Cattle to cattle transmission of bovine tuberculosis: Risk factors and dynamics

In a series of studies the roles of cattle exposure to bovine tuberculosis (bTB) from other cattle and from the farm and the impact of test and cull control are presented. One of the studies is of four counties in England, two with a long history of herd breakdown (HBD) with bTB and two with a history of HBD from the mid 1990s. We conclude that in an unstable situation bTB testing leads to a delay in detection of herds with cattle that are positive to the skin test. At least some of the HBD was most likely to be caused by purchase of infected cattle. In a second study, farms that were depopulated during FMD in 2001 and subsequently restocked were considered. Reformed herds that included cattle purchased from herds located in areas that were tested at less than 2-year test intervals for ten years before 2001 were at greater risk of HBD. This was the only risk for HBD unless these herds were located on farms where the previous herd had had a HBD between 1995 and 2000 when there was a residual farm effect that persisted after the original herd was removed. These two effects, cattle movement and non-cattle environment respectively, were independent. In our third study, cattle in endemic areas of bTB were more likely to be reactors when they had been present at a previous bTB test when a reactor had been detected. There were herds where there were no reactor cattle for many years of tests and where homebred cattle were at a very low risk of becoming a reactor. Additionally, we have conducted theoretical studies of the outcomes of test and cull programmes and the distribution of numbers of reactors obtained at HBD.