Further Adventures of the Tubercle Bacillus

In the past decade, tuberculosis (TB) has recaptured the interest of the medical community and the public because of its impolite refusal to go away when asked. The numbers are now familiar: 2 billion people worldwide with latent infection, 8 million new cases a year, and up to 2 million annual deaths.¹ Equally alarming is the emergence of drug resistance to TB on all continents and the explosive interaction between Mycobacterium tuberculosis and the human immunodeficiency virus.

Despite the untold millions of cases of TB that have occurred through the centuries, surprisingly little is known about the transmission characteristics of the tubercle bacillus.^{2 - 3} It took the landmark studies by Riley et al in the 1950s,^{4 - 5} using untreated patients with cavitary disease, a cleverly designed system of air ducts, and hundreds of down-wind guinea pigs, to establish that TB was indeed spread through the air. Since then, there has been remarkably little scientific experimentation to elucidate other factors that favor the transmission of this organism. Rather, much of what is now known has been the result of careful and thoughtful investigations of various "accidents of nature"—outbreaks, mishaps, and odd moments when TB was transmitted, inexplicably and against all odds, to an unsuspecting group. The study by Johnson and colleagues⁶ in this issue of THE JOURNAL can now be added to this list.

The report by Johnson et al suggests that at least 1 worker from a medical waste treatment facility in Washington State acquired TB from an occupational exposure to contaminated medical waste. If correct, this observation would stretch the geographical limits of potential transmission by many orders of magnitude, as the source case apparently never personally came within miles of the medical waste treatment facility. Somehow the culture of tubercle bacillus survived the long journey to the medical waste plant where it then escaped some or all of the numerous attempts to kill it via steaming, shredding, heating, compacting, and electrothermal deactivation, until finally it was aerosolized and inhaled by a worker who was exposed to medical waste.

The report also demonstrates the utility of the still controversial technique of molecular typing in the study of TB and other infectious diseases. Transmission studies have progressed slowly because they have had to rely on relatively few and dated techniques: in 1882, Koch's stain allowed visualization of the tubercle bacillus; in the first part of the 20th century came the use of screening fluoroscopy; the widespread use of the tuberculin skin test in the early part of the century first used in animal herds and later in humans; and in the 1950s, the use of drug-susceptibility profiles. There is now the fifth major tool, molecular typing, which emerged in the 1990s and has revolutionized scientific understanding of the natural history of infection.⁷ As Johnson et al show, this tool is so convincing that the investigators can feel relatively confident that a worker at a medical waste site far from the source case did indeed contract this particular strain of TB from that person's TB culture.

If the worker contracted TB infection in this manner, it is significant for 2 distinct reasons: First, it demonstrates the tenacity of the tubercle bacillus, thereby adding another mode of transmission to an already crowded list. Prior to the routine pasteurization of milk, ingestion of Mycobacterium bovis was responsible for a significant proportion of cases of clinical TB.⁸ In recent decades, most TB has been spread directly from person to person via air, powered by coughing and less commonly by singing.³ Despite its wide prevalance, numerous studies have demonstrated that M tuberculosis is not particularly contagious, infecting only 30% of tuberculin-negative contacts living in the same house with a source case who was acid-fast bacilli smear-positive.⁹ The current study by Johnson et al⁶ provides evidence that the transmission of the tubercle bacillus need not be aerosolized by coughing, but rather it can be aerosolized by the "blowback" of a clogged waste containment shredder.

Second, and of greater immediate implication, the report serves as a reminder that workers in various jobs outside of the health care profession are at risk for occupationally acquired infections. Because health care workers generally initiate, fund, analyze, and publish studies, including those in the area of occupational infections, the large majority of the published work is decidedly "iatrocentric." However, countless other occupational groups are at risk for specific infections, some of which are deadly. For example, the recent outbreak of the Nipah virus among pig farmers in Malaysia is a reminder of the potential risk of this occupation.¹⁰ Infections, such as brucellosis¹¹ and Streptococcus suis meningitis,¹² have been described among individuals working in pig slaughterhouses and processing plants.

Other non–health care workers similarly are at risk for infections contracted on the job. Forest workers, gardeners, and park rangers may develop such infections as sporotrichosis, Lyme disease, and lymphocytic choriomeningitis.^{13 - 14} Workers in day care centers have been long recognized to have an increased risk for such infections as cytomegalovirus and parvovirus B19 and when the workers care for children in diapers, the Norwalk virus and hepatitis A.¹⁵ And, of course, commercial sex work carries with it a significant risk of workers acquiring sexually transmitted diseases,¹⁶ all of which are preventable with routine use of condoms.

Inevitably, countless other infections are acquired occupationally but escape detection. This may be because the presenting syndrome is nonspecific and therefore not completely diagnosed, such as with upper respiratory tract infections. Lack of detection also may occur because many workers do not have the opportunity to congregate and swap stories. There is no equivalent for pig farmers, for example, of the hospital cafeteria where workers can hear each other's news, including updates about various medical conditions.

The problem of occupationally acquired infections in non–health care workers is a vast one that certainly warrants additional investigation. But how would such studies be designed? Who would conduct them, pay for them, and enforce their findings? Are studies needed for every possible incident and for every job, complete with a data table demonstrating the relative risk assigned to every imaginable gesture made in the course of a normal workday? When is the point of diminishing return reached? Furthermore, can every job or any job be risk free? These are difficult questions with no simple answers. No matter how many studies are done, elements of yet another job will inevitably place a worker unexpectedly at risk for either a recurring pathogen such as M tuberculosis or a newcomer like the Nipah virus.

So what is to be done? The most important way to start to address these challenging issues is by starting. The field of occupationally acquired infections among nonhospital workers essentially is an area where very little is known. Also, it is an area where the limits of time, resources, and real risk will impose a practical boundary on what can and should be done. By embarking on the task of investigating this area, researchers and clinicians will need thoughtfulness, open-mindedness, and trust among the workers, their employers, and the federal regulators who are entrusted with the health of the workforce—a troika not known for its harmony. As to the prospects for success of such an uneasy alliance, one can only hope that the importance of the task will inspire the participants toward a high level of cooperation. The result will be invaluable—development of effective strategies to prevent occupational infection, such as TB, from occurring in non–health care workers.