mechanism, an activity that is still the pivotal part of the program today. Animals with tuberculous lesions were traced back to their original herds and all cattle that had been in contact with the infected or exposed individual were tested for tuberculosis. Using this approach, the prevalence of bovine tuberculosis continued to decline, and by 1978 the reactor rate was 0.03 percent. In 1991 the reactor rate was 0.02 percent.
Given this slow but steady decline in disease rates, one might conclude that continued testing and slaughter would eventually result in eradication. However, within the past 10 years three persistent sources of infection have been identified, and their impact on the eradication program must be considered.
In recent years, the number of feeder cattle entering the United States from Mexico has increased dramatically. Unfortunately, although these cattle may be certified as having negative tuberculin skin tests, a small percentage are in fact tuberculous and may pose a risk to the U.S. cattle population.
Bovine tuberculosis in cattle is now concentrated in a small number (approximately 10) of dairies in Texas and New Mexico near the Mexican border. The official test and slaughter program has not been successful in clearing infection from these herds. In these large dairies (milking more than 2,000 cows each) slaughter of all cattle that had contact with a reactor would impose large economic losses.
The number of ranched and farmed Cervidae, such as elk and deer (and wild ungulates in general), and their movement around the country has increased dramatically. Some of the original animals in the founding herds were infected with M. bovis, and these animals were traded and transported widely, spreading infection. Moreover, the infection spread rapidly because there was little awareness of its existence in Cervidae and there were no requirements or recommendations for routine testing. Until recently, there have been no federal or state requirements for the testing of Cervidae or provisions for compensation if slaughter of exposed animals is indicated. As a result, the bovine tuberculosis problem in these exotic species went largely undetected. Infected captive ungulates may spread the disease back to cattle, to wild ungulates, and to humans.
It is important to consider bovine tuberculosis in the context of the wider North American setting. Enhanced international trade, increased livestock mobility, and public health concerns about disease in animals are impacted by the biology and methods of transmission of M. bovis. Canada is essentially free of bovine tuberculosis and may soon declare that eradication has been achieved in its national herd. Recently, tuberculosis in nonbovine ungulates has led to implementation of a test and slaughter campaign to stamp out that source of infection. So far as is known, Canada has no wildlife reservoir save one infected isolated bison herd in northern Alberta. In contrast, Mexico has only recently initiated a country-wide eradication campaign; the extent of tuberculosis in Mexico's national herd is not known. The recent formation of a National Commission for the Eradication of Bovine Tuberculosis signals the Mexican government's commitment to control the disease.
Little evidence exists to suggest the presence of a wildlife reservoir of bovine tuberculosis in the continental United States. Although some species of animals in zoos are infected, these animals must be treated differently than domestic or wild species because of their scarcity and value. In any