Increasing and More Commonly Refractory Mycobacterium Avium Pulmonary Disease, Toronto, Ontario, Canada

Daan Raats; Sarah K. Brode; Mahtab Mehrabi; Theodore K. Marras


Emerging Infectious Diseases. 2022;28(8):1589-1596. 

In This Article

Abstract and Introduction


In mid-2014, Public Health Ontario Laboratories identified coincident increasing Mycobacterium avium isolation and falling M. xenopi isolation in the Toronto, Ontario, Canada, area. We performed a retrospective cohort of all patients in a Toronto clinic who began treatment for either M. avium or M. xenopi pulmonary disease during 2009–2012 (early period) or 2015–2018 (late period), studying their relative proportions and sputum culture conversion. We conducted a subgroup analysis among patients who lived in the Toronto-York region. The proportion of patients with M. avium was higher in the late period (138/146 [94.5%] vs. 82/106 [77.4%]; p<0.001). Among M. avium patients, conversion was lower in the late period (26.1% vs. 39.0%; p = 0.05). The increase in the proportion of patients with M. avium pulmonary disease and the reduction in the frequency of sputum culture conversion is unexplained but could suggest an increase in environmental M. avium exposure.


Pulmonary infection with nontuberculous mycobacteria (NTM) is a chronic, often progressive, debilitating disease. Most published data show that the frequency of NTM pulmonary disease (NTM-PD) is increasing worldwide,[1–6] as are its substantial medical costs.[7,8] The cause of this rise has not yet been elucidated. NTM are widespread in the environment but disease is uncommon, suggesting that host susceptibility is critical, although exposure magnitude is also likely key.[9–11] Some observations indicate that the Mycobacterium avium complex (MAC) might be a main driver for the increased occurrence of NTM-PD.[2,5,12]

In Ontario, Canada, a rising prevalence of NTM-PD has been demonstrated previously, and, in the most recent years that have been studied, that increase was driven largely by an increase in MAC.[2] More recently, in the spring of 2014, the Public Health Ontario Laboratory (PHOL) observed a sustained increase of >50% in the total number of M. avium isolates from pulmonary samples and persons with positive cultures for M. avium.[13] A coincident reduction in M. xenopi isolates occurred without change in other NTM species. Curiously, this occurrence was only observed in the city of Toronto and the region immediately north (Regional Municipality of York), located between Lake Ontario and Lake Simcoe, which together encompass an area of 2,392 km2 and had ≈4.1 million inhabitants as of 2018. In Ontario, at least 95% of NTM isolates are identified at the PHOL,[14] permitting population-based study. Laboratory techniques at PHOL did not change at the time of increased isolation. Although the sudden increase in isolation frequency could suggest increased environmental exposure, the reason remains unclear.

Whether and how those changes relate to treatment outcomes of patients with NTM-PD caused by M. avium and M. xenopi has not been evaluated. In recent years, we observed that patients with M. avium pulmonary disease (Mav-PD) more often had microbiologically refractory disease and that we were encountering fewer patients with M. xenopi pulmonary disease (Mx-PD). On the basis of those observations, we studied relative proportions, culture conversion, culture reversion, and clinical treatment success of patients with Mav-PD and Mx-PD before and after 2014.