lymph tissue that harbors large concentrations of prions.

State officials have launched a program to kill all the deer in the 400-sq-mi outbreak zone in an attempt to eradicate the disease. The controversial plan pleases nobody and is opposed by those who believe the disease should be allowed to run its course. It has won grudging support among those who see any less-thorough response as squandering an irretrievable opportunity to clear out the disease and, in a decade perhaps, restore the deer population and the hunting traditions. As with so many aspects of TSEs, it will take a long time for the story to unfold.

by Thomas M. Zinnen is Biotechnology Policy & Outreach Specialist, University of Wisconsin-Madison Extension, Biotechnology Center, 425 Henry Mall, Madison, WI 53706.