Bovine spongiform encephalopathy (BSE) is probably the rarest disease in the United States. But despite its rarity, BSE has had tremendous economic impact. The export market for American beef was dramatically reduced when the first U.S. case of BSE was made public in late 2003. That market has not yet completely recovered.

Only three cases of BSE have been diagnosed in the United States. One of these occurred in an animal imported from Canada. The other two were from very old (more than 10 years old) animals that were born and raised in the United States. The Canadian animal was diagnosed with the typical form of BSE, which appears to be the same strain that ravaged the United Kingdom during its epidemic. The U.S.-born cases were both diagnosed with a form of the disease called “atypical” BSE. BSE is rare, and among BSE cases, atypical BSE is very rare. So, although it remains poorly understood, atypical BSE is currently the subject of very active research by U.S. Department of Agriculture (USDA) scientists.

The United States also has native cases of the other common transmissible spongiform encephalopathy (TSE) diseases, scrapie and chronic wasting disease (CWD). The USDA has performed a number of experimental inoculations of scrapie and CWD into domestic cattle and other species to gauge likelihood of the spread of these diseases to other animals. There is no evidence to suggest that CWD or scrapie can spread to cattle, or that either of these TSEs is responsible for producing the BSE or atypical BSE found in the country.

A recent court case (Creekstone Farms vs U.S.D.A., et al.) has raised interest in BSE testing. There is currently no live-animal test for BSE. All approved tests require a sample of brain material (obex) for testing. Most tests digest the brain material with an enzyme (proteinase K) to remove material that would otherwise interfere with detection of the infectious agent. The conformation-dependent immunoassay (CDI) uses a non-destructive method of separating the infectious agent from interfering material. In all tests antibodies are used to detect the infectious material. The UDSA has developed improved antibodies for the CDI and is working to develop a more sensitive test using mass spectrometry. If a blood test for BSE could be developed, it would simplify testing of live animals. This is currently impossible, because scientists have been unable to detect BSE in blood of cattle.

Well before the first native case, the United States adopted regulatory measures to prevent the spread of BSE, including active testing of animals for the presence of BSE and a feed ban. The United States tested more than 400,000 animals and found only three cases of BSE. BSE is spread by consumption of feed containing infected ruminant-derived material. The feed ban instituted in the U.S. government in 1997 very effectively prevents infectious material from being fed to domestic cattle.

An expert panel of the World Organization for Animal Health (OIE) recently used a science-based approach to evaluate the measures the United States employs to control and monitor BSE. After reviewing the data, this panel recommended that the United States be designated as a “controlled risk” for BSE. This recommendation probably will be formally adopted by the OIE in May of this year.