Since 1990, USDA has conducted a science-based surveillance program to detect BSE in the United States. While initially based on experiences in the U.K., focusing on detecting animals with clinical disease, the United States followed OIE guidelines in establishing its later program. This was further enhanced by applying national targets to smaller regions of the country.
Subsequently international focus has increasingly included the testing of targeted, high-risk cattle as the most effective way to detect BSE-infected animals. This now is the basis of agreed international guidelines published by OIE. In the U.S. BSE surveillance program, animals targeted for BSE testing include those exhibiting signs of central nervous system disorders, non-ambulatory animals (those that can not walk) and other animals exhibiting symptoms consistent with BSE that die on-farm. The program also focuses on cattle older than 30 months of age. Since tests can only detect abnormal prion protein a few months prior to clinical disease9, testing younger animals has limited or no value.
USDA maintains an ongoing BSE surveillance program and currently tests approximately 40,000 high-risk cattle annually, a number that exceeds the OIE’s recommended testing levels for the risk status assigned to the United States by the OIE. The ongoing BSE surveillance program is designed to detect BSE at a prevalence level of one case per 1 million adult cattle. All U.S. cattle are inspected by a USDA inspector or veterinarian before going to harvest, with high-risk animals identified for BSE testing. Meat from cattle being tested for BSE is held until the test results are confirmed.
In June 2004, USDA instituted a one-time expanded testing program to determine the incidence of BSE in the United States. From June 1, 2004 through Aug. 20, 2006, USDA tested 787,711 cattle and found two BSE positives, both in older cattle and both with a previously unrecognized form of BSE that was different from the typical form.
As a result of this surveillance, the prevalence of BSE in the United States is estimated to be less than one infected animal per 1 million adult cattle.
Prevention of risk to cattle
The United States began prevention measures for BSE in 1989 by banning the import of ruminants and ruminant products from countries where BSE had been found. Because BSE spreads through contaminated feed, the beef industry agreed to a voluntary ban on the use of ruminant-derived protein supplements in cattle feed in 1996, and the Food and Drug Administration (FDA) made the feed ban a law in 1997. Feed bans are a critical firewall in preventing the spread of BSE, but inspection and enforcement are important because feed bans may be subject to problems such as cross-contamination at feed mills or during transportation, mislabeling of feed or misuse of feed on farms. Although some reduction in infectivity can result from the rendering process that produces MBM, experimental studies have shown that cattle can become infected with doses as low as 1mg of infected brain tissue.6 This highlights the importance of excluding cattle-derived MBM from ruminant feed.
Prevention of risk to humans
In naturally occurring cases, BSE has only been detected in the brain, spinal cord and retina, peripheral nerves and dorsal root ganglia12,13,14, as well as optic nerve14 in affected cattle. In experimentally infected cattle, infectivity has been found in the distal ileum, dorsal root ganglia, trigeminal ganglia, and tonsil as well as in brain and spinal cord15 (click for info). The tonsil result has been reconfirmed and shown to represent the presence of a consistent, if low level, amount of infectivity at several time points during the course of incubation.16 The status of bone marrow remains inconclusive after detection as just a single time point in incubation17, and there currently is little evidence to suggest infectivity in muscle with evidence for a trace of infectivity in just one study that requires further verification. The greatest risk is associated with the brain and spinal cord, which, as indicated earlier in relation to testing, only become infectious late in the incubation period. Peripheral nerves and dorsal root ganglia do not appear to be infectious before the brain, with the likely exception of nerves that run from the intestine to the brain.8 Therefore, clinically affected animals represent the greatest risk of all.
In January 2004, USDA added a BSE prevention measure to the existing import ban and feed ban. A further hurdle that protects public health is the required removal of specified risk materials (SRM) – parts of the animal that have been identified as having the potential to carry the BSE agent in infected animals. SRM are banned from human food and include brain, spinal cord, eyes, dorsal root ganglia, trigeminal ganglia and vertebral column for cattle older than 30 months of age and tonsil and distal ileum (lower part of the small intestine) of all cattle.
Other BSE firewalls implemented by USDA’s Food Safety Inspection Service (FSIS) include:
- Ban on non-ambulatory animals: Because inability to walk can be a symptom of BSE, USDA prohibits all animals that are unable to walk from entering the human food supply.
- Product holding: Cattle tested for BSE cannot be marked by FSIS inspectors as "inspected and passed" to enter the food supply until confirmation the BSE tests are negative.
- Advanced meat recovery: FSIS expanded a prior prohibition on spinal cord from being allowed in product produced from a technology called advanced meat recovery (AMR) to include a prohibition on dorsal root ganglia, clusters of nerve cells connected to the spinal cord along the vertebral column.
- Air-injection stunning: FSIS banned the practice of air-injection stunning to ensure that portions of the brain are not dislocated into the tissues of the carcass as a consequence of humanely stunning cattle during the harvesting process. U.S. plants already had phased out air-injection stunning, but FSIS banned the practice to ensure it would not be used in foreign plants, which must meet FSIS requirements in order to export product to the United States.
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