Risk of Travel-Associated Tuberculosis
- Tuberculosis Division, International Union Against Tuberculosis and Lung Disease, Kirchlindach, Switzerland
- Reprints or correspondence: Dr. Hans L. Rieder, Tuberculosis Division, International Union Against Tuberculosis and Lung Disease, Jetzikofenstr. 12, 3038 Kirchlindach, Switzerland (TBRieder{at}cs.com).
Abstract
Infection with Mycobacterium tuberculosis might be acquired at home or during travel. The risk is determined by exposure frequency to a source case and the duration of the exposure. Thus, whether travel increases the background risk depends on origin, destination, and duration of travel. Infection might be acquired indoors or outdoors, but the overall risk seems small, whatever the setting. Bacille Calmette-Guérin vaccination and preventive therapy have both been discussed as possible preventive interventions, but the disadvantages associated with both approaches appear to outweigh any benefits. Because the risk of acquisition of infection with M. tuberculosis is small, the most rational approach is likely to delay intervention until a traveler presents with clinically active tuberculosis, as is done with any other patient.
The risk of becoming infected with Mycobacterium tuberculosis depends on 2 factors: first, relevant exposure to a source of infection, and second, the probability of becoming infected if there is exposure. Exposure risk is determined by the epidemiologic profile of tuberculosis in the population segment within which one preferentially moves. Tuberculosis incidence varies not only among [1] but also within countries [2].
Indoors, tubercle bacilli are expelled into a finite volume of air unless there is ventilation [3]. Furthermore, they may remain viable and suspended in the air for a prolonged period of time. Inside exposure is rare, but if it happens, exposure time and density of infectious droplet nuclei are often, respectively, prolonged and high.
In contrast, tubercle bacilli outdoors are rapidly dispersed and rendered quickly nonviable by sunlight or even sky light [4]. Outside exposure is usually brief and by chance, and the distance between the source and the exposed makes the risk of transmission very small during a single encounter. However, it might be a relatively common event, depending on the population characteristics and the setting. Most simply put, the risk of becoming infected is the result of the density of infectious droplet nuclei in the available air and the duration of inhalation of that air. Confined air systems with no or poorly filtered air exchanges pose the major risk for indoor transmission, be it at home [5], on a ship [6, 7], on a train [8], in an office building [9], or in a health care institution [10].
These 2 types of exposure risks coexist and determine the resulting prevalence of infection with M. tuberculosis in a population. Therefore, cumulative probability of becoming infected in a lifetime is determined by the risk of infection in both closed and nonclosed settings, and the importance of each contributor will vary in different settings [11]. These general principles of transmission also apply to the risk of acquiring infection with M. tuberculosis during travel.
Indoor Transmission During Travel
A high level of attention has been given to transmission during air travel [12]. However, epidemiologically, the overall public health importance of such events must be very small, for several reasons. The prevalence of transmissible tuberculosis in the air travel population might be in the order of 5–100 cases per 100,000 passengers, depending on the route of the plane. Considering the average number of people on an airplane, there is perhaps 1 encounter per 3–60 flights.
Should exposure to a potential transmitter of M. tuberculosis on a plane nevertheless occur, the ventilation system on modern airplanes will prevent tubercle bacilli from random distribution aboard it, because the laminar flow assures that air entering overhead leaves the cabin near the floor at approximately the same seat row. Furthermore, the efficient ventilation system rapidly dilutes the density of infectious particles [12]. Most important, the exposure time to such air is, by necessity, very limited by the duration of the flight. The example of transmission from a flight attendant to other cabin personnel is instructive in that way [13]. A flight attendant with undiagnosed cavitary tuberculosis exposed and infected several coworkers. Given the constraint of movement for cabin personnel, exposure was probably physically so close that it outpaced the ventilation system. Still, ⩾10 h were required to measure an appreciable elevation of the prevalence of infection with M. tuberculosis above the expected background prevalence. Most such events are likely to go unnoticed. Even where such incidents became known in well-conducted epidemiologic investigations, transmission from passenger to passenger has, as expected, only rarely been documented [12, 14, 15]. Air travel is both individually and from a public health point of view a negligible risk factor for transmission of M. tuberculosis. However, precisely because such casual encounters will only rarely become known, these few specific reports also point out that they do quite logically occur, albeit with a low frequency, and that each single one of which carries a very low (but distinct) risk that may accumulate over a lifetime.
Outdoor Transmission During Travel
The risk among long-term travelers is a different matter. Transmission during travel usually requires repeated, but brief, casual exposure outdoors. In a population-based study, the risk of acquiring infection with M. tuberculosis was measured among low-budget, long-term travelers from The Netherlands who largely visited Africa, Asia, and Latin America [16]. The study essentially confirmed that the risk of acquiring infection with M. tuberculosis among these travelers was essentially the same as that estimated for the general population in the host countries—that is, an annual risk of infection with M. tuberculosis of 1%–3% [17].
Thus, the risk of infection with M. tuberculosis increased from ∼10 latent infections per 100,000 person-years in The Netherlands to an incidence of ∼2000 latent infections per 100,000 person-years during travel, an impressive relative risk but still a modest absolute risk. In the Dutch study, it was not unexpected that the risk of infection for health care workers with direct patient contact was 3-fold larger than that for other travelers [16].
- Received March 9, 2001.
- Revision received May 8, 2001.
- © 2001 by the Infectious Diseases Society of America
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