Bovine Tuberculosis

Bovine TB is a serious disease in the UK and in other countries. It causes animal welfare issues and financial hardship for farmers whose cattle either contract or are at risk of contracting the disease. There have been some attempts at modelling bTB, but it is difficult due to several factors not least the fact that the disease exists in both cattle and some wildlife species - notably badgers. However, modelling is possible and there have been recent steps forwards in this area. Regrettably the results of models do not seem to have been used to inform control strategies for bTB as they were for example during the outbreak of foot and mouth disease.

Our approach to this problem has been to use spatially explicit individual-based models at a scale approximately that of a UK county. We have incorporated bTB infections in both cattle and badgers and have allowed the disease to be passed between cattle, between badgers and between the two species. We have included realistic husbandry details for cattle and realistic ecology on badgers. We have published one paper so far reporting the model (it can be found here), in this we showed that the model replicated known patterns. These include the trajectory of bTB in the UK, the change in the size of the UK badger population and the change in bTB during and after the foot and mouth epidemic which prevented both TB testing and cattle movement. We therefore believe that our model captures the essential details of bTB in the UK. Key results of this paper were:

  • That the practice of winter housing cattle will exacerbate TB infections, this is not because winter housing per se is bad, but it is due to the fact that cattle are brought into close proximity of each other and therefore disease transmission between cattle is more likely. This matters because it is well known that the test for TB in cattle is imperfect (what test is not), therefore it will miss some infected cattle whcih will remain in the herd and pass infection onto uninfected individuals
  • The practice of badger culling does reduce TB infection in cattle but only very marginally
  • The shorter the interval between tests (as long as they are equally effective), the more likely it is that infected cattle will be detected and removed. Reducing the test interval is an effective way of controlling TB

A more recent paper in which we conducted a time series analysis of the data that DEFRA make public on their website demonstrated that there were significant differences in trends of TB prevalence and incidence between the regions of GB. In Wales both of these are declining, in West of England they showing some signs of declining but in North and East England they are increasing. We note that Wales has had annual TB testing since 2010, the West of England is now mostly on annual testing the North and East are in general not on annual testing. We also showed that in Wales increasing the rate of tests does not reveal more cases of TB suggesting that testing is now being conducted at the optimal rate to detect TB cases. In England there is a positive relationship between the number of tests conducted and the numbers of cases detected.

Our work has received quite a lot of attention some if it amusing, but depressing (see here). The Guardian coverage can be seen here and here. Our own Storify summary is here.

Here is a video of me trying to explain what we do.

© Matthew Evans 2015