Bovine tuberculosis

 CAB International 2018. Bovine Tuberculosis

106 (eds M. Chambers, S. Gordon, F. Olea-Popelka, P. Barrow)

• Email: [email protected]

8.1 Introduction

Infection with Mycobacterium bovis is an ongoing
problem both to human and animal health,
costing billions annually in economic losses
(Skuce et al., 2011; Muller et al., 2013). While
eradication in some countries has met with suc-
cess, infection of animals and humans with
M. bovis is still reported globally. Understanding
the virulence mechanisms that allow M. bovis to
survive in vivo, cause disease and transmit to
new (and diverse) hosts will be key to the ulti-
mate eradication of M. bovis infection.
M. bovis is a member of the Mycobacterium
tuberculosis complex (MTBC), the grouping of
genetically related mycobacterial species that
cause tuberculosis in mammals (Frothingham
et al., 1994; Smith et al., 2006). Theobald Smith
was the first to demonstrate that the causative
agent of tuberculosis in cattle, and indeed other
animal hosts, was not the same as the human
bacillus, a finding that ultimately led to the
description of the bovine-adapted species
M. bovis (Smith, 1898; Karlson and Lessel,
1970). The hallmark member of the MTBC,
M. tuberculosis, shares 99.95% nucleotide
sequence identity with M. bovis (Cole et al.,
1998; Garnier et al., 2003). M. bovis is the best-
studied member of the MTBC that infects wild
and domesticated animals, but it is certainly not
the only one, with Mycobacterium microti found
in voles (Wells, 1937, 1946), Mycobacterium

caprae in goats and cattle (Aranaz et al., 1999),

Mycobacterium orygis in antelopes (van Ingen

et al., 2012), Mycobacterium pinnipedii in seals

(Cousins et al., 1993), Mycobacterium mungi in

mongooses (Alexander et al., 2010), Mycobacte-

rium suricattae in meerkats (Parsons et al.,

2013), and the eponymous Dassie bacillus

(Cousins et al., 1994). A core differentiating fea-

ture of the MTBC species is their varying host

preference – M. bovis has been found in the wid-

est range of hosts, including humans, cattle,

badgers, goats, sheep and deer (Skuce et al.,

2011; Muller et al., 2013; Palmer, 2013; Pes-

ciaroli et al., 2014). This nomenclature for the

members of the MTBC does not, however, define

host range and it hinges upon whether the host

in question is one to which the associated patho-

gen is adapted to and can successfully propagate

within (a ‘primary’ or ‘maintenance’ host) or

one to which the pathogen has been introduced

to but is not adapted to for propagation

(a ‘ secondary’ or ‘spillover’ host).

When considering the virulence of the

mycobacteria, there are a number of concepts

that need to be considered. Firstly, one can define

virulence as the degree of severity of pathology

caused by a pathogen, that is, its ability to inflict

damage on the host. However, the resulting

pathology is not solely due to the efforts of the

pathogen, but instead it is the interaction

between the host and the pathogen that results

in damage to the host, either due to the direct

8 Molecular Virulence Mechanisms of

Mycobacterium bovis

Alicia Smyth and Stephen V. Gordon*

School of Veterinary Medicine, University College Dublin, Dublin, Ireland