Bovine tuberculosis

252 F. Olea-Popelka et al.

(DeJesus et al., 2017) will be required to expand
the identification of M. bovis factors involved in
host–pathogen interaction. A 2017 update to
the annotation of the M. bovis AF2122/97
genome, the reference genome sequence for
M. bovis (Malone et al., 2017), is welcomed; this
annotation needs to be revised on a regular basis
to ensure that functional information on anti-
gens, virulence factors, biosynthetic pathways,
etc. is collated into a single data source for the
community.

16.4.2 Immunological

A significant impediment to the development of
improved TB vaccines for animals is that no
single correlate of protection has been identified
for TB (as indicated in Chapters 11 and 14),
although a number have shown promise in this
regard. A confusing aspect is that many of these
markers for protection also serve as indicators of
disease post-challenge. This means that each
vaccine type, dose and method of delivery must
be tested empirically using a host challenge
model before eventually being evaluated for
efficacy in settings of natural infection acquisi-
tion. It is important that bacillus Calmette–
Guérin is field-tested in different environments
and husbandry systems as this may help explain
any variations in vaccine efficacy. This makes
vaccine research and development for animal TB
both time-consuming and costly and places limi-
tations on the statistical power to detect a pro-
tective effect, either due to constraints on the
number of infected animals that can be held in
biosecure laboratory facilities, or limitations
posed by a comparatively low force of infection
in the natural setting.
Another immunological consideration is
the need to define the target dose of vaccine for
each species and route of potential administra-
tion. In the case of an oral, bait-delivered vac-
cine for sylvatic species, the dose is impossible to
control precisely and there is always the risk that
non-target species may access the vaccine. At
best, this results in vaccine wastage, at worst it
could result in the induction of misdiagnosis of
TB in livestock should they become exposed to
sufficient quantities of the vaccine.

Finally, there is often little to no empirical

data to inform the optimal size or duration of an

animal vaccination programme, a fact exacer-

bated by the difficulty of defining the practical

duration of immunity. This makes long-term

commitment or buy-in from stakeholders diffi-

cult to secure.

16.4.3 Practical

Major challenges for the vaccination of wildlife,

in particular, are the identification of the means

for cost-effective and reproducible vaccine deliv-

ery coupled to the need for a single dose vaccine,

as in many cases there will be no certainty of

accessing the same animal to administer a sec-

ond dose of the vaccine.

Oral bait TB vaccines have been shown to

be effective in a number of wildlife species, but

more research is needed to improve formulations

with appropriate attractants, systems for opti-

mizing bait distribution and avoiding bait uptake

by non-target species.

16.4.4 Safety

Although the majority of vaccines under con-

sideration for TB control in animals are likely to

be safe, this cannot be assumed and needs to be

demonstrated formally for at least the target spe-

cies, in order for a licence to be granted from the

national competent authority. Where there is

the possibility of exposure of non-target species

to the vaccine, e.g. for an oral, bait-delivered vac-

cine, it may prove necessary to evaluate the

safety of the vaccine to each species that is at

risk of exposure.

16.5 Challenges to Disease Control

In some countries, elimination of TB in livestock

is unlikely to occur without some form of control

targeted to wildlife vectors of the disease as high-

lighted by Fox and co-authors in Chapter 7. This

raises the question as to who ‘owns’ or is ulti-

mately responsible for wildlife? Is it the govern-

ment, landowners or even the public? Where