Bovine tuberculosis

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

Brooks-Pollock and colleague’s national
level network model from 2014 provided an
important mid-way ground between assuming a
constant risk from wildlife and a fully specified
two-host model (Brooks-Pollock et al., 2014). In
this model the environmental reservoir is
dynamic, increasing proportionally to the local
prevalence of disease in cattle and decaying at a
constant rate. This does not quite amount to an
implicit assumption that disease will not persist
within the environment in the absence of cattle
infection – the estimated rate of decay may in
principle be small enough that persistence is
ensured over any reasonable timescale of inter-
est. The mechanism is therefore likely to be able
to account statistically for the background risk
of infection, but lacks the detail necessary to
make predictions about the impact of interven-
tions that target transmission between badgers
and cattle, or on the badger population itself.
The major contributions of the Brooks-
Pollock model are a framework within which
alternative transmission models may be system-
atically estimated from data (using approximate
Bayesian computation) and a more robust quan-
tification of the risks of cattle movements
between herds to spread of the disease than was

previously possible. Brooks-Pollock found that

the herd reproduction ratio – the number of

expected new breakdowns resulting from a given

breakdown – was highly skewed. The majority

of breakdowns lead to no transmission to other

herds, but there is a fat tail of super-spreading

herds responsible for multiple new incidents.

This variability, driven by the trading practices

and demographic structure of herds comple-

ments the variation seen in the within-herd R 0

and numbers of disclosed reactors at the herd

level (Fig. 4.4). Herds that trade frequently are at

a lower risk of within-herd transmission due to

the relatively short duration of time individual

animals remain within the herd – but these

same herds may well pose a greater risk of trans-

mission between herds.

4.4 Conclusion

Control of bovine TB in cattle is difficult, but in

situations without sympatric host species tuber-

culin testing-based slaughter programs have

been successful in eliminating disease. Control

in populations with multiple host species

involved will always be more challenging.

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