44 A.J.K. Conlan and J.L.N. Wood

This sentiment could equally well apply to
bovine tuberculosis, but there are important
contrasts in the progression of disease in
humans and cattle. In human tuberculosis, only
a small fraction (~10%) of infected individuals
that react positively to a tuberculin skin test will
ever develop symptomatic disease. The rate of
progression of tuberculosis in humans depends
on the age at infection, with children more likely
to develop active tuberculosis (Comstock et al.,
1974). However, older people may also develop
disease due to endogenous reactivation of infec-
tion that was acquired decades earlier in life
(Comstock, 1982). This dichotomy is reflected in
mathematical models of the transmission of
Mycobacterium tuberculosis by explicitly model-
ling these two groups as separate model com-
partments relating to ‘fast’ and ‘slow’ progressors
(Blower et al., 1995).
Such distinct heterogeneity in progression
is assumed not to be important for M. bovis

infection in cattle (Francis, 1947) and this is

reflected in the structure of mathematical mod-

els for the transmission of M. bovis, which focus

rather on the relationship between immunologi-

cal status with respect to diagnostic tests and

infectiousness. Our understanding of the pro-

gression and routes of transmission of M. bovis

between cattle is underpinned by a long history

of experimental and field studies. The common-

alities and inconstancies of this body of work

have been reviewed extensively by previous

authors (Francis, 1947; Menzies and Neill,

2000; Goodchild and Clifton-Hadley, 2001) and

are also challenged by recent work on the distri-

bution of lesions in reactor animals (Brooks-

Pollock et al., 2013; Downs et al., 2016). Here,

we review the key findings of these studies

within a conceptual model of disease progres-

sion (Fig. 4.1) that synthesizes the assumptions

commonly used to develop mechanistic trans-

mission models for M. bovis in cattle.

Fig. 4.1. Conceptual model of progression of bovine tuberculosis infection and relationship of model
compartments to surveillance and control measures. S, susceptible; O, occult/unreactive; R, reactive;
I, infectious; A, anergic.

False

positives

S O R I A

Diagnostic test

sensitivity

Diagnostic test

specificity

Slaughterhouse

efficiency

Culture

efficiency

‘True’

reactors

Lesions

Culture