Environmental Reservoir of the Mycobacterium avium Complex

MAC disease, at least in humans, is not contagious. In a summary of his experiences with Battey-type pulmonary disease, Corpe (1964, p. 381) comments:

We have never found a source case.

In spite of the fact that well over 95% of the patients we see have been or are married, we have never seen either a husband or a wife also clinically ill with the disease. We have never seen two cases in the same family. This is an entirely different epidemiologic picture than is observed in Mycobacterium tuberculosis infections.

The epidemiology of MAC in humans differs from tuberculosis because the source of infection is different (Falkinham, 1996). MAC is typically acquired from soil or water. Despite some success in using molecular techniques to match patient specimens with isolates from environmental sources (von Reyn et al., 1994; Lande et al., 2019), the vast environmental reservoir of the MAC confounds such studies. These mycobacteria are abundant in diverse geographical regions, soil types, aquatic ecosystems and urban water distribution systems (Falkinham, 2002). Mycobacterium avium DNA was even detected in samples from the space station Mir (Kawamura et al., 2001). Opportunities for exposure are so extensive that it is extraordinarily challenging to identify the time and place of infection and then, months or years later, actually isolate the causative MAC clone from that site. Such efforts are further complicated by the fact that infections can be polyclonal (Arbeit et al., 1993; Wallace et al., 1998). The ecological activities of the MAC are largely unknown. In aquatic systems, including swimming pools, hot tubs and municipal pipes, these mycobacteria form biofilms, which enhance resistance to disinfectants and other antimicrobial agents (Vaerewijck et al., 2005). Planktonic cells sloughed from a biofilm can be aerosolized or ingested, and thus contribute to MAC infections.

Aerosolized bacteria from contaminated heatercooler units triggered an international outbreak of M. chimaera disease in cardiac surgery patients (van Ingen et al., 2017). Not all environmental MAC are free-living. Via a process reminiscent of mammalian macrophage infection, M. avium can invade and replicate within protozoa (Cirillo et al., 199 7; Steinert et al., 1998). The intracellular space is a refuge that provides the mycobacteria with nutrients and protects them from biocides (Steinert et al., 1998). Experiments with tissue culture and animal models even suggest that amoeba-grown bacteria are more virulent than those propagated in standard culture media (Cirillo et al., 1997). As such, MAC-infected protozoans may be an important ‘environmental’ reservoir for human and animal disease (Samba- Louaka et al., 2018).

In birds, MAC disease (specifically M. avium subsp. avium) is considered to be contagious (Dhama et al., 2011). Transmission of avium tuberculosis is similar to that of paratuberculosis. Infected animals shed large amounts of organism that contaminate the environment and can then be inhaled or ingested by healthy animals. The risk of exposure increases if hygiene practices are inadequate and/or where numerous animals are closely confined, e.g. zoo aviaries.

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Source: Behr Marcel A., Stevenson K., Kapur V. (eds.). Paratuberculosis: Organism, Disease, Control. 2nd edition. — CAB International,2020. — 439 p.. 2020

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