BSE is one of four known naturally occurring prion diseases. Others include scrapie of sheep, chronic wasting disease of elk and deer, and transmissible mink encephalopathy. These diseases have many characteristics in common. Fortunately, for control of BSE, they differ in their transmission efficiency. Scrapie and CWD are transmitted from animal to animal by contact within the flock or herd. In contrast, BSE and TME, based on abundant research information, can transmit from cow to cow or mink to mink only through the recycling of protein from an infected animal. The fortunate consequence of this fact is that BSE transmission is more easily controlled.
There is a sporadic form of the human prion disease, Creutzfeldt-Jakob disease, for which the cause is unknown. The occurrence of variant CJD in the 1990s and its strong association with BSE has raised concern of the safety of beef. There also is considerable misunderstanding as to the actual levels of risk.
To determine the cause of vCJD in humans, scientists have to rely on studies of disease patterns and relationships, rather than experimental research. These studies have generated convincing evidence that BSE transmission to humans, resulting in vCJD, occurs through ingestion of BSE-contaminated products, specifically nervous-system tissue or other specified risk materials from BSE-infected cattle. So, assuming that is the mode of infection, how efficient is the process of transmission to humans, or what is the level of risk?
The BSE outbreak in the United Kingdom peaked in 1994. During 1990 through 1994, there were approximately 137,000 cases of BSE within the UK. In 1994, the first cases of vCJD were discovered in the UK. During the subsequent eight years, approximately 130 cases of vCJD have been documented within the UK, which has a population of 60 million people. Once bans on feeding ruminant protein to cattle were implemented, new cases of BSE and vCJD dropped off dramatically.
The tragic loss of human life due to vCJD cannot be overstated. While not wanting to trivialize this loss, these numbers show that the risk of BSE transmission to humans, on a relative basis, is extremely low.
With feed bans and other firewalls in place, and the lack of other routes of BSE transmission, the levels of BSE should continue to decline globally. With this decline in BSE, the risk of transmission to humans also will decline.
In summary, the news concerning BSE is currently positive. Transmission of BSE is understood, appropriate control practices are in place, and BSE and vCJD should continue to decline. A note of caution is necessary, though. We must remember that when dealing with biology, change and unpredictability is the rule. There have been reports of “new” strains of BSE. To date, these strains do not appear to have resulted in important changes in BSE that would affect control measures, such as new routes of transmission. It is important that the industry and those involved in BSE research and policy remain vigilant.
Don Knowles, DVM, PhD, USDA-ARS Animal Disease Research Unit
Terry McElwain, DVM, PhD, Washington Animal Disease Diagnostic Laboratory, Animal Health Research Unit, College of Veterinary Medicine, Washington State University