Is cough really necessary for TB transmission?

Abstract

Cough has long been implicated in the production of infectious aerosol leading to transmission of tuberculosis (TB). However, prevalence studies frequently identify radiographic evidence of TB in subclinical individuals in the absence of reported coughing. Elucidating the role of cough in transmission depends on understanding the physical process of aerosolizing and expelling mycobacterium tuberculosis (Mtb) bacilli. In the last decade, human aerosol studies have progressed with improved precision of particle detection and greater sophistication of experimental protocols. Combining principles of respiratory physiology, the site and mechanism of aerosolization of respiratory lining fluids during phases of the respiratory cycle has been investigated in detail. Additionally, recent success in the direct detection of naturally generated Mtb aerosols has allowed more detailed characterization in terms of their rate of production and size distribution. We propose that TB transmission depends on the coincidence of the site of aerosol generation with the presence of Mtb bacilli. This review will examine the evidence for site of aerosol production during cough and respiratory activities in conjunction with the characteristics of detectable Mtb aerosols and locations of tuberculosis infection. Furthermore, we propose respiratory activities that are likely to optimise aerosol sampling for investigation of transmission.

Fig. 1

Aerosol creation in terminal bronchioles by the bronchiole fluid film burst mechanism (Modified from Johnson et al. ref 18).

Fig. 2

Schematic representation of the respiratory cycle illustrating the closure point (CP) between the functional residual capacity (FRC) and the residual volume (RV). The CP volume can be experimentally identified using the ¹³³Xe bolus method [26] and indicates the depth of expiration when airways closure begins. Terminal bronchiole closure and subsequent reopening occurs most prominently at expiratory volumes beyond the closure point. The blue line indicates expiration to CP and the red line indicates expiration beyond CP leading to greater aerosol production (Modified from Almstrand A et al. ref. 20). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Illustration of aerosol creation by shear forces in large bronchi.

Fig. 4

Laryngeal aerosol creation by vocal fold vibration and closure and reopening cycles.