Above, a collection of headlines from recent years.

A brief history of BSE

First recognized in 1986, bovine spongiform encephalopathy (BSE, or mad cow disease) is one of several similar but not identical diseases known as transmissible spongiform encephalopathies (TSEs). TSEs are characterized by progressive degeneration of the central nervous system. All have long incubation periods (4 to 8 years average in cattle, and up to 30 years reported in humans), evoke no immune response and are inevitably fatal.
Different TSEs affect several different mammalian species including humans (Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia, kuru), mink (transmissible mink encephalopathy), sheep and goats (scrapie), and mule deer and elk (chronic wasting disease). About 15% of human TSEs are heritable, with 85% considered sporadic (about one person per million per year will be diagnosed with a sporadic TSE on every continent worldwide).
In the mid-1990s, U.K. scientists linked the consumption of food containing BSE-diseased cattle protein from specific organ systems to a new human disease, vCJD (variant Creutzfeldt-Jakob disease). As many as 900,000 U.K. cattle might have been infected with BSE between 1984 and 1995, with close to 800,000 carcasses entering the human food chain prior to recognition of the problem as a foodborne zoonosis (disease shared by humans and animals).
Studies implicated the feeding of meat-and-bone meal contaminated with the abnormal BSE prion protein to young cattle. Changes in rendering practices in the United Kingdom in the late 1970s and early 1980s may have allowed the infectious particle, a prion (“proteinaceous infectious particle,” see page 206) to be amplified in the production of meat-and-bone meal. The export from the United Kingdom of rendered animal feed products contaminated with BSE prions is considered the source of BSE affecting native cattle in other countries. The use of meat-and-bone meal as a protein supplement for ruminants (four-stomached animals including cattle, sheep and goats) had been common practice for more than 50 years prior to the emergence of BSE. Historical tissues have been examined and there is no evidence that the disease existed prior to 1985.
The United States responded as early as 1989, well before there was any evidence that this was a zoonotic disease, with import bans designed to keep the prion contaminant out of the country. Between 1989 and the present, there have been several waves of regulation by the U.S. Department of Agriculture and the U.S. Food and Drug Administration, some in response to requests from livestock and professional animal-health organizations to minimize the nation’s risks of importing this disease. Others, such as the slaughter surveillance program initiated in 1990, were designed to monitor the food chain.
After confirmation of the first U.S. BSE case in December 2003, the United States vastly expanded its slaughter surveillance program. While testing is mandatory for cattle condemned at slaughter, the majority of the program is voluntary. Surveillance now includes as many cows as possible from the highest-risk populations, such as downer cattle — those unable to stand and walk on their own. (While BSE-affected cattle become downers as the disease progresses, other downers are disabled for a host of reasons having nothing to do with BSE.) From June 2004 through September 2005, more than 470,000 samples had been examined with only one positive. Scientists in the California Animal Heath and Food Safety Laboratory at UC Davis perform more than 10% of these tests.
A key to containing the outbreak in the United Kingdom included the killing and stringent disposal of positive animals and a ban on using protein from ruminants in protein supplements for other ruminants. The United States banned ruminant-source protein in ruminant feeds in 1997 (following a voluntary ban in 1996).
One U.S. animal was confirmed with BSE in December 2003. This older dairy cow was imported to the United States from Canada and was likely exposed to prions while a calf in Canada. A second positive animal was identified in 2005 in Texas as part of the slaughter surveillance program. This beef cow was about 12 years old and investigations are under way relative to its source of exposure. All tissue from these animals was destroyed with no resulting contamination of the animal or human food chain.
— D.J. Klingborg and B.I. Osburn.