52 A.J.K. Conlan and J.L.N. Wood
4.3.2 Persistence of infection
within herdsThe dynamic link between testing and the poten-
tial for transmission is increased further by the
intensification of testing that results from disclo-
sure within a herd. Herds are repeatedly tested at
short intervals of at least 60 days until they test
clear. The duration of breakdowns is incredibly
heterogeneous. Although the majority of break-
downs resolve within 240 days (a maximum of
four short interval tests), approximately 30%
breakdowns will be prolonged, with some herds
remaining under movement restrictions for
years (Karolemeas et al., 2010). While the
intrinsic dynamics of transmission of bovine TB
no doubt contribute to this variability, the stron-
gest risk factor for breakdowns in Great Britain
to become prolonged is the confirmation of a
reactor animal by visible lesions and culture.
Such laboratory confirmation has a systematic
effect of increasing the number of clear tests
required to clear restrictions and demanding the
use of a ‘severe’ interpretation of the SICCT test
where an increase in sensitivity is traded for a
reduction in specificity (i.e. the expected number
of false positive tests).
The effectiveness of testing also plays a role
in understanding the high rate of recurrence,
which is another important measure of within-
herd persistence. By far the greatest, and most
consistent, risk factor for a herd having a break-
down of bovine TB is a past history of infection
(Skuce et al., 2012; Broughan et al., 2016). In
Great Britain between 2003 and 2005, approxi-
mately 38% of breakdowns recurred withinFig. 4.4. Distribution of reactor animals disclosed at the beginning of a herd breakdown in PTI 1, 2 and
4 historical testing areas in Great Britain (2003–2005).
0.40.30.20.10 1 2 3 4 5 6 7 8 9 10+PTI 1 PTI 2 PTI 4