Managing Bovine Tuberculosis: Successes and Issues 241
Vaccination of cattleVaccination of neonate calves with live M. bovis
bacillus Calmette–Guérin (BCG) vaccine pro-
vided 56–68% protection against subsequent
exposure to natural M. bovis infection (Ameni
et al., 2010; Lopez-Valencia et al., 2010). Fur-
ther to this Ameni et al. (2010) identified that
more of the vaccinated cattle would have passed
the standard meat inspection process. Lopez-
Valencia et al. (2010) found that fewer vacci-
nated calves had positive PCR nasal swabs and
suggested that they may pose a lower TB trans-
mission risk. Buddle et al. (2005) identified that
calves orally vaccinated with a lipid formulated
BCG had a significant level of protection against
TB compared with unvaccinated calves. Nugent
et al. (2017) identified an efficacy of 64% for oral
BCG vaccinated calves that were up to 9 months
old and exposed to natural, mostly wildlife, TB
infection.
Even if BCG vaccination effectively protects
no more than about half of vaccinated cattle,
this would still provide a significant advantage in
reducing cattle TB incidence in low to medium–
high GNI economies able to facilitate vaccina-
tion programmes. While vaccination with BCG
sensitizes cattle to the tuberculin test, this would
be immaterial for its use in these countries, as
they are unlikely to be TB testing cattle on any
large scale. Vaccinating female calves will reduce
their later risk of passing M. bovis in milk,
thereby potentially reducing the number of
humans exposed to infection. Where resources
are limited, vaccinating female calves alone may
thus be a well-targeted strategy to reduce human
health risks.
In order to maximize the efficacy of BCG
vaccination, calves need to be TB free to start
with. This means that before and for a period
after calves are vaccinated, they should not have
access to colostrum or milk that may contain
M. bovis. At a practical level, this may be difficult
to achieve unless colostrum and raw milk can be
pasteurized before being fed to calves intended
for vaccination, or ensuring that calves are not
raised on TB test-positive cows.
TB testingExposure to M. bovis can be diagnosed in
livestock by a range of in vivo (intradermal
tuberculin) and in vitro (whole blood and sero-
logical) tests as outlined in Chapter 12. Serologi-
cal tests work best if the animal’s immune
system has been stimulated with tuberculin
some 10 to 14 days earlier. Milk can also be
tested for TB using a serological test that pro-
vides similar result to that for sera, provided few
animals are involved (Buddle et al., 2013). Test-
ing is better at diagnosing infection at a herd
than animal level. Most test evaluations have
been carried out in cattle, so the available data
on diagnostic utility may not be equally applica-
ble to other species such as goats, deer and
camelids.
As identified in the Chapters 12 and 13 on
diagnostic tests, evaluation is based on measures
of sensitivity and specificity. Unfortunately, no
diagnostic test has a sensitivity and specificity of
100%. Test measurement can be adjusted to
increase either sensitivity or specificity, but an
increase in one parameter will generally result
in a decrease in the other. For disease control
purposes, when TB prevalence is high, it is pref-
erable to maximize test sensitivity. However, the
consequent lower test specificity will lead to
more non-infected animals giving false-positive
test results. Livestock owners may have difficulty
seeing the value in this, especially when cattle
are their main asset and there is a reduction in
value or a requirement to slaughter test-positive
animals.
The other factor that needs to be considered
is test cost. The intradermal tuberculin skin test
is currently cheapest, but requires skill to apply,
and animals need to be presented twice: first to
inject the tuberculin, and then 3 days later to
read the test by examining the injection site. This
may not suit all grazing arrangements. Whole
blood and serological testing only need animals
to be presented once, for a blood sample. Testing
milk samples may be simpler, but not all animals
may be lactating. However, blood, sera and milk
samples currently need to be sent to a laboratory
with sophisticated equipment and trained staff
to undertake the testing. Whole blood testing
requires blood samples to be submitted to the
laboratory within 24 hours. These factors need
practical and logistic consideration before decid-
ing upon which test to use for a particular
situation.
Before introducing a testing programme,
the proposed test and its implications need to be