Tuberculosis control measures cannot be expected to prevent all worker exposure to tuberculosis, especially in areas with moderate to high rates of tuberculosis. Although control measures have helped end workplace outbreaks of the disease and prevent transmission of M. tuberculosis, they cannot be expected to prevent all exposures. Not all individuals with infectious tuberculosis have evident symptoms or signs of the disease, so workers may be exposed to them for some time before tuberculosis is suspected and a diagnosis is made. In addition, opportunities will exist for exposure in emergency departments and elsewhere before infectious individuals are recognized and isolation protocols can be applied and completed. Conscientious implementation of guidelines does not guarantee that transmission will never occur, but it appears to reduce risk significantly, especially in high-prevalence areas.
In communities with little or no tuberculosis, the effectiveness of control measures is necessarily limited. If success in community control of tuberculosis continues, more communities can be expected to join this low-prevalence group. Nonetheless, given the mobility of the U.S. population including immigrants from high-prevalence countries, it can be expected that people with infectious tuberculosis will occasionally appear in low-prevalence communities and their health care facilities. For example, in 1997, just 2 percent of the U.S. population lived in counties that had had no reported cases of tuberculosis in 5 years (Geiter, 1999).8
As noted in Chapter 5, workers in low-prevalence areas who encounter someone with infectious tuberculosis may be at higher risk of exposure than their colleagues in high-prevalence areas. They are less likely to be familiar with and alert to the disease’s signs and symptoms and may be less likely to have protective engineering controls in place in the emergency departments and other areas where such individuals are first encountered.
In the late 1980s and early 1990s, institutional departures from recommended tuberculosis control policies and procedures were widespread.
By using CDC data and a definition of tuberculosis elimination as no cases in 5 years, 587 of 3,142 (19 percent) counties in the United States could be considered tuberculosis free as of 1997 (Geiter, 1999). Of the counties with no tuberculosis from 1993 to 1995, 75 percent had no cases in the next 2 years and an additional 19 percent only had one case in either 1996 or 1997. Another definition of tuberculosis elimination is a case rate of less than 1 per 1 million population. Starting from a case rate of 74 per 1 million population in 1997, it would take 50 years to reach the elimination target if case rates were declining at an average annual rate of 5 percent and 41 years to reach the elimination target if case rates were declining at an average annual rate of decline of 10 percent. The average yearly rate of decline in recent years has been about 7 percent.