Bovine tuberculosis

198 S.I. Wanzala and S. Sreevatsan

reactivation, which may occur months or years
after the initial infection due to a compromised
immune system (van Crevel et al., 2002;
Kaufmann, 2004; Kleinnijenhuis et al., 2011;
Ramakrishnan, 2012). The various manifesta-
tions of infection with mycobacteria are a reflec-
tion of the delicate balance between the bacteria
and the host defense mechanisms (van Crevel
et al., 2002). Recent research by (Palmer et al.,
2016) examined cytokine expression of TNF-α,
IFN-γ, TGF-β, IL-17A and IL-10 in experimen-
tally infected calves and found a moderate but
positive correlation between the level of cyto-
kine expression and cell size or number of nuclei
in giant cells of granulomas. Their work demon-
strated that these giant cells contribute to the
‘cytokine milieu necessary to form and maintain
granulomas’ (Palmer et al., 2016).
Another cytokine is IL-17 (produced by
Th17 cells), which has been demonstrated to
play a role in tuberculosis immunopathology as
well as other chronic illnesses. Antigen-specific
in vitro expression of IL-17A has been correlated
to both increased disease severity and vaccine-
induced protection in cattle experimentally
infected with M. bovis (Palmer et al., 2016). In
tuberculosis, IL-17 cytokines play key roles in
initiating both protective and harmful inflam-
matory responses and the use of Th17-
associated cytokines have been suggested as
possible biomarkers of infection and protection
in the immune responses to bovine tuberculosis
(Waters et al., 2015).

13.6. Cellular Immune Responses for

Potential Biomarker Application

Mycobacteria are intracellular pathogens and
the host mounts a successful response through a
strong cell-mediated response by the adaptive
immune system, which, in the case of bovine TB,
also acts as the immunological diagnosis of
infection (Vordermeier et al., 2000; Goosen et al.,
2014). Several cellular immune responses have
been the target for biomarker application. BCG
vaccination is one example, though it has given
variable responses in both humans and cattle
and studies to improve the protection conferred
by the BCG vaccine are in progress (Vordermeier
et al., 2016b). One such method is the

heterologous prime-boost strategy that involves

use of supplemental or booster vaccine where

the immune system is primed with BCG after

which it is boosted with subunit vaccines con-

taining protective antigens present in BCG

( Vordermeier et al., 2016a). Other methods

include complete replacement of BCG with

attenuated M. bovis strains leading to over-

expression of antigens or the use of genetically

modified BCG strains with improved immunoge-

nicity (Waters et al., 2009; Vordermeier et al.,

2016a).

Identification of genes deleted in BCG when

compared to M. bovis by comparative genomics

identified key targets in cattle and humans, for

example, the M. bovis proteins early secretory

antigenic target (ESAT-6) and the culture filtrate

protein (CFP-10) located on the RD1 region of

M. bovis and in virulent strains of M. tuberculosis

(Mahairas et al., 1996; Vordermeier et al., 2000,

2016a). In terms of diagnostics, ESAT-6 and

CFP-10 can differentiate between M. tuberculosis-

infected and BCG-vaccinated humans and

ESAT-6 could differentiate between M. bovis-

infected and BCG-vaccinated cattle; these pep-

tides, through the Bovigam PC-EC assay (BEC)

and the Bovigam PC-IHC (BHP), have been har-

nessed to improve the specificity of IGRAs,

though the sensitivity of BTB diagnosis has been

sub-optimal (Goosen et al., 2014). A study by

Goosen et al. (2014) in African buffalo demon-

strated that monocyte-derived chemokine IFN-

γ-induced protein 10 (IP-10) was a useful

marker of immune activation by M. bovis anti-

gens when using the bovine IP-10 ELISA, and

they recommended that the diagnostic potential

of IP-10 for BTB in cattle be re-evaluated using

species-specific reagents.

13.7 Human Tuberculosis

In humans, for MTB complex organisms to be

detected from sputum, it implies that the airways

are close to or have necrotic foci of infection

(Gardiner and Karp, 2015). Thus, for diagnosis

with sputum to be possible it means that most

likely the patient has had active disease for quite

some time, often with severe damage to the

lungs (Gardiner and Karp, 2015). The most

common method used is the direct detection of