Mycobacterium tuberculosis and the search for the impossible truth

Epidemiol. Infect. (2015), 143, 949–950. © Cambridge University Press 2014 doi:10.1017/S0950268814001794 COMMENTARY Mycobacterium tuberculosis and the search for the impossible truth T. F. BREWER* University of California, Los Angeles, CA, USA Received 11 June 2014; Accepted 25 June 2014; ﬁrst published online 27 August 2014 The ability of accidentally inoculated Mycobacterium tuberculosis to cause skin and soft tissue infections has been known for almost two centuries, yet it was, and remains, a rare form of tuberculosis (TB) [1]. Because M. tuberculosis is assumed to have very limited survival outside the host, environmental reservoirs are not felt to have any role in TB transmission [2]. When inoculation TB cases are reported, they tend to be iso- lated or to include a limited number of cases over a short period of time [3]. In contrast, non-tuberculous mycobacteria (NTM) are ubiquitous in the environment, and are readily iden- tiﬁed from water, soil, instruments and other sources [4]. They can be resistant to disinfection, and have been responsible for a range of skin and soft tissue infec- tion outbreaks through a variety of inoculation proce- dures including tattoos, surgical procedures, injections and acupuncture among others [5]. In one epidemic associated with video-assisted surgical instruments, 1051 possible cases of Mycobacterium massiliense wound infection were identiﬁed across 38 Brazilian hos- pitals over an 11-month period [6]. Among mycobac- teria, the members of the M. tuberculosis complex and NTM are presumed to inhabit two different worlds. What then is one to make of the recent report by Wang and colleagues in this Journal reporting 13 cases of acupuncture-associated inoculation TB occur- ring over a 4-month period in patients attending a southeastern China clinic? [7]. The epidemiological evi- dence supports a point source for the outbreak, and the microbiological evidence supports M. tuberculosis as * Address for correspondence: T. F. Brewer, MD, MPH, University of California, Los Angeles, Box 951405, Los Angeles, CA 90095, USA. (Email: tbrewer@conet.ucla.edu) the causative agent. But given conventional wisdom that M. tuberculosis should not exist in the environment for any meaningful length of time, one must consider alternative explanations. The ﬁrst would be that these cases were unrelated; however, the cases all attended the same clinic, developed infections at the site of known injections, had – when tested – the same strain of M. tuberculosis and new cases stopped occurring when the clinic switched to disposable needles. The conclusion that these cases were unrelated seems improbable. Another possibility was that NTM were misiden- tiﬁed as M. tuberculosis. All 13 patients were M. tu- berculosis culture positive, and the 10 patients who underwent interferon-gamma release assay testing using the T-SPOT.TB test were also positive. Five iso- lates were genotyped at the Disease Control Centre of Zhejiang Province and found to be M. tuberculosis, Beijing type. Misdiagnosis of NTM also seems im- probable. Another possibility is laboratory cross- contamination. Twelve patients had positive stains for acid-fast bacilli at aspiration or surgical drainage, and granulomatous reactions were found on histopatho- logical examination of biopsies from all patients. The cases also were spread over a 4-month time period, further making laboratory cross-contamination unlikely. To paraphrase the great ﬁctional detective Sherlock Holmes, once the improbable has been ruled out, one must consider the impossible, i.e. that M. tuberculosis can survive in the environment for extended periods of time and remain capable of causing disease. Mycobacterium bovis, another member of the M. tu- berculosis complex, has previously been shown to be able to survive in the environment for extended peri- ods of time, and soil-contaminated M. bovis may be responsible for ongoing outbreaks of disease in cattle in the UK, for example [8, 9]. Mycobacterium canettii,