Bovine Tuberculosis in Other Domestic Species 83
M. bovis infection in pigs is the result of
exposure from contaminated pastures, feeding
of contaminated milk or contact with wildlife
including carrion (O’Reilly and Daborn, 1995;
Cousins, 2001; Bailey et al., 2013; Nugent et al.,
2015). The presence of such a source of infec-
tion in combination with opportunities of expo-
sure has become scarce in many developed
countries as they have eradicated bovine TB
from their cattle population and contact with
infected feed or wildlife is practically impossible
under the strict biosecurity measures applicable
to commercial intensive pig productions. The
bulk of recent knowledge of the epidemiology of
M. bovis in domestic pigs therefore relies on
reports from free-ranging and feral pig popula-
tions in Spain and New Zealand. In western
Spain, extensively bred Iberian race pigs were
found to be infected with M. bovis in the absence
of cattle but in contact with a large population
of wild boar that are maintenance hosts of
M. bovis (Naranjo et al., 2008). The lesions in
both domestic pigs and wild boar were mainly
located in the respiratory tract and included
open lesions, indicating a respiratory route of
infection between them (Parra et al., 2003).
Under specific conditions like those described
here, which are characterized by a high pig pop-
ulation density and frequent contact with a
sympatric reservoir of M. bovis exhibiting the
same species-specific behaviour, it appears pos-
sible that certain populations of domestic pigs
are capable of transmitting M. bovis.
In pigs experimentally challenged with
high infective doses of up to 10^8 colony forming
units, expansive lesions with liquefied necrotic
centres and large numbers of extracellular
bacilli as well as macrophages containing
M. bovis in the airways were observed (Bolin
et al., 1997). If under similar natural conditions
live pigs may transmit M. bovis in their respira-
tory secretions, there is currently no evidence
that the rate of intra-species transmission is suf-
ficient to establish infection in the pig popula-
tion. Therefore, the presence of respiratory
lesions alone may not prove a maintenance sta-
tus but it classifies such populations as spillover
hosts with amplifier potential.
Similarly, generalized tuberculosis was
observed in the majority of semi free-ranging
Sicilian domestic black pigs sampled from
infected herds (Di Marco et al., 2012). Like in
other domestic pigs, the M. bovis prevalence
increased with age and the most common site for
macroscopic lesions was the head but the
appearance and dissemination of lesions dif-
fered. The lesions in black pigs mostly lacked the
fibrous layer of encapsulated lesions found in
other pigs, suggesting that these pigs may be
able to effectively excrete mycobacteria via aero-
sol or in their faeces (Di Marco et al., 2012).
A contrasting situation has been reported
from New Zealand where strong evidence exists
that feral pigs easily contract M. bovis from
infected possums through scavenging. Possums
are the main wildlife reservoir of bovine tuber-
culosis in New Zealand (Nugent, 2011). A series
of well-documented investigations have been
conducted on pig-related factors in the transmis-
sion cycle of bovine tuberculosis. The release of
15 domestic pigs as sentinels in an area where
wildlife tuberculosis was widespread demon-
strated that pigs are very susceptible to M. bovis
as infection was confirmed in all pigs, and in
some as early as 2 months after release. Visible
lesions in the mandibular lymph nodes were
common to all pigs, whereas only a few animals
showed involvement of thoracic lymph nodes,
suggesting oral rather than a respiratory route
of infection (Nugent et al., 2002). This is in line
with other studies involving farmed slaughter
pigs that found a similar distribution of lesions
with a predilection for the head lymph nodes
(Cousins et al., 2004; Bailey et al., 2013). In pigs
experimentally infected with moderate to low
doses of M. bovis, the earliest lesions were detect-
able approximately 3 weeks after inoculation
and contained large numbers of bacilli which
gradually decreased over time. At 2 months most
lesions had developed into well- circumscribed
often encapsulated granulomas containing few
bacilli and with a potential to heal (Francis,
1958; Bolin et al., 1997; Bailey et al., 2013). The
white–yellow caseocalcerous tubercles are simi-
lar to those found in cattle but generally do not
exceed 40 mm in diameter (Cousins et al., 2004;
Bailey et al., 2013). Occasionally, generalized
cases with dissemination of lesions to the liver,
spleen and lungs and associated lymph nodes
have been reported. Affected lungs may present
with caseous bronchopneumonia but in other
organs lesions appear fibrotic and less calcified.
These findings gave rise to the view that
feral pigs in New Zealand are foremost spillover