Preface

Approximately 100 years have elapsed since the tuberculosis skin test was adopted in the United States to detect Mycobacterium bovis infection in cattle. This organism was the principal cause of tuberculosis in cattle and, at the time, was a serious threat to human health as well. Cattle transmitted the disease to each other, most likely by aerosol routes, and to humans, chiefly via infected raw milk. The idea behind the tuberculin skin test was to identify animals infected with M. bovis, remove them from the herd for immediate slaughter, and repeat this process until the herd, then the area, the state, and eventually the nation would be free of this disease. Officially, the national campaign began in 1917. In the early years, the prevalence of bovine tuberculosis decreased considerably; by 1935 the prevalence had been reduced to about 3 percent from the 1917 level of about 5 percent of the U.S. cattle population. By 1940 only 0.5 percent of cattle and by 1952 only 0.1 percent were reactors to the skin test. Concomitantly, the introduction of meat inspection and pasteurization of milk helped minimize the threat of M. bovis to human health.

In the 1960s, surveillance of slaughtered animals became the chief means of finding tuberculosis in cattle. Because the prevalence was so low, this approach was deemed effective and less costly than continuing the individual animal tests. Now the question became how best to continue the program to achieve national eradication?

In 1970 a team of researchers developed a model to investigate alternative strategies for eradication. The results indicated that if the program was halted the disease might become even more prevalent than it had been in 1917. Continuing the program as it existed in 1970 was expected to achieve eradication by 1995, and a very aggressive program of detection and depopulation of infected herds was predicted to lead to eradication by 1980. By about 1983, the estimate of prevalence was approximately two animals per 10,000 with about 12 newly infected herds found each year. This level of disease marks the point at which the eradication program apparently has stalled; in the last 10 years about 10 to 15 newly infected herds have been discovered each year. The great majority of infected herds are located in the El Paso, Texas, and New Mexico milksheds.

In addition to the lack of progress in eradicating bovine tuberculosis, the program now faces both continuing and new threats to its success. First, the number of cattle herds is decreasing and their average size increasing; this is particularly true in areas such as Texas and New Mexico. Indeed, it is in these large dairies that infection persists and in which the eradication program failed to remove the last vestiges of infection. Depopulation of these herds has not been possible largely because public funds were insufficient to compensate producers when infected and exposed animals were designated for slaughter. Second, the number of Mexican cattle entering the United States (as feeders, as dairy animals, and as roping/rodeo steers and bulls) has increased. Concomitant with this increase was a rise in the number of confirmed M. bovis infections in feedlot steers. This led to concern regarding the potential reintroduction of bovine tuberculosis in the U.S. national cattle herd. Third, tremendous growth in the number of farmed ungulate (Cervidae) herds, other than cattle, and in movement of animals between herds presented a new disease threat. No surveillance for bovine tuberculosis in species other than cattle was undertaken until



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LIVESTOCK DISEASE ERADICATION: EVALUATION OF THE COOPERATIVE STATE–FEDERAL BOVINE TUBERCULOSIS ERADICATION PROGRAM Preface Approximately 100 years have elapsed since the tuberculosis skin test was adopted in the United States to detect Mycobacterium bovis infection in cattle. This organism was the principal cause of tuberculosis in cattle and, at the time, was a serious threat to human health as well. Cattle transmitted the disease to each other, most likely by aerosol routes, and to humans, chiefly via infected raw milk. The idea behind the tuberculin skin test was to identify animals infected with M. bovis, remove them from the herd for immediate slaughter, and repeat this process until the herd, then the area, the state, and eventually the nation would be free of this disease. Officially, the national campaign began in 1917. In the early years, the prevalence of bovine tuberculosis decreased considerably; by 1935 the prevalence had been reduced to about 3 percent from the 1917 level of about 5 percent of the U.S. cattle population. By 1940 only 0.5 percent of cattle and by 1952 only 0.1 percent were reactors to the skin test. Concomitantly, the introduction of meat inspection and pasteurization of milk helped minimize the threat of M. bovis to human health. In the 1960s, surveillance of slaughtered animals became the chief means of finding tuberculosis in cattle. Because the prevalence was so low, this approach was deemed effective and less costly than continuing the individual animal tests. Now the question became how best to continue the program to achieve national eradication? In 1970 a team of researchers developed a model to investigate alternative strategies for eradication. The results indicated that if the program was halted the disease might become even more prevalent than it had been in 1917. Continuing the program as it existed in 1970 was expected to achieve eradication by 1995, and a very aggressive program of detection and depopulation of infected herds was predicted to lead to eradication by 1980. By about 1983, the estimate of prevalence was approximately two animals per 10,000 with about 12 newly infected herds found each year. This level of disease marks the point at which the eradication program apparently has stalled; in the last 10 years about 10 to 15 newly infected herds have been discovered each year. The great majority of infected herds are located in the El Paso, Texas, and New Mexico milksheds. In addition to the lack of progress in eradicating bovine tuberculosis, the program now faces both continuing and new threats to its success. First, the number of cattle herds is decreasing and their average size increasing; this is particularly true in areas such as Texas and New Mexico. Indeed, it is in these large dairies that infection persists and in which the eradication program failed to remove the last vestiges of infection. Depopulation of these herds has not been possible largely because public funds were insufficient to compensate producers when infected and exposed animals were designated for slaughter. Second, the number of Mexican cattle entering the United States (as feeders, as dairy animals, and as roping/rodeo steers and bulls) has increased. Concomitant with this increase was a rise in the number of confirmed M. bovis infections in feedlot steers. This led to concern regarding the potential reintroduction of bovine tuberculosis in the U.S. national cattle herd. Third, tremendous growth in the number of farmed ungulate (Cervidae) herds, other than cattle, and in movement of animals between herds presented a new disease threat. No surveillance for bovine tuberculosis in species other than cattle was undertaken until

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LIVESTOCK DISEASE ERADICATION: EVALUATION OF THE COOPERATIVE STATE–FEDERAL BOVINE TUBERCULOSIS ERADICATION PROGRAM 1984, when 21 bison herds in 10 states were found to be infected. Subsequently, infected elk and deer herds were identified, and questions arose about the extent of the infection in the nonbovine domesticated ungulate population and how easily it could be detected and controlled. Largely for these reasons, and at the request of the U.S. Department of Agriculture (USDA), the Board on Agriculture of the National Research Council (NRC) formed the Committee on Bovine Tuberculosis to undertake a comprehensive review of the Cooperative State-Federal Bovine Tuberculosis Eradication Program, which operates under the auspices of the USDA-Animal and Plant Health Inspection Service (APHIS). The specific charges were to assess the feasibility of eradication; evaluate what changes in program standards, procedures, and regulations, including the additional financial resources, would be needed to achieve eradication; review the literature for more effective ways of detecting bovine tuberculosis than the tuberculin skin test; assess the need to expand the tuberculosis program to farm-ranched nonbovine species; and assess the adequacy of requirements to prevent reintroduction of bovine tuberculosis from livestock entering the United States, particularly cattle coming from Mexico. The study committee met three times and on two of these occasions heard from a number of delegations including producer groups, industry representatives, veterinary associations, and other scientists. The committee also had the opportunity to tour one of the major entry ports for Mexican cattle near El Paso, Texas, and listen to the concerns of the Mexican cattlemen's associations. Chapter 1 of this report provides a historical perspective on tuberculosis and explores cross-infections with M. bovis, causing disease in cattle and humans, and with M. tuberculosis,causing disease principally in humans and to a much lesser extent in cattle. Today it is difficult to appreciate the tremendous problem M. bovis caused in cattle and the human illness it produced, particularly in children who consumed raw contaminated milk. Although the majority of infected cattle appeared normal, their productivity was greatly reduced. At the same time, tuberculosis in humans was the leading cause of death in 1900 and continued to produce major problems, particularly in young people, for the next 30 to 40 years. The prevalence of disease led the U.S. government to propose and implement the national eradication program in 1917. Chapter 2 discusses etiology, diagnosis, and detection of tuberculosis in both animal and human populations and how this affects the nature and success of the eradication program. Considerable attention is given to recent scientific developments because of their potential future value; however, this is not intended to downplay the significance of the other factors. Chapter 3 outlines the regulatory authorities and fiscal aspects of the eradication program, describes the status of bovine tuberculosis since 1983, and places the U.S. program in the context of the programs of its neighbors, Canada and Mexico. In Chapter 4, the committee summarizes the biologic and economic considerations affecting the choice of disease control objectives and the design of the disease control program. Bioeconomic models of bovine tuberculosis in the United States, Canada, Australia, and Ireland are reviewed, and conclusions about the benefits and costs of alternative strategies are drawn. Recommendations on revision of bioeconomic models to reflect contemporary conditions are offered. Finally, in Chapter 5 the committee outlines its specific recommendations, the single most important one being that the USDA should press onward with an aggressive campaign to eradicate M. bovis infection from the national cattle and farmed ungulate herds and take steps to prevent the reintroduction of bovine tuberculosis in imported, chiefly Mexican, livestock. During the committee's deliberations, the USDA has forged ahead and implemented a number of

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LIVESTOCK DISEASE ERADICATION: EVALUATION OF THE COOPERATIVE STATE–FEDERAL BOVINE TUBERCULOSIS ERADICATION PROGRAM modifications that, as it turns out, are consistent with our findings. These recommendations address three major problems: eradication of tuberculosis from large dairies; control and eventual eradication from farmed ungulates; and prevention of reinfection via imported Mexican cattle. Perhaps the most beneficial aspect of the committee 's deliberations has been the constructive comment and criticism on the U.S. eradication program by a diverse and talented group of experts. I have appreciated the opportunity to work with the study committee members, and I thank each of them for their support. S. Wayne Martin, Chair Committee on Bovine Tuberculosis

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LIVESTOCK DISEASE ERADICATION: EVALUATION OF THE COOPERATIVE STATE–FEDERAL BOVINE TUBERCULOSIS ERADICATION PROGRAM This page in the original is blank.