234 P. Livingstone and N. Hancox
livestock infection is not identified, additional
surveys may need to be undertaken, as it is pos-
sible that the prevalence of infection in the
maintenance host may vary over time.
Given there is sufficient ecological and epi-
demiological information to show an association
between infection in a particular wildlife species
and cattle that is indicative of a maintenance
host, then this finding needs to be investigated
further. It is likely to require undertaking a
broader epidemiological and ecological investi-
gation, concentrating on areas where there have
been other unexplained TB breakdowns in cattle
herds. In addition, areas where the wildlife spe-
cies is present, but TB is absent from cattle herds,
should also be investigated. These investigations
will help to determine the distribution of infected
wildlife species, their TB prevalence, their rela-
tionship to TB breakdowns in cattle herds, and
the exact mode of disease transmission from
wildlife species to cattle (and vice versa) as iden-
tified by O’Brien et al. (2011). Over time these
surveys will also provide data to determine
whether TB infection in wildlife is spreading geo-
graphically. It may be possible to undertake a
longitudinal study to assist in proving that the
infected wildlife is a maintenance host for TB
(Morris and Pfeiffer, 1995).
Evidence from a number of studies indi-
cates that for TB to be maintained in wildlife spe-
cies – that is, when the basic reproduction rate
of the disease R 0 is ≥1 (Palmer, 2013) – then the
local density of the wildlife host reflects an eleva-
tion of environmental carrying capacity caused
by some form of ecosystem disturbance or
modification.
Thus, badger densities appear to have
increased over time in Ireland following pasture
improvement and abandonment of traditional
game-keeping and hunting (Smal, 1995). An
increase in badger density has also been reported
in England, possibly due to climate change
( Macdonald and Newman, 2002). As well as
density effects, human activity may have inad-
vertently caused changes to a wildlife species
social structure, enabling greater interspecies
and possibly intraspecies interaction as sug-
gested for badgers by Wilson et al. (2011). In
New Zealand, the possum experienced ecologi-
cal release from its natural controls when it was
deliberately introduced from Australia and
released into vacant mammalian browsing
niches. Following release and redistribution of
possums for their fur value, the species became
ubiquitous at often high population densities
due to its tolerance of a wide range of condi-
tions, generalized omnivorous diet and freedom
from predation or competition (Clout and
Ericksen, 2000; Efford, 2000). In Michigan and
Minnesota, supplementary feeding of white-
tailed deer increased deer congregation and
interaction at feed dumps, which has facilitated
TB transmission (O’Brien et al., 2006; Palmer,
2013). In south-central Spain, wild boar aggre-
gation at artificial watering or feeding sites has
been associated with an increased risk of TB
(Vicente et al., 2007).
Measures to manage or reduce infection
levels in wildlife populations thus need to take
account of factors such as increased densities
and social interaction that appear to have facili-
tated some species becoming TB maintenance
hosts with R 0 ≥1. Management options may
include density reduction by culling as for
possums in New Zealand (Warburton and
Livingstone, 2015) and badgers in Ireland
(Sheridan, 2011). Vaccination to protect wildlife
against TB has been investigated in a number of
species including possums and badgers (Buddle
et al., 2011) white-tailed deer (Waters and
Palmer, 2015) and wild boar (Garrido et al.,
2011). Combinations of testing, culling and
vaccination are under consideration for badgers
in Northern Ireland (Department of Agricul-
ture, Environment and Rural Affairs, 2016).
Consideration of wildlife management options
requires engagement with farmers, wildlife
organizations, interest groups and the wider
public, who may all have relationships or con-
cerns with the species under evaluation. Wildlife
management also needs to be effectively inte-
grated with cattle TB control measures. Failure
to do so may lead to setback or collapse of a cat-
tle TB programme (Wilson et al., 2011; O’Brien
et al., 2011; Livingstone et al., 2015a).
TB programmes that include controlling
infection in a wildlife source are subject to simi-
lar broad success factors as those identified for
programmes where cattle are the sole functional
disease host. Nevertheless, some changes in
management approach and emphasis will be
required to deal with the complications intro-
duced by wildlife ecology, TB epidemiology and
greater public interest in findings and outcomes.