82 A.L. Michel
zoonotic risk to consumers in rural areas of
Sudan where it is customary to eat raw sheep
liver, lung, rumen reticulum and omasum
(Tag el Din and el Nour Gamaan, 1982).
As far as other members of the MTBC are
concerned few have been associated with lesions
resembling tuberculosis in goats. Recently,
M. microti was isolated from a dairy goat in
France that was most likely infected from a
badger carrying the same spoligotype (Michelet
et al., 2016).
Even though the disease is considered to be
endemic in goats in Spain, prevalence data are
limited to targeted studies in specific areas. This
is because tuberculosis in goats is not covered
by national control strategies and eradication
programmes (Shanahan et al., 2011; Kassa
et al., 2012; Napp et al., 2013; Harwood, 2014).
For this reason, there is no compulsory testing of
small ruminants for M. bovis and no internation-
ally recognized test exists today. As long as the
control of tuberculosis in small ruminants
remains voluntary, the disease can be expected
to spread and the prevalence to increase.
The relative higher frequency of outbreaks
observed in goats compared to sheep is attrib-
uted to husbandry factors rather than a differ-
ence in susceptibility. The previously held belief
that goats and sheep are purely spillover species
has been challenged and the maintenance
potential of goat and sheep flocks has been
reported (Malone et al., 2003; Muñoz Mendoza
et al., 2012; Napp et al., 2013).
6.3 M. bovis Infection in
Domestic PigsFrom all mycobacterial infections, pigs are most
susceptible to M. avium complex (MAC), espe-
cially M. avium subsp. hominisuis (Mah) and
occasionally M. avium subsp. avium (Maa) (Mijs
et al., 2002; Cvetnic et al., 2007; Agdestein et al.,
2014; Pérez de Val et al., 2014). Within the
MTBC, M. bovis is most frequently associated
with tuberculosis in pigs. The historical signifi-
cance of M. bovis infections in pig herds globally
is difficult to establish from a review of the pub-
lished literature because earlier reports classified
all cases caused by M. bovis and by MAC as swine
tuberculosis (Myers and Steele, 1969).
Despite the similarity of clinical and patho-
logical characteristics associated with either
MAC or MTBC in pigs, it is of utmost importance
to distinguish between these causative agents as
they play a different role in the epidemiology and
control of tuberculosis-like lesions. Infection of
pigs with M. avium or M. bovis results in localized
indistinguishable granulomatous lesions mostly
located in the mandibular and mesenteric lymph
nodes without prior manifestation of clinical
signs (Matlova et al., 2004; Cvetnic et al., 2007).
Therefore, their detection is limited to observa-
tion of tuberculosis-like lesions in slaughter pigs
at the abattoir. Country- and setting-specific pre-
dominance of either Mycobacterium organism is
observed and depends on the epidemiological
situation of bovine TB in cattle and wildlife as
well as on pig husbandry factors. Mah is a ubiq-
uitous environmental organism and Maa is
distributed by birds. Therefore, infection in pigs
can have a wide geographical distribution given
exposure of pig herds to the environment in out-
door production systems (Lahiri et al., 2014), or
through the use of untreated feed and bedding
materials containing Mah (Matlova et al., 2004;
Johansen et al., 2014). Mah causes opportunis-
tic infections in swine resulting in condemna-
tions at the abattoir with economic losses, but
there is also growing concern about possible
zoonotic transmission to humans (Tirkkonen
et al., 2007; Leão et al., 2014) where it can cause
pulmonary disease (Lahiri et al., 2014) and
lymphadenitis (Despierres et al., 2012). In some
studies, co-infection of M. bovis and M. avium
strains has been observed and may be underdi-
agnosed in others due to the competition of
these bacteria in culture and the paucibacillary
nature of lesions (Santos et al., 2010;
Barandiaran et al., 2015).
Infection of pigs with M. bovis occurs in
countries where bovine tuberculosis is endemic
in cattle or wildlife, while successful control and
the prevalence in pigs is likely to reflect the local
situation in the reservoir (Corner et al., 1981;
Bernard et al., 2005; Barandiaran et al., 2011;
Muwonge et al., 2012; Bailey et al., 2013;
Broughan et al., 2013). Elimination of bovine TB
in the reservoir leads to a decline and later the
disappearance of M. bovis from pigs and to a rel-
ative increase of M. avium-derived mycobacterial
disease in swine (Lesslie et al., 1968; Schliesser,
1985; Möbius et al., 2006).