Role of Wildlife in the Epidemiology of Mycobacterium bovis 95
higher prevalence than females, which is attrib-
uted to behavioural differences, with the males
involved in aggressive behaviour associated
with territorial defence (Gallagher and Clifton-
Hadley, 2000). Following detection of bacterial
excretion, males succumb to infection sooner
than infected females (Graham et al., 2013;
Tomlinson et al., 2013).
Badgers live in social groups typically com-
prised of three to ten individuals (Vicente et al.,
2007a), and their behaviour strongly influences
M. bovis transmission. Aerosol transmission is
proximity dependent, and groups living with
communal use of underground chambers
(within setts) provides the opportunity for dis-
ease spread. Scent plays a key role in badger
communication and all adult badgers scent
mark territories, by defaecation, urination and
anal and sub-caudal excretion, including at
latrine sites (Neal, 1986). Bite wounds are
commonly obtained during territorial defence
and mating behaviours, especially in males
( Gallagher and Nelson, 1979; Cheeseman et al.,
1989). Environmental contamination from
latrine use and inoculation from biting behav-
iours provide potential intra-specific trans-
mission routes. To further complicate the
relationship between badger behaviour and
transmission, M. bovis culture-positive badgers
show increased ranging behaviour compared to
test-negative individuals (Garnett et al., 2005),
are more socially isolated within their own
groups, and display an increase in intergroup
contacts (Weber et al., 2013).
7.1.3 Wild boar and feral pigsHigh prevalences of M. bovis infection have been
recorded in wild suid populations. In Spain, for
example, where wild boar populations densities
can reach 90 individuals per km^2 (Acevedo et al.,
2007), M. bovis prevalence up to 100% has been
recorded (n = 14) (Vicente et al., 2006). There is
geographic variation in M. bovis levels, for
example prevalence values of 0–40% have been
recorded in Australia, where densities are typi-
cally below 11 individuals per km^2 (Naranjo
et al., 2008).
No sex differences in prevalence have been
found, but the probability of infection does
increase with age (Vicente et al., 2013) and roles
in transmission are likely to be age dependent.
Martín-Hernando et al. (2007) found a high pro-
portion of juveniles had severe lesions in multi-
ple anatomical areas, and the potential to
excrete mycobacteria by several routes. As these
are the age groups when dispersion is at a maxi-
mum, they have the potential to contribute to
spatial spread. In contrast, piglets are thought to
play a limited role in transmission, as in experi-
mental studies transmission between infected
and uninfected piglets is rare (Gortazar et al.,
2015). Piglets have the potential to acquire
infection by pseudovertical transmission, as
tuberculous lesions have been found in the
mammary glands of sows (Martín-Hernando
et al., 2007). The distribution of tuberculous
lesions vary across boar populations with high
M. bovis prevalence; it is likely that individuals
with generalized infection and high lesion scores
play a disproportionate role in initiating new
infections (Martín- Hernando et al., 2007).
It is possible that infections occur through
both respiratory and food-borne routes, as wild
boar have been found with tuberculous lesions
only in the thoracic region (usually bronchial
lymph nodes) or with only abdominal lesions
(usually mesenteric lymph nodes) (Martín-
Hernando et al., 2007). In addition to the geo-
graphical variation in disease prevalence, there
is also spatial variation in disease progression.
For example, lesions in the thoracic region are
displayed in almost 50% of M. bovis-infected
wild boar in Mediterranean Spain (Martín-
Hernando et al., 2007), while fewer than 10% of
infected wild boar in Atlantic Spain consistently
present thoracic lesions (Muñoz-Mendoza et al.,
2013). In contrast, the tuberculous lesions in
feral pigs in New Zealand are mainly found in
the head, in association with the lymph nodes
(Nugent, 2011). Such variation in pathogenesis
is indicative of differing infection sources.
Suids are implicated as spreaders of M. bovis
(Martín-Hernando et al., 2007; Naranjo et al.,
2008). There is potential for mycobacteria to be
excreted in the saliva as wild boar have a high
proportion of tuberculous lesions in mandibular
lymph nodes and tonsils, and mycobacteria have