86 A.L. Michel
observation of gross lesions alone may be
an overestimate of M. bovis-associated camel
tuberculosis. In Ethiopia, the prevalence of
tuberculosis- like lesions in two studies was 10%
and 4.5%; of these, only 22% and 21%, respec-
tively, were caused by M. bovis, corresponding to
a final bovine tuberculosis prevalence of less
than 1% (Mamo et al., 2011; Kasaye et al., 2013).
As established for small ruminants and
pigs, there is agreement between various studies
in different countries that age was found to be
positively associated with the prevalence of
tuberculosis in camels (Mamo et al., 2011;
Narnaware et al., 2015). In view of their life
expectancy, which ranges from 22 to 35 years,
the cumulative risk of old camels to be infected
with and to transmit M. bovis is significantly
higher than in other domestic animals (Wosene,
1991).
Clinical signs of tuberculosis in camels may
be absent in early stages, but later on generally
include anorexia and progressive but often rapid
emaciation (Narnaware et al., 2015). Pathologi-
cal changes caused by M. bovis present as granu-
lomatous, caseous-necrotizing lesions that are
macroscopically comparable to those in cattle.
Microscopically, the only marked difference is
the scarcity of giant cells in the lesions. Calcifica-
tion and fibroplastic reactions may be observed
(Elmossalami et al., 1971; Bush et al., 1990).
The lesions are predominantly found in the
lungs and mediastinal lymph nodes, retropha-
ryngeal and mesenteric lymph nodes with occa-
sional cases of dissemination to the kidneys,
liver, spleen, heart and pericardium, mesentery,
trachea, pancreas and peripheral lymph nodes
(Bush et al., 1990; Chartier et al., 1991; Kasaye
et al., 2013; Narnaware et al., 2015). Acid-fast
organisms are either absent or scant.
The exact route of M. bovis transmission
between camels is unknown, but the lung-
centred pathology strongly suggests that camels
contract the pathogen via aerosols either during
close contact or through dust particles (Wernery
and Rüger-Kaaden, 2002). In exceptional cases
of generalized tuberculosis with kidney involve-
ment, shedding of bacilli in the urine has been
reported (Dekker et al., 1962). There are no
reports on possible vertical or pseudovertical
transmission of M. bovis in camels, neither has
tuberculous mastitis been described. This may
suggest that M. bovis excretion in milk is an
unlikely event reserved for generalized cases
under intensive husbandry conditions. There is
reason to believe that the arid climate and the
traditional nomadic, semi free-ranging hus-
bandry system prevailing in most camel-owning
communities neither facilitate close, prolonged
contact nor the confined conditions that are
conducive to high transmission rates by aerosol.
Their current status as spillover host may there-
fore be seen as a function of the incapacitating
circumstances for M. bovis transmission rather
than a low degree of susceptibility, similar to the
varying roles of wild boar as hosts of M. bovis in
Mediterranean Europe. However, with the
increasing global demand for camel milk the
production in intensive camel farms in East
Africa has tripled in the past 50 years and pres-
ently contributes 7% of the milk produced in
sub-Saharan Africa (Schwartz, 2013). Further
intensification of camel production in the
absence of tuberculosis surveillance and control
programmes may certainly pave the way for a
changing role of M. bovis in camel health and
production.
Tuberculosis is a serious disease concern
and cause of death in zoological collections.
While M. tuberculosis is the more common cause
of tuberculosis in this setting, both localized and
generalized cases have been described which
were caused by M. bovis (Bush et al., 1990) but
also by M. caprae (Erler et al., 2004; Pate et al.,
2006), M. pinnipedii and M. orygis in a camel in
the UAE (Kinne et al., 2006; Wernery et al.,
2007; Wernery and Kinne, 2012).6.6 ConclusionDomestic animals are susceptible to M. bovis
resulting in an extensive host spectrum. Where
differences in the frequency of outbreaks are
observed, those have been attributed to species-
specific behaviour or husbandry practices rather
than to differences in host susceptibility. Overall,
tuberculosis in domestic animals other than
cattle is a sporadic event and, with the exception
of small ruminants and water buffalo, strongly
associated with spillover of infection from cattle
or wildlife. Therefore, in regions where bovine
tuberculosis has been successfully eradicated
from cattle or where contact between reservoir