34 H. Yahyaoui Azami and J. Zinsstag
used in human TB treatment (Scorpio and
Zhang, 1996; McLaughlin, 2012; Bobadilla-del
Valle et al., 2015).
In most developing countries, no microbio-
logical identification of TB causative agents is
made before the administration of treatment.
Considering the natural resistance of M. bovis to
pyrazinamide, in addition to the re-emerging
mutations in M. bovis genome, which cause
resistance to other TB drugs (McLaughlin,
2012), a human infection with M. bovis could be
considered as one of the causes contributing to
the relapse of TB patients. Consequently, there is
an urgent need to quantify the exact burden of
zoonotic TB among human TB patients in devel-
oping countries and, more importantly, among
the groups that are the most at risk of contract-
ing M. bovis from cattle.
3.4 Bovine Tuberculosis:
Transmission and Risk Factors for
Cattle and HumansBovine TB is a zoonosis caused by M. bovis, a
Gram-positive bacteria belonging to the Myco-
bacterium tuberculosis complex. The most impor-
tant host for M. bovis is cattle (Amanfu, 2006);
however, this species infects a wide range of
domestic and wild animals as well as humans
(Palmer, 2013; Pesciaroli et al., 2014).
A brief description of factors impacting
transmission of M. bovis among cattle and to
and between humans is presented in this section
to highlight the need of an OH approach to con-
trol bovine and zoonotic TB. For more details
regarding M. bovis transmission between cattle
see Chapter 4 and for M. bovis public health
implications see Chapter 2.
Several risk factors are linked with bovine
TB infection in cattle. The risk of bovine TB
infection has been described to increase with age
(Brooks-Pollock et al., 2013), while local breeds
have been linked with lower prevalence of
bovine TB (Moiane et al., 2014). The risk of
bovine TB infection regarding gender has been
observed to be linked to livestock management
practices and cultural behavioural habits related
to each country (Humblet et al., 2009). In devel-
oping African countries, imported cattle are
usually kept under intensive conditions, a factor
that has been previously described as a risk
factor for bovine TB infection (Elias et al., 2008).
In addition, intensive breeding is usually prac-
ticed in larger herds, a factor that has been
shown to increase the risk of bovine TB infection
( Humblet et al., 2009). The type of production
could also be a risk factor for bovine TB,
as described in a cohort study in New Zealand
from 1980 to 2004, where dairy herds were
observed to have a higher risk of infection com-
pared to fattening schemes (Porphyre et al.,
2008).
Two routes of transmission have been
described in humans: for adult and older
patients, airborne transmission is the most
common route causing pulmonary TB, while
in younger patients, foodborne transmission
occurs more often, which may lead to extra-
pulmonary tuberculosis (Hlavsa et al., 2008).
Consumption of unpasteurized milk has been
recognized as a major risk factor (Cosivi et al.,
1998). However, the transmission of M. bovis to
humans can be enhanced by other factors, such
as HIV co-infection (Grange, 2001; Hlavsa et al.,
2008). Person-to-person transmission of
M. bovis has been previously reported in
immune-deficient patients (Roring et al., 2002;
Evans et al., 2007), as well as in immune-
competent patients as described in France in
2009 (Sunder et al., 2009). The transmission of
M. bovis between animals and humans depends
on many risk factors, which vary from one epi-
demiological context to another. In developing
countries, the livestock management system is a
very important risk factor for bovine TB trans-
mission. As the economy of a country grows,
the livestock keepers tend to move from more
extensive pastoral systems to more intensive
livestock management for dairy production. In
such systems, animals are closer together in less
ventilated spaces and with less sunlight. Such
intensified production systems provide a more
favourable environment for the persistence of
the disease, as M. bovis is more easily transmitted
(Shitaye et al., 2007; Elias et al., 2008).
Moreover, human TB due to M. bovis has
been suggested as an occupational hazard after
the isolation of M. bovis from 5 abattoir workers
among 3000 abattoir workers during a 2-year
period in Australia (Robinson et al., 1988). In
Pakistan, human TB caused by M. bovis was
found in livestock keepers and abattoir workers.