Nontuberculous mycobacteria

In the current issue of the Canadian Respiratory Journal, Al-Houqani et al (1) (pages 19–24) examined the isolation prevalence of nontuberculous mycobacteria (NTM) in Ontario in 2007, and compared it with previous experience. Isolation prevalence essentially refers to the number of positive sputum cultures for these organisms related to the number of patients and the population at large. The data were from the Toronto Public Health Laboratory (Toronto, Ontario), which processes at least 90% of these sputum specimens for the province. The same group previously examined the same question over the years 1997 to 2003, and found that the prevalence increased from approximately nine patients per 100,000 population to 14 patients per 100,000 population (2). In 2007, it increased to approximately 19 patients per 100,000 population, which, needless to say, was highly significant.

Nick R Anthonisen.

What is going on here? Changes in isolation techniques did not occur during either study period (1,2). Either colonization and/or disease due to these organisms were, in fact, increasing, or physicians were finding more of them without such an increase. If NTM disease/colonization were not increasing, but physicians were requesting more cultures based on general awareness, the increase in isolation prevalence should have been accompanied by an increase in negative cultures, but this did not occur. Indeed, the fraction of samples that were positive – approximately 10% – did not change between 2002 and 2007. Thus, given the same set of patients, the findings cannot be based on physicians simply ordering more tests. The increase may also be due to differing patterns of behaviour by physicians in following patients. Positive cultures must represent both incidence and prevalence and, if the former did not change, but more physicians obtained more sputum samples during follow-up, then isolation prevalence would increase. Unfortunately, data for the years between 2003 and 2007 were not available; therefore, the frequency of repeat offenders could not be measured. The authors discount this because before 2003, it was uncommon for one patient to have positive cultures in more than one year, but admit that this may not be currently relevant. On the other hand, it may be that physicians improved their discriminatory powers and, consequently, discovered more cases. The obvious cause for this may be an increased frequency of computed tomography lung scans yielding patterns similar to the ‘tree and bud’ suggestive of NTM infection. This, it seems to me, is a reasonable hypothesis, but there is nothing in the data from Al-Houqani et al that suggests that this, in fact, occurred; the frequency of positive sputa did not increase as it perhaps should have with improved methods of detection.

The isolation prevalence of NTM of 19 patients per 100,000 population in Ontario was higher than reported elsewhere, but others have shown that it is increasing, although the other results are not as current as those of Al-Houqani et al. What does this mean in terms of disease burden? As implied above, isolation of these organisms does not necessarily signify illness because it could result either from colonization without disease or, possibly, contamination with ubiquitous organisms. Smear-positive sputa have been associated with disease, and the frequency of positive smears remained constant in Ontario, implying an increase in this index of disease. Previously, the same group estimated that approximately 25% of patients with positive cultures have disease – also implying an increase (3). Al-Houqani et al believe that disease due to these organisms is likely increasing, led by an increase in the isolation of avian mycobacteria. The reasons for such an increase are unclear, and may relate to the increasing prevalence of immunocompromised hosts – something that cannot currently be measured. Regardless of the reason, an increase is bad news. Disease due to these organisms is difficult to ascertain with certainty and very difficult to treat (4). Currently, management of individual patients with NTM is much more difficult than management of pulmonary tuberculosis.