Cull or cure? bovine TB and badgers

By Dave Armstrong - 07 Aug 2013 10:25:7 GMT
Cull or cure? bovine TB and badgers

The familiar badger, Meles meles, of the British countryside, but for how much longer?; Badger image; Credit: Shutterstock

The bTB or bovine tuberculosis debate has gone on forever. The European badger, Meles meles has been blamed by several groups for transmitting it between cattle within Britain as well as around its own species. To categorise this political dynamite into usable information is a brave and commendable act. The authors use Data, Expert opinion, Supporting evidence and Projections as their 4 categories.

Cattle, humans and other livestock are the major infectees of bTB . Pasteurisation of milk presents human infection, but slaughter of infected cattle causes much financial loss. For 20 years, bTB incidence has been increasing, as has the slaughter and farmers' annoyance. Wildlife "reservoirs" of infection have been found, with the badger clearly identified in Britain and vaccines made available for both cattle and badgers.

In the past badger culling was used and its effects studied from 1998 to 2006 in a trial (RBCT). It seems now about to begin this year, with 2 pilot projects, despite great public objection. The limitations of this paper are fully understood, it seems, by the authors as they explain how it can be extended.

Social issues are mixed with natural science in the evidence. Consideration of the CAP (Common Agricultural Policy) of the EU as well as farmers' implementation of control measures in Britain is considered. Australia and New Zealand have identified 2 species as "reservoirs." This means that the path seems simple to follow- simply copy successful practice. The epidemiology of the disease, however, is complex, with culling and vaccination options quite variable in effect. Different regions of Britain have completely different incidence figures, from absence to consistently high. Cattle movement alone can account for the disease spreading to relatively unaffected areas. In fact, almost every single incidence of bTB in Tuberculosis Free Areas is down to cattle movement. Elsewhere, The US and the other countries mentioned all have persistent bTB infection within wildlife populations.

The natural science evidence seems unable to direct policy alone, but equally, the policy maker must be made aware of what is evidence, and what is politics. One major fact is that high badger and cattle infections occur where both populations are high(e.g. in the south and west.)Confusingly, high badger density on its own is not connected to high incidence of bTB.

It's possible that badgers transmit bTB within their populations, but it's likely that they continually receive infection from cattle. 25% of badgers seem to have been infected in high incidence areas. The method of infection is almost impossible to guess, as there are so many possibilities of indirect or close, even aerosol contact. Many other species can naturally carry the bacterium to cattle, but the badger is more or less alone in its association. Only deer species are assessed locally as potentially infecting cattle, but any "2-hoofed" mammal seems capable. The most likely possibility would be companion animals catching bTB infection form cattle and passing it to a human. Less developed countries still have bTB infection in humans, and it is said to lead to human TB.

Control programmes would be better integrated in both cattle and badger populations in an area. The solution for a high-incidence area would likely differ from that for a low-incidence area. Prevention being better than cure, the effects of badger/cattle isolation mechanisms on farms are yet to be studies. As far as cost is concerned, it would seem less expensive to develop such a separation system at feed stores, and barns, badger setts and latrines. Control by culling in the RBCT study, using 30 setts, showed a relative reduction in new cattle infections, which then increased gradually. In 2003 however, culling terminated as the bTB incidence was obviously higher in neighbouring areas!

It's possible that "perturbation" of the badger causes it to remove to new areas and then cause new bTB infections. It certainly caused badger bTB infection to increase, but whether this continued in the population after culling stopped is unknown. The impact of badger culling would only be significant if large numbers were killed over a very large area. Badgers would not become regionally extinct, on the favourable side, but most noticeable is the negative effect of any short-term or small-scale culling.

As culling seems to leave the choice of massive effort highly unpopular reactions, or smaller attempts with negative effect, vaccination would seem an improvement. However, EU regulations currently prohibit the human-like BCG vaccine, a licence should be relatively simple to operate, at least within the British Isles. Neonatal calves are injected, with a possible back-up a year later. One danger is the flagging of false positive test results with the standard SICCT test for bTB. Given several problems of differential testing procedures, that is quite a problem for early testers! Effects of vaccines on current herds are likely to be limited, because only the calves will be protected to start with.

The alternative of badger vaccination is licensed by the EU, with oral types soon to be available. Trials indicate that most of the population can be reached with baited food. Perhaps there is hope for the future, without violent public reaction and equally aggressive government moves in support of angry farmers.

Charles Godfray of Oxford University and his many colleagues publish the paper today in the Proceedings of the Royal Society B: Biological Sciences journal.