Role of Wildlife in the Epidemiology of Mycobacterium bovis 99
national parks have a responsibility to protect a
number of the species that host M. bovis, compli-
cating control options. Due to the deleterious
impact of culling, control strategies have focused
on preventing the movement of infected wildlife
to curtail further spread. However, if M. bovis
diagnosis in wildlife increases movement restric-
tions, endemic areas (e.g. Kruger National Park)
can become conservation islands. As movement
of endangered species between national parks
becomes restricted, conservation efforts can
suffer from the limited exchange of genetic
resources (Michel et al., 2006). Such dilemmas
in how to control M. bovis in species-rich ecosys-
tems of high conservation importance are fur-
ther complicated by the paucity of understanding
on the true roles that each host species play.
However, control in Southern Africa remains
critical as M. bovis threatens the survival of
endangered species (Michel et al., 2006) and
poses a zoonotic risk, especially in HIV-endemic
communities surrounding conservation areas
(Hlokwe et al., 2014).
7.3 Transmission at the Livestock–
Wildlife InterfaceGiven the diversity in host species and transmis-
sion routes, transmission of M. bovis at the live-
stock–wildlife interface is perhaps unsurprising.
However, the role of wildlife in the epidemiology
of livestock disease can be controversial. There
are many studies that quantify wildlife–livestock
interactions, both direct (e.g. Bohm et al., 2009)
and indirect (e.g. Hutchings and Harris, 1997),
that may represent routes of M. bovis transmis-
sion. However, there is often little if any direct
evidence of transmission, let alone quantifica-
tion of the rates of transmission. The best evi-
dence supporting the role of wildlife in the
epidemiology of M. bovis in livestock comes from
the results of disease control actions.
Local eradication of M. bovis in cattle and
national TB-free status has been achieved fol-
lowing the implementation of disease control
methods applied to livestock species alone (More
et al., 2015; and see Chapter 14). These successes
have resulted from national control strategies
based on test-and-cull aligned with livestock
movement restrictions and implemented using
the skin test (Radunz, 2006; More et al., 2015;
and see Chapter 11). Reduced performance and
failure of this strategy is associated with rich
M. bovis host communities and known wildlife
maintenance hosts (Bessell et al., 2012;
Hardstaff et al., 2014).
The dynamic of wildlife–livestock transmis-
sion varies within and between ecosystems, and
is dependent on a multitude of factors including
species composition, densities, contact rates,
behaviours, susceptibility, pathology and excre-
tion. Wildlife are thought to play an important
role in livestock M. bovis outbreaks in New Zea-
land, where contact with infected possums (both
direct and indirect) is believed to be a driving fac-
tor in a majority of M. bovis breakdowns in live-
stock (Hutchings et al., 2013). In the advanced
stages of infection, possums can become debili-
tated, unable to climb and wander about in day-
light, leaving them exposed to contact with
inquisitive livestock (Paterson and Morris,
1995). Inter-species transmission is thought to
occur when dead or terminally ill possums are
inquisitively licked or sniffed by livestock (Sauter
and Morris, 1995). Consequently, lethal control
of possum populations has been implemented,
at a cost of US$40million per year (since 1994).
This has contributed to the >95% reduction in
M. bovis-infected cattle and managed deer herds
in New Zealand (Buddle et al., 2015). While it is
clear that that the control strategy was a suc-
cess, the contribution of possum control cannot
be isolated as livestock testing and movement
restrictions were also in place.
In the UK and Ireland, badgers are the wild-
life species thought to play the biggest role in the
epidemiology of livestock disease, with bi-
directional transmission between badgers and
cattle (Jenkins et al., 2007b). Strain typing of
isolates from badgers and cattle populations has
revealed close linkages in the spatial distribution
of M. bovis strain types in the two host species
(Olea-Popelka et al., 2005; Woodroffe et al.,
2005). This finding that spatially overlapping
cattle and badger populations have similar
M. bovis strains is consistent with inter-specific
transmission, or both hosts being infected from a
common source. Badgers often forage for food
items (such as earthworms) on pasture used by
cattle, and cattle graze badger latrines and other
sites contaminated with urine, faeces or infected
pus from bite wounds and scent marks