Removing TB badgers
Journal of Applied Ecology, Volume 36,Number 4, September 1999, pp.
Controlling TB in cattle by removing infected
badgers - constraints imposed by live test sensitivity
Bovine tuberculosis is a serious disease of cattle
caused by the bacillus Mycobacterium bovis. In south-west England,
badgers Meles meles sustain endemic M. bovis infection and almost
certainly transmit the disease to cattle. When tuberculosis outbreaks
have occurred in cattle, the Ministry of Agriculture, Fisheries and Food
(MAFF) therefore culled badgers to try to avert further outbreaks.
To limit the number of badgers killed, MAFF has
assessed a possible new strategy (the ‘live test strategy’) that used a
serological test to identify and remove infected badgers. However,
because the test correctly identified only 41% of truly infected
badgers, individuals were pooled according to the setts at which they
were sampled. All badgers were culled at setts where one or more
seropositive animals were caught.
On average, 1·9 ± 1·4 (SD) badgers were sampled at
each sett. Using a simple model, we show that this level of sampling
still gives a low (24–37%) probability of detecting infection at a given
Badger social groups typically occupy more than one
sett. We allocated setts to social groups by using Dirichlet
tessellations and field signs to predict territory borders. On average,
3·3 ± 2·8 badgers were sampled in each group. Our model shows that this
increase in sample size gives probabilities of detecting M. bovis in
truly infected groups of 43–62%, which is still likely to be
Culling badgers according to the setts where they
were trapped led to incomplete removal of social groups; some
seronegative badgers were released in 61% of groups containing
seropositive animals. As infection is clustered within groups, it is
likely that some infected animals were released even though they tested
seronegative. Incomplete removal might also cause social disruption that
could accelerate the transmission of M. bovis between social groups.
We conclude that the live test strategy, as
implemented, would be unlikely to reduce the overall prevalence of M.
bovis infection in badgers, and thus the risk to cattle. Furthermore,
the poor sensitivity of the serological test makes it unlikely that
modifications to the live test protocol could increase its
bovine tuberculosis; Meles meles; Mycobacterium
bovis; wildlife disease