Bovine TB: Research project summary
Project SE3037: A quantitative risk assessment of the role of wild deer in the perpetuation of TB in cattle
Project duration: 11 monthsThe aim of this work was to develop a quantified risk assessment in order to assess the role that wild deer play in the perpetuation of bovine TB in cattle. To this end, a 3-species disease spread model was developed to capture and explore the underlying factors that have been identified as playing a part in the transmission of the disease between badgers, cattle and deer.
On the basis of the basic parameterisation and the modelling work undertaken, it is concluded that in most circumstances (even in areas of high cattle and deer density) the direct contribution of deer to cattle infection is small (between 1 and 5%). This is principally because:
- prevalence of bovine TB within the deer population appears to rise more slowly than within the badger population, even in circumstances in which deer are the initially infected species
- badger contact is more probable and more infectious
- even if deer prevalence is high, it tends to result in more badger cases, which then perpetuate the disease without much further contribution by deer
While the presence of deer does speed up the process of cattle infection, it is only as a second order effect. In the absence of badgers, there is little growth in deer prevalence or corresponding cattle prevalence.
Estimates of the deer to deer transmission rate suggest that it is significantly lower than a number of the other key transmission rates, and it is shown here that even at higher values of deer transmission (combined with favourable badger and cattle interaction), deer are unlikely to contribute to more than 20% of cattle infections in highly dense cattle and deer regions.
However, it is less clear whether deer are simply a spill-over host since the model predicts that:
- prevalence in the deer population does tend to rise with time
- migration of infected deer into infected regions does appear to be sufficient to trigger new outbreaks
Evidence that would support deer being a spill-over host is principally based on the fact that deer only appear to contribute in a minor way to the general level of infection within badgers.
While population density and contact rates have been shown to have a significant impact on the dynamics of the disease, in general the contribution by deer has been shown to be high only when cattle prevalence is low (i.e. while the relative contribution of deer can be significant its absolute contribution is small). The only significant circumstances in which deer become a much more important factor is when population densities increase significantly (>30 deer/km2 over a significant area).
Back to Research projects
Page last modified:
July 7, 2008