In 1965 the direction of the program was changed to slaughter surveillance instead of repetitious whole-herd testing. The finding of a tuberculous animal at slaughter initiated a traceback with intradermal testing of the herd of origin and any other cattle that might have been exposed. By 1965 New Hampshire had no M. bovis infections for 12 consecutive years and became the first state to be designated accredited free.

As the overall frequency of tuberculosis decreased, a supplementary test was required to decrease the number of false-positive tests and the number of animals unnecessarily slaughtered. Thus in 1973 the Uniform Methods and Rules (UMR) were changed so that animals with a caudal fold response in routine testing would be retested by the comparative cervical test (CCT). In 1978 the infection rate was 0.03 percent, and in 1991 the infection rate had fallen to 0.02 percent with 293 tuberculous carcasses found out of 36,000,000 head slaughtered (U.S. Department of Agriculture, 1992b).

Despite the dramatic progress in the early years of the eradication campaign, from 1983 to 1992 an average of 12 newly infected cattle herds were detected each year (Figure 3-1). This plateau of infected herd discovery reflects no apparent progress toward the goal of eradication. The prevalence of 0.02 percent has remained steady for approximately 10 years in the nation's cattle population. If species other than cattle were included in the calculation, however, this prevalence rate

FIGURE 3-1 Number of newly infected and exposed herds. Source: USDA Bovine Tuberculosis Eradication Program.

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