No. 7: Frequently Flying XDR/MDR TB—Reason Gajillion to Avoid Air Travel

It was a confluence of infectious disease and the explosive celebrity of the hapless, clueless common man. Beyond inspiring another “Law and Order” episode, itinerant lawyer Andrew Speaker introduced the wider world to 2 ominous forms of tuberculosis—multidrug resistant (MDR) and extensively drug resistant (XDR). His story highlighted the difficulty of gauging the contagiousness of TB in the non-sick sick and negated the idea that educated, infected citizens will act responsibly.

Just a few days before Speaker knew that his bronchoscopy culture grew Mycobacterium tuberculosis, the CDC published a tally of known XDR-TB cases within the United States in its March 23 issue of MMWR. From 1993 to 2006, the US National TB Surveillance System identified 49 individuals with the extensively resistant pathogen—that is, TB resistant to first-line drugs isoniazid and rifampin, any fluoroquinolone, and at least 1 second-line injectable agent (amikacin, capreomycin, or kanamycin). Most were located in New York City (19) or California (11).

Transmission of TB, which is almost exclusively airborne, depends on 3 major factors (Marrie TJ, ed. Community-Acquired Pneumonia. New York, NY: Springer; 2001.): the contagiousness of the index case, the case’s proximity to others, and the susceptibility of others (eg, immunocompromise due to HIV coinfection). The contagiousness of a case depends on the following:

·         Site of disease (pulmonary infection obviously provides the greatest opportunity for airborne transmission);

·         Extent of disease (heightened risk of transmission is associated with laryngeal involvement);

·         Treatment (both its existence and effectiveness); and

·         Behavior of the individual (for example, risk is increased with frequent and forceful coughing).

When the sputum smear for AFB is negative but the culture is positive, contagiousness is considered negligible after 2 weeks of effective treatment (providing that the TB is drug susceptible). Risk of transmission also depends on the duration of exposure and the environment of contact: a cramped plane cabin on an overseas flight represents a bad scenario. In Speaker’s case, he hopped on no less than 7 flights in May—one of which was an Air France/Delta flight from Atlanta to Paris with 433 passengers and another of which was a Czech Airlines flight from Prague to Montreal with 191 passengers. Despite his risky behavior, passengers sitting in proximity to Speaker on either plane were reported to be uninfected as of late November.

An editor’s note accompanying the MMWR article advised that the identified XDR-TB cases are an underestimate, owing to incomplete data and the pitfalls of drug-sensitivity testing (DST). Approximately 20% of reported TB cases do not have positive cultures that enable DST, and only 22% of reported MDR cases were associated with complete DST results, thereby enabling the identification of XDR TB. Conversely, the consistency of DST results from laboratory to laboratory has been called into question: TB grown from Speaker and that of another publicized individual were downgraded from XDR to MDR.

The XDR variant likely resulted from the aggressive treatment of MDR-TB cases during the last part of the 20th century. But the relative contribution of person-to-person transmission of XDR TB and that of XDR TB emerging in an MDR-infected person are presently unknown.