Role of Wildlife in the Epidemiology of Mycobacterium bovis 97
high in some groups and low in others. Higher
M. bovis prevalence is found in males, which
have larger home ranges and make contact with
more unrelated deer (Cosgrove et al., 2012).
There is a positive correlation between deer den-
sity and disease prevalence (Hickling, 2002),
with a decrease in disease prevalence in both
white-tailed deer and elk following population
reductions (Shury and Bergeson, 2011). As
transmission is density dependent it is affected
by population management measures, with sup-
plemental feeding leading to an overall increase
in M. bovis transmission and more spatially
homogenous disease distribution. The transmis-
sion implications of supplemental feeding have
been highlighted by Palmer et al. (2004a), who
demonstrated indirect experimental M. bovis
infection in deer through the sharing of feed.
7.2 Multi-Host Complexes
and the Ecology of M. bovis
Dynamics and PersistenceThe previous section discussed M. bovis infection
in key host species. For each species, transmis-
sion is dependent on the pathology of infection,
excretion potential, the minimum infective dose
and interactions between infected and suscepti-
ble individuals. However, numerous host species
often co-exist, and maintenance of M. bovis at
the system level is dependent on multi-species
complexes. On a simplistic level, hosts are often
described as either maintenance hosts or spill-
over hosts. Maintenance hosts can maintain
infection in a given area via intra-specific routes
of transmission, without any inter-specific
transmission (domestic or wild). In contrast,
persistence of infection in spillover hosts
requires continuous inter-specific transmission.
Although both maintenance and spillover hosts
can act as disease vectors, it is generally consid-
ered that the M. bovis hosts with greatest impli-
cations for disease control are maintenance
hosts with high potential to transmit to other
species. However, it is at least theoretically pos-
sible for host communities comprised of only
spillover species to maintain infection. In such
cases, control applied to any single species is
likely to exaggerate their perceived role in main-
taining the infection.
The role of maintenance hosts is typified
in New Zealand’s brushtail possums, where
M. bovis can persist in possum populations that
are completely isolated from infected livestock
(Morris and Pfeiffer, 1995). In these areas, epi-
demiology suggests that pigs, ferrets and deer
are spillover hosts (Morris and Pfeiffer, 1995;
Ragg et al., 1995; Jackson, 2002). The predomi-
nance of tuberculous lesions in the head and
gastrointestinal tract of sympatric scavengers in
New Zealand (e.g. wild pigs and ferrets) suggests
that such wildlife are infected through scaveng-
ing carcasses of infected possums (Ragg et al.,
1995; Coleman and Cooke, 2001). The role of
deer as spillover hosts is supported by evidence
that M. bovis in deer was eradicated in areas
where possum population control was carried
out, but levels in deer were maintained in corre-
sponding areas where possum numbers were
not reduced (Palmer et al., 2015). Captive deer
have been observed licking and biting moribund
M. bovis-infected possums (Sauter and Morris,
1995), demonstrating a potential transmission
route.
The role of sympatric wildlife species in dis-
ease transmission and maintenance varies geo-
graphically with land use, ecology and habitat,
and consequent host behaviours and population
densities. While deer are considered spillover
hosts in New Zealand, where hunting pressure is
high and population densities are low, in Michi-
gan, USA, self-sustaining outbreaks in wild cer-
vid populations have been reported (O’Brien
et al., 2002). Their role in maintenance and
transmission is partly due to their high popula-
tion density, with concentrations of deer at arti-
ficial feeding sites that can facilitate transmission
(O’Brien et al., 2011). As a consequence of
density dependent transmission, control efforts
focus on population reduction.
The status of suids as hosts of M. bovis also
varies dramatically, and is dependent on density,
levels of infection in sympatric species, disper-
sion and ecological factors. The highest preva-
lences of M. bovis in wild boar are found in
regions with highest population densities, which
are usually artificially maintained through
intensive game management (i.e. supplemen-
tary feeding and watering, translocating and
fencing). In areas of high suid population den-
sity, intra-specific transmission can be sufficient
for M. bovis to be independently maintained