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What is atypical tuberculosis (TB)?

Q: What is atypical tuberculosis (TB) non-tuberculous mycobacteria (NTM) or Mycobacterium Other Than Tuberculosis (MOTT) infection? How is it treated?

A:The term "Atypical TB" may be used to signify a group of closely related diseases caused by bacterial organisms belonging to non-tuberculous mycobacteria (NTM) or Mycobacterium Other Than Tuberculosis (MOTT). Although they share some common characteristics to the bacteria casing TB namely Mycobacterium tuberculosis, there are significant differences in the spectrum of diseases (e.g. incidence of extra-pulmonary disease such as skin, lymph node, disseminated disease, intestinal involvement, catheter related infections), patient characteristics (e.g. immunocompromised conditions due to HIV or chemotherapy), presentation, clinical course and outcome of disease (e.g. chest infection due to MOTT), specific drug, (e.g. need of macrolides such as Clarithromycin in regimens) and duration of treatment (e.g. longer duration of treatment~ 2 years, in chest infection). MOTT or NTM is not a single organism or a group, but consists of a diverse group of related organisms all belonging to the genus Mycobacteria. Although all of them are acid-fast bacilli, and may closely resemble M. tuberculosis when examined microscopically, they differ from each other in their growth requirements and growth characteristics on culture (e.g. need of light to produce pigments photochromogens, as opposed to scotochromogens, and non-chromogens), time required to grow in culture media (e.g. rapid growers generally grow within a week) and biochemical and genetic characteristics.

The diseases caused by NTM or MOTT have a much more indolent course of disease compared to that of M. tuberculosis. Since many of these organisms are present in the environment isolation of these organisms from clinical specimens may represent colonisation and contamination and may not necessarily correlate with disease. The treatment of these groups of pathogens depends on the specific bacteria involved (as identified by culture), disease or organ affected (chest, lymph node, skin, disseminated infection), and drug susceptibility of the specific pathogen. Some common examples of diseases caused by MOTT or NTM are M. avium-intracellulare complex casing chest infection in immunocompromised patients (e.g. those with HIV infection and low CD4 counts), M. scrofulaceum causing scrofula a form of lymphadenopathy (disease of the lymph nodes), M. marinum (causing swimming pool or fish tank granuloma- a type of skin infection). Although some drugs used in the treatment of tuberculosis are also used in the treatment of MOTT infections (such as rifampicin and ethambutol), the choice of a third agent (e.g. clarithromycin), prophylactic agents (e,g. azithromycin or clarithromycin or ciprofloxacin) are different from those of TB. In some cases surgery (e.g. excision of an affected lymph node) may be required. The treatment recommendations will also depend on whether the patient is HIV positive or negative.


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