Mycobacterium bovis as the Agent of Human Tuberculosis: Public Health Implications 21
1994). It is uncommon that the muscle mass be
affected by tuberculous lesions, and this has
been documented only in the advanced stages
of the disease (Drieux, 1957). In 1957, Drieux
reviewed studies of the isolation of M. bovis from
skeletal muscle in cattle with advanced TB. The
majority (but not all) of the studies reviewed by
Drieux failed to isolate M. bovis from muscle.
However, two of the studies did recover M. bovis
from muscle in a high proportion of cases tested.
It should be noted that organs that do not dis-
play visible lesions of TB at post-mortem inspec-
tion may nevertheless carry M. bovis (Food
Safety Authority of Ireland, 2008).
Although it is well recognized that meat
consumption poses a considerably lower risk in
the transmission of M. bovis when compared to
the consumption of dairy products, it is none-
theless a risk that has been recognized in certain
settings. For example, in Ethiopia, Ameni et al.
(2003) showed that almost 99% of meat con-
sumed in areas where bovine TB was endemic
was either consumed raw or semi-cooked. Fur-
thermore, the authors showed that there was a
general lack of awareness of the risk associated
with this behaviour. In West Africa, Hambolu
et al. (2013) showed that 22% of meat handlers
in Nigeria ate ‘Fuku elegusi’ (the practice of eat-
ing raw, visibly TB-infected parts of the lungs in
order to convince customers to buy these parts)
and only 28% of meat handlers were aware
that eating ‘Fuku elegusi’ could be a source of
M. bovis infection and zoonotic TB in humans.
The magnitude of risk associated with such
behaviour remains to be established and, thus,
the role of meat products as a source of M. bovis
for humans deserves further attention, particu-
larly in geographical areas in which cultural and
eating habits could facilitate transmission.
2.3.3 Occupational risk of
contracting M. bovisThe risk for zoonotic TB disease increases in
areas where bovine TB is endemic and people
live in conditions that favour direct contact with
infected animals. M. bovis poses an occupational
risk throughout the world to farmers, pastoral-
ists, veterinarians, zoo keepers, slaughterhouse
workers, butchers and other types of workers
with frequent direct contact with animal or ani-
mal products (Fanning and Edwards, 1991;
Dalovisio et al., 1992; Cosivi et al., 1998;
Adesokan et al., 2012; Torres-Gonzalez et al.,
2013; Michel et al., 2015; Khattak et al., 2016).
Aerosol infection has been suggested in farmers
in contact with elk (Fanning and Edwards,
1991) and zoo keepers exposed to rhinoceroses
infected with M. bovis (Dalovisio et al., 1992).
Cutaneous TB due to M. bovis infection has been
documented in hunters handling infected wild
animals (Wilkins et al., 2008) and infected pos-
sums (Gallagher and Bannantine, 1998). In
Ontario, Canada, veterinarians became infected
while working in abattoirs as part of a depopula-
tion campaign of infected deer and elk herds
(Liss et al., 1994). Adesokan et al. (2012) con-
firmed undetected pulmonary TB due to M. bovis
among livestock traders in Nigeria. In Mexico,
Torres-Gonzalez et al. (2013) reported a high
prevalence of latent and pulmonary TB among
people working with cattle infected with
M. bovis, especially in people working in non-
ventilated spaces. In these settings, the epidemi-
ology and risk factors for zoonotic TB differ
significantly from those for airborne disease
caused by M. tuberculosis.2.3.4 M. bovis human-to-human
transmissionAlthough an uncommon route of transmission
of M. bovis, person-to-person transmission has
been reported (Yates and Grange, 1988; LoBue
et al., 2003; Smith et al., 2004; Evans et al.,
2007; Sunder et al., 2009; Adesokan et al.,
2012; Malama et al., 2014; Sanou et al., 2014;
Buss et al., 2016). In the US, the analysis of
8 years of data (2006–2013) by Scott et al.
(2016) concluded that although the ingestion of
unpasteurized dairy products was the main
mode of transmission of M. bovis to humans,
airborne transmission was suggested by their
data. Pulmonary TB caused by M. bovis via air-
borne transmission among people therefore
appears possible and deserves further investiga-
tion as a source of secondary transmission. As
mentioned in section 2.3.3, there is also evi-
dence for occupational airborne infection.