© 2002 American Thoracic Society
Tuberculosis transmission—rogue pathogen, rogue patient, or the vagaries of change?To the Editor:In his recent editorial, "Tuberculosis transmission—rogue pathogen or rogue patient?", Bishai rightly stressed that more than one factor may determine the extent of tuberculosis outbreak (1). That some patients can be unusually effective disseminators of infectious droplet nuclei was clearly demonstrated by Riley and colleagues in 1959 (2), and it is likely, though less certain, that virulence varies among strains of Mycobacterium tuberculosis. It is well known that some strains are less virulent for guinea pigs than others (3, 4), but there is no solid evidence of similar variability for humans. However, there is a third factor that can account for variation in the extensiveness of tuberculosis outbreaks. Anyone who has worked with stochastic models of epidemics knows that chance can also play a part in determining how extensive an outbreak can be. For most, the proportion of susceptibles who become infected will be relatively close to the modal value, but on rare occasions none or only a few will be infected, or at the other tail of the distribution, all or nearly all susceptibles will be infected. A recent extensive tuberculosis outbreak on the Kentucky–Tennessee border, originally suspected to be due to a rogue pathogen or an unusually infectious patient, may well have represented one of the few in which a very large proportion of the population became infected as the result of a sequence of chance occurrences (5, 6). The complete question thus becomes, "Rogue patient, rogue pathogen, or the vagaries of chance?" Rogue patients can be suspected by clinical and behavioral characteristics—laryngeal lesions, frequent cough, failure to cover coughs, contacts with others in closed poorly ventilated spaces, etc. Now that strains can be identified by their DNA "fingerprints," it will be possible to see if extensive outbreaks are associated with certain strains. But to identify a truly rogue pathogen will require that it be associated with extensive outbreaks on an unexpectedly high proportion of occasions to rule out the possibility that chance was the real rogue.
Johns Hopkins Bloomberg School of Public Health Baltimore, Maryland REFERENCES
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