An important challenge in understanding how, when and where tuberculosis (TB) transmits to and between cattle is that infections are not immediately apparent. In most cases, infections are detected when cattle are tested using a skin test which measures each animal's immunological response to the bacterium that causes TB in cattle (Mycobacterium bovis). The skin test indicates the presence or absence of infection, which can be later confirmed after a post-mortem examination of the animal, but it does not provide information on when the infection occurred. Thus, those investigating the disease learn that an infection occurred, but subject to caveats about test performance, the insights are limited to the knowledge that the infection happened between the date of the current skin test indicating infection and the date of the most recent past skin test (if any) showing no evidence of infection.
This limitation means that careful statistical analysis is required in order to investigate any seasonality in the risks of infection. This project will investigate this issue directly to determine whether cattle appear to be at higher risk of becoming infected with M. bovis in some months than others. If there were big differences in risks by month, then advice to farmers regarding biosecurity measures (designed to limit opportunities for disease spread) could be improved by highlighting when extra vigilence would be most beneficial.
Not being able to determine precisely when cattle became infected also complicated interpretation of the results of the Randomised Badger Culling Trial (RBCT), which tested two potential badger culling strategies to determine whether they reduced the amount of TB in cattle. This project will also investigate, using statistical models, what the information collected from skin tests performed on cattle in the RBCT means in terms of how badger culling affected the risks of cattle TB infections over time, in RBCT trial areas and on nearby land.
Further analysis of RBCT data will also investigate if/how badger and cattle herd densities immediately surrounding herds affected their TB risks and the impacts of badger culling on these TB risks.
Finally, the project will investigate ongoing trends in TB incidence among cattle herds in proactive and survey-only trials areas following the completion of a Defra-funded contract which ends on 31 March 2009. The results will be reported to Defra in August 2009 and February 2010.
The results of these investigations will be submitted to scientific journals where they will be subject to review by independent scientists prior to publication. Furthermore, members of the project team will give presentations of the results both to other scientists and to stakeholder groups.