Bovine tuberculosis

The Pathology and Pathogenesis of Mycobacterium bovis Infection 123

potential. Even though the disease has been
known for centuries, its pathogenesis is still not
fully understood and the mechanisms of diagno-
sis and control in different scenarios are under
constant revision. This chapter reviews the
pathology of M. bovis infection in cattle and
other animal species and the main aspects of the
pathogenesis of the immune response in the
host after infection.

9.2 Macroscopic Pathology of

M. bovis Infection

The typical gross pathological lesion of bovine
tuberculosis is known as a tubercle, which is a
circumscribed yellowish granulomatous inflam-
matory nodule of variable size that is more or
less encapsulated by connective tissue and often
contains a central core of necrotic tissue with
varying degrees of mineralization (Aranday-
Cortés et al., 2013; Domingo et al., 2014). The
localization of the granulomatous nodules
depends largely on the route of infection. Adult
cattle typically show respiratory lesions, circum-
scribed to the lung parenchyma (Fig. 9.1) and
regional lymph nodes within the thoracic cavity.
The association of lung and mediastinal lymph
nodes in the pathogenesis of tuberculosis has
been described many years ago (Ghon, 1912) as
illustrated by the identification of the primary
complex, or Ghon complex, defined as the pres-
ence of both primary pulmonary lesion and
caseous lymph node lesion.

The primary lesion is often localized within

the dorsal area of one of the lung lobes. This

lesion generally progresses towards an encapsu-

lated and mineralized lesion. However, if the

infected animal is immunocompromised, or the

immune response is ineffective, the primary

infection may generalize during the initial stages,

in a process known as ‘early generalization’.

Generalization via haematogenous or lymphatic

dissemination can also occur after re-infection or

in the post-primary phase, therefore called ‘late

generalization’ (Domingo et al., 2014).

The primary lesion can progress and gener-

alize, inducing a ‘miliary’ form with abundant

nodules of small size throughout the lung and

pleura (primary generalization). The lesion can

grow, showing different forms depending on

the development and involvement of adjacent

tissues, including: (i) an ‘acinar’ form showing

numerous small yellowish nodules affecting pri-

mary pulmonary lobules; (ii) a ‘cavernous’ form

when the bronchial lumen is dilated due to the

accumulation of caseum coming from the lesion

or when the caseum breaks out into a bronchus;

and (iii) an ‘ulcerative’ form in the trachea and

bronchi when bacilli infect small erosions within

the airway epithelium.

Typical tubercles can also be observed in

extra-thoracic lymph nodes in the head and

neck area (parotid, medial and lateral retropha-

ryngeal and submandibular) (Aranday-Cortés

et al., 2013; Dean et al., 2014, 2015; Ameni

et al., 2017; Salguero et al., 2017). In calves,

bovine tuberculosis is usually transmitted by

ingestion and lesions involve the mesenteric

Fig. 9.1. Gross pathology of Mycobacterium bovis infection in cattle. (a) Multiple sub-pleural lesions can
be observed in the dorsal part of the right middle lung lobe. (b) After sectioning, multiple coalescing gran-
ulomatous lesions observable with caseous necrosis in the centre and inflammatory reaction surrounding
the areas of necrosis.