incidentally. The source cases often had HIV infection or AIDS and had atypical radiographs and negative sputum smears that made it easier for them to go undetected and untreated. A majority of the health care workers who developed active multidrug-resistant tuberculosis had HIV infection or AIDS, as did most of those who died. The estimated duration and consequences of reported exposures varied widely. In one outbreak involving an autopsy on a person with unsuspected tuberculosis, all five of those present for the 3-hour procedure—including one person present for only 10 minutes—subsequently had skin test conversions and two developed active tuberculosis (Templeton et al., 1995). Two developed active tuberculosis. Other reports also indicate that transmission of M. tuberculosis can occur during relatively short periods of exposure (e.g., 2 to 4 hours).
Some studies of hospital outbreaks of tuberculosis have reported information useful in assessing the likelihood of occupational versus community transmission of tuberculosis. Most of these studies have also attempted to assess the effects of implementing tuberculosis control measures consistent with the 1990 or 1994 CDC guidelines. The discussion below focuses on evidence of workplace transmission of M. tuberculosis. Chapter 6 reviews evidence on the effects of tuberculosis control measures.
After an outbreak of multidrug-resistant tuberculosis at St. Clare’s Hospital and Health Center (New York City) in the early 1990s, researchers compared tuberculin skin test conversion rates for different occupational categories (nurse, physician, laboratory, housekeeping, social service, and finance) (Louther et al., 1997). They found the highest rate of skin test conversions among housekeeping employees. A multivariate analysis showed that significant differences in conversion rates by job category remained after adjustment for differences in age, BCG vaccination status, country of birth, gender, and the tuberculosis incidence in the zip code area of residence. In the multivariate analysis, residence was not associated with risk of conversion. As discussed in Chapter 6 and Appendix D, that study also reported decreases in conversion rates following the implementation of tuberculosis control measures.
Following an outbreak at the Cabrini Medical Center (New York City), researchers compared rates of skin test conversion for workers on wards admitting patients with tuberculosis with rates for workers on wards that did not admit such patients (Maloney et al., 1995). For the 18-month period before tuberculosis control measures were implemented, conversion rates were 16.7 percent for the former group and 2.8 percent for the latter group, a statistically significant difference. Following the introduction of infection control measures from June through October 1991, rates fell on wards that admitted patients with tuberculosis but not on other wards.