6.3 Virulence of the MTC and Risk of Infection to Humans and Animals
The two distinct lineages of the RD9-deleted clade of the MTC appear to differ significantly in their virulence and the risk that they pose to humans and their domestic animals. Mycobacterium bovis, the dominant member of the Eurasian lineage, has become established in animal populations throughout the world and causes severe disease in numerous species.
Other members of this lineage are also the cause of TB in multiple hosts, and M. pinnipedii and M. orygis have been documented to cause TB in a number of species, including humans and cattle (Cousins 2003; Loeffler et al. 2014; van Ingen et al. 2012). Moreover, M. bovis and M. pinnipedii are highly virulent in experimental animal TB models (Cousins 2003; Dannenberg Jr and Collins 2001).In contrast, members of the African RD9-deleted lineage appear to display a less virulent phenotype, and the risk of transmission between host species may be lower. In humans, infection with M. africanum WAF2 is less likely to result in active TB than infection with M. tuberculosis (de Jong et al. 2008). Also, certain WAF2 strains show significant attenuation in laboratory animal models (de Jong et al. 2010), and despite the close association between humans and livestock, M. africanum is rarely isolated from domestic animals.
Similarly, the dassie bacillus, M. mungi, and M. suricattae are highly associated with specific host species. The genes encoding ESAT-6 and CFP-10 have been deleted from these pathogens, and such a loss has been associated with the attenuation of virulence of the vaccine strain M. bovis BCG and of M. microti (Mostowy et al. 2004). It is notable that the only known hosts of two members of this clade are mongoose species (the banded mongoose and the meerkat) and that the only known transmission of the dassie bacillus to a novel host has been to a meerkat (Mostowy et al. 2004).
This phenomenon might merely reflect an epidemiological link between these hosts, but it might equally be indicative of a limited host range of these pathogens, possibly dependent on immunological susceptibility to mycobacterial disease (Clarke et al. 2016). While these organisms have not, to date, been associated with the infection of humans or livestock (Alexander et al. 2015), they are able to induce fatal disease in susceptible hosts, and the movement of potentially infected animals should be undertaken with care.Since infection by members of the MTC typically presents as an insidious chronic disease, the risk for introduction into new populations is significant, for example, the risk presented by M. pinnipedii-infected sea lions (Neophoca cinerea) in zoos as a source of zoonotic disease (Cousins 2003). Similarly, the detection of the dassie bacillus in hyraxes in zoos outside of South Africa demonstrates the potential for introduction of these organisms to novel locations (Cousins et al. 1994; Mostowy et al. 2004). Zoos and other private facilities that import exotic wildlife may unknowingly import novel diseases that are not detected during the typical quarantine period. Since the susceptibility of native wildlife in these locations is undetermined, there is a potential threat of possible spillover to these species and to other zoo animals.
6.4