Correspondence

Transmission of Tuberculosis during a Long Airplane Flight

N Engl J Med 1996; 335:675-676August 29, 1996

Article

To the Editor:

The article by Kenyon et al. (April 11 issue)1 is reassuring to the air traveler, but I question two classification decisions made by the authors that may have affected their results and conclusions.

According to the Centers for Disease Control and Prevention, indurations of 5 mm or larger on tuberculin skin testing should be classified as positive in close contacts of a person with infectious tuberculosis.2 In the study by Kenyon et al., the induration had to be 10 mm or larger for the test to be classified as positive. Would the number of positive skin tests, which indicated the number of people infected, have been larger if the conventional recommendation had been used? In addition, the classification of Contacts 6, 7, 10, and 11 as having a previous infection is inconsistent with the published data on the likelihood of infection with multidrug-resistant Mycobacterium tuberculosis among contacts thought to be newly infected.3 If this standard of classification had been used instead, 6 of the 55 people (11 percent) seated in the same cabin section as the index patient but not within two rows of her seat would have been classified as newly infected, and the rate ratio based on seat location would have been less than 3.

A pitfall for both researchers and practitioners in classifying such contacts is to attribute too many of the reactive skin tests in foreign-born persons to previous, rather than recent, infection. Without an adequate explanation of these deviations from the public health standards for assessing tuberculosis, it is not clear that the authors' findings and conclusions are supportable.

Mark J. Tracy, M.D., M.P.H.
County of San Diego Department of Health Services, San Diego, CA 92110

3 References

1. 1

   Kenyon TA, Valway SE, Ihle WW, Onorato IM, Castro KG. Transmission of multidrug-resistant Mycobacterium tuberculosis during a long airplane flight. N Engl J Med 1996;334:933-938
   Full Text | Web of Science | Medline

2. 2

   Core curriculum on tuberculosis: what the clinician should know. 3rd ed. Bethesda, Md.: Department of Health and Human Services, 1994.
   

3. 3

   Management of persons exposed to multidrug-resistant tuberculosisMMWR Morb Mortal Wkly Rep 1992;41:59-71
   

To the Editor:

In our opinion, the data provided by Kenyon et al. do not prove that multidrug-resistant M. tuberculosis infection can be transmitted during travel in an airplane. The fact that six persons who had sat in the same section of the plane as the index patient had positive tuberculin skin tests (four of whom had skin-test conversions) with no other risk factors for tuberculosis suggests but does not prove that multidrug-resistant M. tuberculosis was transmitted to these persons. The only way to prove this would be to demonstrate by a molecular biologic method1 that the same strain of multidrug-resistant M. tuberculosis that infected the index patient produced tuberculosis in a passenger or crew member on the flights. Without such proof, it is not reasonable to state that this study “provides evidence of the transmission of M. tuberculosis from passenger to passenger and from passenger to flight crew aboard commercial aircraft.”

The results of this study could cause unnecessary alarm among the general public. As the authors suggest, the top priority of tuberculosis-control programs is still to identify and ensure the complete treatment of all patients with active tuberculosis. What is most alarming is to learn that a tourist with overwhelming tuberculosis did not receive the health care she desperately needed during her stay in the United States.

José M. Aguado, M.D.
José T. Ramos, M.D.
Carlos Lumbreras, M.D.
Hospital 12 de Octubre, 28041 Madrid, Spain

1 References

1. 1

   Tabet SR, Goldbaum GM, Hooton TM, Eisenach KD, Cave MD, Nolan CM. Restriction fragment length polymorphism analysis detecting a community-based tuberculosis outbreak among persons infected with human immunodeficiency virus. J Infect Dis 1994;169:189-192
   CrossRef | Web of Science | Medline

Author/Editor Response

The authors reply:

To the Editor: The objective of using the cutoff points for tuberculin skin testing referred to by Dr. Tracy is not to determine who has a positive test but to increase the likelihood that persons at high risk for tuberculosis will be clinically evaluated as candidates for preventive therapy and to reduce the likelihood that persons with reactions to tuberculin not due to M. tuberculosis (e.g., persons with nontuberculous mycobacteria or recipients of bacille Calmette–Guérin vaccine) will not receive unnecessary and potentially toxic medications.1,2 Our epidemiologic investigation had a different objective — namely, to determine whether the transmission of M. tuberculosis occurred during air travel. We therefore established a cutoff point of 10 mm for the size of the induration, favoring specificity over sensitivity, before collecting data. These are important distinctions to make when determining outcomes based on tuberculin skin testing, given the limitations of this method.2 We did not classify passengers and crew as close contacts, because of the relatively short duration and low intensity of exposure. If we had used 5 mm of induration as the cutoff point, only four persons would have been added to the number with positive tests, and this change would not have affected our interpretation of the results. The other four passengers Dr. Tracy refers to were classified as having a low or low-to-intermediate likelihood of infection with multidrug-resistant M. tuberculosis, since their exposure was limited and their infection was more likely to have been the result of prior exposure to predominantly drug-susceptible M. tuberculosis in their native countries, where tuberculosis is highly endemic.3

With regard to the use of DNA fingerprinting, an isolate of M. tuberculosis from a clinical specimen is required to produce a DNA fingerprint. No specimens could be obtained from the asymptomatic passengers and crew members with positive tuberculin skin tests, since they did not have active disease. Our evidence of M. tuberculosis transmission on an aircraft met the commonly accepted criteria for causal inferences in epidemiology: strength of association, consistency, temporality, a dose–response effect, and biologic plausibility.4

Thomas A. Kenyon, M.D., M.P.H.
Sarah E. Valway, D.M.D., M.P.H.
Ida M. Onorato, M.D.
Centers for Disease Control and Prevention, Atlanta, GA 30333

4 References

1. 1

   Screening for tuberculosis and tuberculous infection in high-risk populations and the use of preventive therapy for tuberculous infection in the United States: recommendations of the Advisory Committee for Elimination of Tuberculosis. MMWR Morb Mortal Wkly Rep 1990;39:1-12
   Medline

2. 2

   Huebner RE, Schein MF, Bass JB Jr. The tuberculin skin test. Clin Infect Dis 1993;17:968-975
   CrossRef | Web of Science | Medline

3. 3

   Management of persons exposed to multidrug-resistant tuberculosisMMWR Morb Mortal Wkly Rep 1992;41:59-71
   

4. 4

   Rothman KJ. Modern epidemiology. Boston: Little, Brown, 1986:16-20.
   

Citing Articles (1)

Citing Articles

1. 1

   Gwangpyo Ko, Kimberly M. Thompson, Edward A. Nardell. (2004) Estimation of Tuberculosis Risk on a Commercial Airliner. Risk Analysis 24:2, 379-388
   CrossRef