Attempts to control tuberculosis in cattle by removing infected badgers: constraints imposed by live test sensitivity

1. Bovine tuberculosis is a serious disease of cattle caused by the bacillu s Mycobacterium bovis. In south-west England, badgers Meles meles sustain e ndemic M. bovis infection and almost certainly transmit the disease to catt le. When tuberculosis outbreaks have occurred in cattle, the Ministry of ag riculture, Fisheries and Food (MAFF) has therefore culled badgers to try to avert further outbreaks. 2. To limit the number of badgers killed, MAFF assessed a possible new stra tegy (the 'live test strategy') that used a serological test to identify an d remove infected badgers. However, because the test correctly identified o nly 41% of truly infected badgers, individuals were pooled according to the setts at which they were sampled. All badgers were culled at setts where o ne or more seropositive animals were caught. 3. 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 sett. 4. Badger social groups typically occupy more than one sett. We allocated s etts to social groups by using Dirichlet tessellations and field signs to p redict territory borders. On average, 3.3 +/- 2.8 badgers were sampled in e ach group, Our model shows that this increase in sample size gives probabil ities of detecting M. bovis in truly infected groups of 43-62%, which is st ill likely to be unacceptably low, 5. Culling badgers according to the setts where they were trapped led to in complete 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 soci al groups. 6. We conclude that the live test strategy, as implemented, would be unlike ly to reduce the overall prevalence of M. bovis infection in badgers, and t hus the risk to cattle. Furthermore, the poor sensitivity of the serologica l test makes it unlikely that modifications to the live test protocol could increase its cost-effectiveness.