Vaccination of Domestic and Wild Animals Against Tuberculosis 215
Options for preventing the transmission of
M. bovis from infected badgers to cattle are lim-
ited to minimizing the potential for contact
between them (biosecurity), reducing the num-
ber and density of infected badgers via selective
and non-selective culling, and vaccination, with
the objective of reducing the force of infection
within and between species (reviewed by
Gormley and Corner, 2013). Badgers are pro-
tected by law in the UK and Ireland, which limits
the public acceptability and practicality of dis-
ease control through culling. Trials to ascertain
the impact of reducing badger populations of
bovine TB in England and Ireland have delivered
complex and sometimes conflicting results that
likely reflect subtle differences in the epidemiol-
ogy of the disease locally (O’Connor et al., 2012).
Options for effective biosecurity may be limited
by what is practical and feasible and the recogni-
tion that the environment may be contaminated
by M. bovis leading to opportunities for indirect
infection (King et al., 2015). Vaccination of
badgers against TB has the potential to be an
effective TB control measure, especially in com-
bination with other control measures (Abdou
et al., 2016). As in the case of possums in New
Zealand, the key attribute of a successful TB vac-
cine for badgers would be to prevent the spread
of M. bovis infection to other wildlife or domestic
animals. Prevention of infection is of lesser
importance compared to that for domestic spe-
cies. At present, the developed vaccine agent for
tackling TB in badgers is BCG.
The use of BCG to vaccinate badgers
against TB in the UK was licensed by the UK
Competent Authority (Veterinary Medicines
Directorate) in 2010 as BadgerBCG™ and is
available for use by vets and trained lay vaccina-
tors under prescription from a veterinary
surgeon. Licensing of BadgerBCG™ required
evidence of vaccine safety and efficacy, obtained
from laboratory and field studies. The laboratory
studies showed that vaccination of badgers by
injection with BCG was both safe and signifi-
cantly reduced lesions of TB caused by M. bovis
(Lesellier et al., 2006, 2011). Protection was
incomplete, in that M. bovis infection of vacci-
nated badgers still produced either visible
pathology or M. bovis was isolated from organs
at necropsy. The problem with the experimental
studies was that high challenge doses were nec-
essary to ensure reproducible infection and
therefore the protection produced by BCG vacci-
nation may not reflect the level of protection
against more natural challenge involving a
range of infectious doses. When tested in a field
study in wild badgers over 4 years, the level of
protection using BCG was consistent with the
protection induced directly by BCG in the experi-
mental studies. Uninfected animals identified as
negative in a range of diagnostic tests were less
likely to become immunologically/serologically
positive after vaccination than were non-
vaccinated control badgers (Chambers et al.,
2011; Carter et al., 2012). In addition, unvacci-
nated badger cubs taken from a vaccinated
social group were much less likely to react to TB
if more social group members had been vacci-
nated previously. This is remarkable and most
likely an effect of herd immunity where trans-
mission rates are effectively reduced in those
groups of animals in which a higher percentage
are vaccinated.
A practical limitation to the extensive use of
BadgerBCG™ is the need to trap badgers before
the vaccine can be injected. A form of BCG that
could be delivered orally to badger populations
via impregnated food baits would reduce this
limitation and make more extensive vaccine
deployment feasible. A wide variety of oral baits
have been developed and assessed for their
palatability and attractiveness to both captive
and wild badgers (M. Chambers, unpublished
results). BCG has been incorporated into the
most promising of these, including encapsula-
tion in the same lipid matrix used to deliver BCG
orally to possums. BCG retains viability for pro-
longed periods in frozen baits and for sufficient
time in simulated and actual environmental
conditions (i.e. temperature and humidity) to
make vaccination via bait feasible. Administra-
tion of BCG orally to captive badgers, either
directly to the back of the throat, or indirectly
via ingested bait has been shown to protect
badgers against experimental challenge with
M. bovis (Murphy et al., 2014; M. Chambers,
unpublished results). A dose of 7.9 × 109 to
8.1 × 109 CFU BCG given to nine badgers orally
was well tolerated. BCG was shed in faeces by
two vaccinated badgers (372 CFU/g and
996 CFU/g) approximately 48 hours later
(M. Chambers, unpublished results). The target
dose of BCG for the oral vaccination of badgers is
yet to be defined.