July 2, 2014

expert reaction to new modelling paper looking at bovine TB spread and control

A paper in Nature used models to separate out the different factors contributing to the bovine TB problem, to establish the routes of transmission of different outbreaks, and to predict which of the factors would be best to address in attempt to more effectively control the spread and incidence of this disease. The SMC held a briefing on the paper.

Prof Rosie Woodroffe, Institute of Zoology, Zoological Society of London (ZSL), said:

“Cattle TB is a terrible problem for farmers in the UK, and its control continues to present major challenges. This new research is thought-provoking and potentially very important for informing decisions about TB control. Like all models, it’s a simplification of reality, but it raises a number of interesting questions which merit further exploration.

“A particularly interesting finding relates to the importance of infection from the environment; this includes a number of environmental sources including badgers, contamination of the farm environment, and possibly other wild or domestic animals. The model suggests that this environmental source on its own is responsible for just 15% of herd infections. While this finding needs to be interpreted with caution, it does echo the findings of a model by Donnelly & Nouvellet which suggested that 6% (and a maximum of 25%) of new herd incidents in cattle were caused by direct transmission from badgers to cattle.

“While perhaps not a major cause of cattle on its own, this environmental effect is found to contribute to 68% of infections when in combination with factors relating to cattle management. What is surprising is that this environmental source appears to decay rapidly (with a half-life of 34 days). The rapidity of this decay resembles the rate of decay of M. bovis in the environment, and suggests that physical contamination of the environment might potentially be a more important source of cattle infection than previously assumed. Were infection from badgers a major source of new infections in cattle, one would expect a much slower decay of the infection in the environment – on a scale of years rather than days. Further investigations of the role of environmental contamination on cattle TB risks would be warranted.

“One of the paper’s conclusions is that a 50% reduction in this local environmental TB risk – intended to simulate large-scale badger culling – is predicted to be insufficient to prevent the continued spread of cattle TB. This finding is important given the massive cost, effort and controversy associated with badger culling. While eradicating TB in cattle in the long term is likely to require efforts to target infection in badgers, this paper predicts that controlling TB in the short and medium term would be more effectively achieved by management targeted at cattle.”

Prof Robbie McDonald of the University of Exeter’s Environment and Sustainability Institute, writing in Nature‘s News and Views section this issue, said:

“Brooks-Pollock and colleagues’ complex modelling approach provides two simple messages: first, that direct, generic controls imposed on cattle provide the greatest leverage for acting on the current mass of the bTB epidemic; and second, that although the environment is an important source of infection, indirectly tackling cattle disease by managing wildlife, by whatever means, is likely to yield unimpressive results and contribute little biologically to controlling a national epidemic.

“Although their work is compelling, the authors’ job is not complete. Policymakers often speak of their use of a “package of measures” and “every tool in the box” in controlling bTB, particularly with respect to controversial policies to cull badgers. Measures aimed at wildlife could contribute to bTB control in Britain, perhaps as part of achieving consensus among stakeholders in conflict or, most importantly, in the eventual transition from control to eradication of infection. But having shown that individual strategies, short of draconian whole-herd culling, will not rapidly turn this epidemic around, Brooks-Pollock et al. now have the perfect opportunity to use their model to test policymakers’ expectations of their various packages and tool-boxes and to gauge their overall cost-effectiveness. UK governments and the farming industry badly need such guidance to frame and implement their overall strategy and to manage expectations among their diverse stakeholders.

“This model highlights the scale of the challenge involved in controlling the bovine TB epidemic. The model suggests that slaughtering the whole herd upon detection of bTB would result, in the year after implementation, in drastic reductions in the number of cattle testing positive for bTB and in removal of restrictions from much of the national herd. However, this is a silver bullet that would cause severe wounding to the cattle industry, because it would come at the cost of a one-off, 20-fold increase in cattle slaughtered in the first year of the policy. Such a cost is reminiscent of the catastrophic scale of Britain’s 2001 experience of controlling foot and mouth disease and is likely to be horrifying to policymakers, farmers and animal-welfare campaigners alike.”

Defra Chief Scientific Advisor Prof Ian Boyd said:

“The study confirms the immense complexity of bovine tuberculosis and the challenges there are with bringing the disease under control. It makes some simplifying assumptions, especially about the role of badgers, and this may compromise the predictions so the results need to be interpreted with care.

“Based on our current understanding of the disease cycle, the more severe control measures suggested by the paper would probably result in a rapid decline in the cattle industry in areas where TB occurs. However, the study reinforces the basis of the current TB control strategy which is designed to cope with complex and diverse routes of infection.”

Lord John Krebs, Principal of Jesus College, and Professor of Zoology, University of Oxford, said:

“The sophisticated model and conclusions described in this paper give further support to the view that culling badgers is not an effective strategy for controlling bovine TB.  Instead the emphasis should be on stopping cattle-to-cattle transmission.  It is to be hoped that Defra takes on board this latest piece of scientific evidence when they formulate their policy for the future.”

Prof Kevin McConway, Professor of Applied Statistics, The Open University, said:

“This study involves building a statistical model, and using it to make some predictions about the effect of possible interventions aimed at controlling bovine TB. As with any statistical model, you’ve got to bear in mind that the model can’t be a full description of real life, and in the real world things might not develop precisely as the model says they will. But, that said, in my view the modelling is state-of-the art and predictions from it are very likely to be useful. The model is informed by considerable amounts of data on real cattle herds, has been constructed by experts, and uses appropriate advanced modelling techniques.

“The model doesn’t pick apart the exact details of how cattle might be infected from environmental sources such as badgers or infected pastures. But its key prediction is that dealing with just one aspect of TB transmission, whether it’s a major badger cull or much greater restrictions on cattle movement, is unlikely to have a major effect on its own. The authors do suggest that some more drastic interventions would have an effect, such as large increases in the amount of TB testing in cattle, or even culling whole herds when infected cattle are found. But it seems clear that there’s no simple solution to this problem.”

‘A dynamic model of bovine tuberculosis spread and control in Great Britain’ by Ellen Brooks-Pollock et al. published in Nature on Wednesday 2 July 2014.

Declared interests

Prof Rosie Woodroffe has research funding from Defra for studies of TB in cattle and badgers.

Prof Robbie McDonald’s comment is taken from his News and Views piece, which accompanies the Brooks-Pollock et al paper in this issue of Nature.

Lord John Krebs was the author of the report that led to the Randomised Control Badger Trial.