The Meat and Bone Meal Factor
Let us start by examining the varied factors responsible for the transmission and potential amplification of the infectious agent to cattle that could possibly result in infection or clinical disease. The first important consideration, accepting the compelling and prevailing hypothesis that meat and bone meal (MBM) is a “reservoir” for infection, is whether or not the United States imported amounts of MBM or feedstuffs contaminated by the infectious agent (a prion). The answer to that theory varies between very unlikely to no, since the United States is the largest exporter of rendered products, producing more than half of all the marketed rendered products globally, and it would be highly unlikely to be in the business of importing MBM. Additionally, using an absolute worst-case scenario by accepting the possibility that a small amount of MBM could have been imported prior to imposed prohibitions instituted in 1989, what was imported had to originate from the United Kingdom, the index source of infection, and presumed to be contaminated. While this is possible, it was highly unlikely. Also, heightening the worst-case assumption of the possibility of contamination of a small amount of imported MBM, or contaminated feedstuffs, the product had to be fed to cattle, obviously, and the infectious titer had to be large enough to infect susceptible cattle, thus contributing to the genesis of an index case. There are a lot of barriers in the process to indicate that MBM originating from a source in the United Kingdom during that interim period could not be a risk factor in the genesis of BSE in the United States.
The data documenting the type and composition of feed imported into the United States between 1980 and 1990 is subject to query, because prior to 1986/1987, and until BSE was associated with having a feed-borne link in 1987, the importance was relatively limited. The USDA, however, reported that no MBM was imported from the United Kingdom between 1980 and 1990(1). Shipping MBM to the United States from the United Kingdom or Europe is economically unfeasible since the United States is mainly an exporter of the product. This is an extremely important determinant in the epidemiology of risk, since MBM was the presumed major “vector” in the transmission of BSE in all the affected countries. For all practical assessments of risk, therefore, MBM was not a factor in the United States.
The Importation Factor
While considerations had to be evaluated on an ongoing basis, the United States in 1989, in establishing interim control measures, banned importation of all ruminants, bovine semen, embryos, and MBM from the United Kingdom based on the emerging epidemiology, and continued the prohibitions to other countries as applicable as the disease manifested in countries contiguous to the United Kingdom and linked to the importation of contaminated MBM from the United Kingdom. Also during this period, the United States established policies for surveillance testing of tissues from BSE “suspect” cases, including cattle exhibiting signs of neurological disease, cattle condemned at slaughter with characteristics of central nervous system disorders, and old cattle that are not ambulatory at slaughter and designated “downer” cattle(2).
The United States did import 334 live cattle between January 1981 and July 1989 from the United Kingdom during the BSE epidemic. Of the 334 animals, 161 were disposed under circumstances that eliminated any potential risk they could have entered the animal or human food chains. The rest of the animals (173) came into the country before the peak of the BSE epidemic and none originated from a herd in which a BSE case is known to have occurred. According to USDA’s documented records, 164 (94.8 percent) of the 173 cattle were beef “breeders” and nine (5.2 percent) were dairy cattle(1).
A Harvard/Tuskegee risk analysis report referenced that during the period 1983 to 1987, a total of 397 predominantly beef breeding cattle were imported from Switzerland, France, Italy, and Belgium. The vast majority of these imports occurred before 1988, thereby posing only a limited risk to the United States(1). This may not satisfy some sectors that are prone or belong to the school of absolutely no risk. Uncertainties are a part of life, but comparative assessments have shown that the risks associated with the importation of live cattle that were incubating the disease were minimal to non-existent, as referenced in the Harvard/Tuskegee risk analysis and other assessments done by USDA’s epidemiologists.
Discussion
While there will always be people or groups in every society that are not satisfied with the policies imposed by government to prevent and control diseases, the record will show that the USDA acted immediately in the case of BSE by making the disease officially reportable under national veterinary legislation. This was done within 10 days of the public announcement of BSE in the United Kingdom, becoming the first country in the world to react to the threat. Some very progressive countries continued to be concerned observers, but did nothing officially to institute control measures similar to the United States. That initial reaction by the USDA was only the beginning of a series of logical controls that followed based on the evolving epidemiology of the disease in the United Kingdom and subsequently the progression of the epidemic to contiguous countries in Europe.
It is also unfortunate, using BSE as an exemplar, that risk is now generally used to heighten negative or undesirable outcomes, and often overlook the positive aspects initiated to control the disease. This is so often the case even in professional assessments, especially when the risk is used almost exclusively to refer to a threat or hazard like BSE. This has occurred in spite of all the conclusive risk analyses that demonstrate otherwise. This is readily exemplified with special reference to the United States where some are still intent to emotionalize risk ad infinitum, regardless of the findings of the Harvard Center for Risk Analysis and the USDA’s own internal analyses of risk, some published as early as 1993.
Assuming BSE is ever successfully introduced in the U.S. cattle population, it would have to be dependent on two major factors: the importation of MBM contaminated by the infectious agent, or the importation of cattle incubating the disease or clinically infected. Both these routes of introduction that could later serve to transmit and amplify the disease have been severely curtailed by the development of logical preventive policies that went into effect as early as 1989. Unless there is a total breakdown in the enforcement of these policies, the possibility of BSE gaining a foothold in the United States is very unlikely.
The Redeeming Factor – Surveillance Testing
The European experience is that if BSE is present in the cattle population, the most expeditious way to determine the presence of it is by testing cattle that are non-ambulatory, dead on the farm, or showing clinical signs of a central nervous system disorder that parallel BSE(3). Embracing this philosophy, the USDA, in spite of the existing minimal risk of BSE in the U.S. cattle population, decided effective June 1, 2004, to collect as many samples as possible from the high-risk population over a period of 12 to 18 months. The process was also developed to assure that “there is statistically appropriate geographical representation of the adult cattle population in the United States”(3).
The surveillance objective heightened two determinants:
1. Assuming all the BSE positive cattle are within the defined high-risk population, sampling and testing 201,000 animals would allow USDA to detect BSE at the rate of one positive in 10 million adult cattle at a 95 percent confidence level. This has already been accomplished.
2. If the number were increased to a total of at least 268,500 samples, this level would permit the detection of BSE at the rate of one positive in 10 million adult cattle at a 99 percent confidence(3). USDA has surpassed this, having tested over 347,000 samples as of this writing.
Conclusion
Debates among scientists and other experts often occur around uncertainties to determine what is hazardous, the ability to identify or calculate the risk, the degree of seriousness of the risk/hazard in terms of likely effects, the consequences of controlling the alleged hazard, including the options to consider the acceptability of the risk/hazard. BSE is a perfect example of this hazard/risk dilemma. This is demonstrated by the cumulative epidemiology of the past 18 years that indicates the United States is either BSE-free or minimal-risk at worst, yet there are still concerns and discussions about the need to “tighten” existing regulations without any changes in the dimensions of risk.
A review of the available data debunks any evidence that MBM, the considered “vector” of the disease, was imported into the United States from the United Kingdom or other BSE-affected countries prior to the imposed prohibitions in 1989. Additionally, there is no supportable evidence to promote the theory that imported cattle from the United Kingdom or Continental Europe could have been a contributory source of introducing the infectious agent to the country. That risk factor has been thoroughly addressed by both the USDA and the independent risk analysis conducted by the Harvard/Tuskegee epidemiologists.
Lastly, the greatest validation of the workability of government’s policies is the accelerated surveillance testing program that went into effect on June 1, 2004, which is described as the redeeming factor. Since the inception of the program, over 347,000 cattle have been tested for BSE with no positives.
With any other disease, the current information would be reason for closure, yet the perpetuation of BSE as a serious threat, in spite of the existing controls, defies logic since the health of animals or man is not compromised or threatened, and begs the question, “Why?”
References:
1. Harvard Center for Risk Analysis and Center for Computational Epidemiology/Tuskegee University. November 26, 2001. Evaluation of the Potential for Bovine Spongiform Encephalopathy in the United States.
2. Franco, D.A. June/July 2004. BSE and the Safety of Beef: A Perspective. Food Safety Magazine. Volume 10, no. 3:22.
3. United States Department of Agriculture, Animal and Plant Health Inspection Service, Veterinary Services. August 2004. Bovine Spongiform Encephalopathy (BSE) Surveillance Plan. Secretary’s Advisory Committee on Foreign Animal and Poultry Diseases.
Tech Topics - June 2005 Render