Transmission of Mycobacterium
tuberculosis from passenger to passenger aboard a long airplane flight was
reported in 1996 (1). Other cases were previously published, in particular by the Centers
for Disease Control which had then suggested criteria for epidemiological investigation in
such cases (2). More recently, in 1998, the World Health Organization (WHO) has published
guidelines on such investigations, emphasising duration of the flight (more than eight
hours), the infectiousness of index patient, and the proximity of those exposed (3).
On 10 May 1998, the Swiss Office of Public Health (Division of
Epidemiology and Public Health) informed the Greek National Centre for Surveillance and
Intervention of an infectious case of tuberculosis who had travelled from Bangkok to
Zurich via Athens on 15 April. He was a young Thai man, with productive cough and blood
stained sputum. He had been admitted to hospital in the University Hospital of Basel,
where abundant acid fast bacilli had been identified in spontaneously produced sputum and M.
tuberculosis resistant to isoniazi had been cultured. Since the first part of his
journey (Bangkok-Athens) had exceeded eight hours, we conducted an investigation to
identify positive contacts.
A list of all 144 passengers from Bangkok to Athens and the crew was
obtained from the airline company. The list of passengers included no addresses or
telephone numbers so we tried to locate them from frequent flyer and tour operators
records. The passengers and crew members were informed by telephone and letter about their
potential exposure to tuberculosis and the possible risks and were advised to have a
baseline tuberculin purified protein derivative (PPD) skin test, and to complete a
questionnaire. PPD tests were to be repeated 12 weeks after exposure if the initial test
was negative. If the baseline test was positive they were to be reviewed by experts for
risk factors for tuberculosis.
Two investigators made about 600 telephone calls and sent 190 letters
to notify passengers, tour operators, and hotels. Twenty passengers were contacted but
only three had a baseline PPD test. Two had negative tests; chest radiography of the
passenger with a positive PPD test showed no evidence of tuberculosis. None of the crew
members responded to our notification, but we insisted that four members of the crew, who
worked in the cabin area occupied by the index patient, were tested. In all four PPD was
All contacts who had a negative baseline test were advised to have a
repeat test for conversion. The crew members were retested with negative results and the
passengers failed to comply.
The limited success of this case investigation reflected several
constraints of such investigations. For several weeks we tried to contact people worldwide
at an estimated cost of about US0.$4000 (3742 Euros) and 300 staff hours with very poor
results among the passengers. One reason for the lack of success is that most airline
companies have limited information about passengers. In addition, the compliance of the
passengers was seriously affected by their diverse international origin (a fact for most
long distance flights) and their distance from the investigators. Both factors also
increase the cost of such investigations.
Our experience has made us skeptical about the cost-effectiveness of
such investigations. Strategies for tuberculosis prevention and control vary between
countries, making the interpretation of results difficult (4). Moreover, the risk of
transmission of M. tuberculosis infection on aircraft appears to be no greater than
in other confined spaces, and seems to be low (2,5).
Air carriers should be encouraged to keep full lists of passengers for
some months after the flight. Furthermore, such case investigations should be restricted
to highly infectious cases (for example, cases of laryngeal tuberculosis or cases of
sputum positive tuberculosis who cough a lot during the flight) or cases
infected with multidrug resistant strains of M. tuberculosis (6).
As our investigation ended, WHO published guidelines for the prevention
and control of tuberculosis transmission during air travel (3). WHOs guidelines also
described difficulties encountered in such investigations. They are extremely labour
intensive, taking time and resources from other important public health activities, as we
found. WHO proposes that the best approach in many cases is to inform passengers and crew
of their potential exposure to M. tuberculosis and to encourage them to seek
medical assessment. WHO also proposes criteria to decide whether to inform passengers and
crew; these criteria include the infectiousness of the person with active tuberculosis,
duration of the flight, time interval between the flight and the notification of the case
to the health authority, and proximity of passengers and crew to the index case.