Transmission and Institutional Infection Control of TB-A Reexamination of Fundamental Strategies

Although the airborne route of Mycobacterium tuberculosis has been known for more than 60 years, and suspected long before that, there remains controversy over exactly how transmission occurs and how to prevent it. From an infectious particle perspective, the Wells-Riley equation simplifies understanding the interrelationship of some but not all relevant factors, and many assumptions are made that we know are untrue, like steady-state, well-mixed conditions, uniform virulence of infectious particles, and uniform susceptibility of exposed hosts. The source strength of infectious individuals is an important unknown factor. Retrospectively, the infection rate is used to calculate the infectious dose generation rate (quanta/h). A recent controversy is the importance of presymptomatic tuberculosis (TB) and quiet breathing as opposed to cough-generated aerosol. Early effective treatment rapidly reduces infectiousness from known and presymptomatic cases but requires active screening of high-risk populations. Environmental controls are essential for reducing transmission from presymptomatic and unsuspected cases. While masks for patients and respirators for healthcare workers are valuable in high-risk situations before appropriate treatment begins, their effectiveness is limited.