Our November Editor’s Choice article raises the question of whether the culling of badgers could increase the risk of TB spread in cattle, as badgers in culled areas travel further. Associate Editor, Andrew Park, looks at context and management implications of Ham et al.’s recent findings.
Bovine tuberculosis (TB) is an important livestock disease in the UK, where it has been increasing since the 1990s and was recently found to affect more than 1 in 20 herds in England. Part of the challenge of eradicating the causative agent, Mycobacterium bovis, is that M. bovis is a generalist pathogen with wildlife hosts capable of introducing or reintroducing infection to cattle herds. In the UK, the European badger (Meles meles) has been estimated to be responsible for over 5% of such introductions in high TB incidence areas. Following these introductions, infection is then amplified by cattle-to-cattle transmission.
Badger culling has been implemented in an attempt to reduce the incidence of TB in cattle. In principle, culling may both reduce the prevalence of disease in wildlife hosts and reduce the contact rates between infected badgers and uninfected cattle herds. However, culling has produced equivocal results as a TB control policy. In some cases, it has been associated with increases in the numbers of suspected or confirmed TB herds.
Badger culling-related increases of TB incidence in cattle are thought to operate by changes in badger behaviour. At low densities following culls, badgers have exhibited increased home range size and overlap, leading to the ‘perturbation hypothesis’ whereby culling actually increases both transmission between badgers and contact rates with cattle. However, the hypothesis has suffered from the lack of before, during and after-cull data on individual movement of badgers. This Editor’s Choice study by Ham et al. provides just that fine-scale data using GPS collars on badgers living inside a cull zone, as well as comparative data on badgers in unculled areas.
The authors found that culling was associated with a 61% increase in monthly home range size and that these nocturnal animals were less active at night, to the tune of around 90 minutes – meaning they were travelling further and faster than their counterparts in unculled areas. These effects were observed during culls and persisted after the culls had ended. Activity levels were predicted to drop as the need to forage above ground for food is reduced by culling, which leads to less competition for resources. The authors note that reduced nocturnal activity of badgers may negatively impact shooting-based control strategies, augmented by the fact that badgers surviving a cull are likely to have a less ‘bold’ behavioural phenotype than their culled counterparts.
In terms of M. bovis transmission, badgers in cull zones visited an average of 45% more fields and showed a 20-fold increase in the odds of trespassing into neighbouring badger territories – increasing both inter- and intra-specific transmission potential. Collectively, these analyses demonstrate the challenges that badger behaviour poses to the control of TB and adds to the disease ecology literature on reasons for cull failures, which include failure to reduce population sizes significantly and demographic booms following culls.
Read the open access Editor’s Choice article, Effect of culling on individual badger Meles meles behaviour: Potential implications for bovine tuberculosis transmission in issue 56:11 of Journal of Applied Ecology.