Biomarkers in the Diagnosis of Mycobacterium tuberculosis Complex Infections 193
(Vordermeier et al., 2016a). Another marker
investigated as a potential predictor of vaccine-
induced protection and memory is IL-2. IL-2
production is also a potential biomarker for
latency and different disease stages in cattle
(Palmer et al., 2000, 2001, 2004; O’Brien et al.,
2009; Vordermeier et al., 2016b).
Mycobacteria have evolved an array of
sophisticated mechanisms for evading the host
immune system and defending itself in the harsh
intracellular environment of the macrophage,
some of which are well understood while others
are yet to be discovered. In the ‘dance of seduc-
tion’ with M. bovis, the host has also evolved
several responses to contain infection, and
together with the pathogen mechanisms will
make up the gist of our discussion.
13.2 Biomarkers DefinedA biomarker can be defined as a characteristic
that can be measured and evaluated as an
indicator of a normal biological process, a patho-
genic process or pharmacologic responses to a
therapeutic intervention (Biomarkers Defini-
tions Working Group, 2001). Biomarkers can be
used as diagnostic tools for the identification of
patients with disease (e.g. elevated glucose con-
centration for diagnosis of diabetes mellitus), for
disease staging (e.g. prostate specific antigen
concentration in blood), as an indicator of dis-
ease prognosis (e.g. anatomic measurement of
tumour shrinkage of certain cancers) or for pre-
diction and monitoring of clinical response to an
intervention (e.g. response to anti-tuberculosis
drug treatment, vaccine efficacy, blood choles-
terol concentration for determination of heart
disease, etc.). Currently, the technologies used in
biomarker discovery include in vitro analyses of
DNA variation (disease susceptibility), circulat-
ing DNA or RNA (disease progression-apoptosis/
proliferative pathways), transcriptomes (disease
induced transcriptional alterations) and pro-
teomics (disease progression).
Discovery of novel biomarkers is essential
for developing new diagnostic tests to aid in
identification of infected animals in disease sur-
veillance for bovine TB, without necessarily
slaughtering herds of 100s to 1000s of animals
for identification of the single infected animal.
Early diagnosis is essential in bovine TB because
clinical symptoms appear very late, when the
disease has significantly advanced and the risk
of infection being transmitted is high.13.2.1 Characteristics of an
ideal biomarkerFor a biomarker to qualify as an ideal diagnostic
tool it has to meet a minimum set of criteria or
target product profiles (Gardiner and Karp,
2015). The ultimate diagnostic test for MTB
complex infections would be one that is highly
sensitive, specific and non-sputum-based with a
clear predictive response to therapy, independent
of the host response (Gardiner and Karp, 2015).
An ideal biomarker-based assay should also have
a very high degree of sensitivity and specificity
(>98%), be non-invasive or minimally invasive,
provide results rapidly, without the need for the
cold chain and be affordable. Haas et al. (2016)
argue that having a combination of biomarkers
would enhance the diagnostic value in different
settings: for instance, having one set of biomark-
ers for differentiating between active and latent
TB (humans)/subclinical TB (animals) and
another set to diagnose TB in comparison with
other diseases. Such efforts are possible through
collaboration between human and bovine tuber-
culosis researchers. The pathway taken for a
prospective biomarker to reach the market is a
tortuous one and potential biomarkers must
progress through sequential testing to confirm
their efficacy in an independent cohort, then
they are further validated in a prospective study.
The biomarkers are also required to undergo
tests on a platform appropriate for their proposed
use; such platforms include TB clinics in endemic
countries (Gardiner and Karp, 2015).
Existing TB diagnostics are inadequate.
Infection with MTB complex organisms presents
unique challenges for diagnostic testing. Cur-
rent diagnostics have several shortcomings;
potential new diagnostics are explored below.13.3 Circulating BiomarkersThe analysis of alterations in circulating protein
profiles or circulating DNA or RNA (including