Risk factors for herd breakdown with bovine tuberculosis in 148 cattle herds in the south west of England

Bovine tuberculosis (bTB) is caused by Mycobacterium bovis. The disease has a long latent period, heterogenous spread, can infect many species and can persist in the environment. In the UK, the rate of herd breakdowns (HBD) with bTB is increasing. A retrospective cohort study of 148 cattle herds was set up to investigate risk factors for HBD from October 2001 to November 2004. Herds were selected from farms located in the randomised badger culling trial (RBCT) and comprised holdings (24%) that were restocked with cattle after the foot and mouth disease (FMD) epidemic in 2001 and holdings (76%) that were continuously stocked throughout the FMD epidemic. Farmers were interviewed between June 2003 and February 2004. Questions on herd and farm management were asked for the period October 2001 to June 2003. Data on herd testing for bTB were sourced from the VetNet database and historic data from 1995 were used in the analysis. A discrete time survival analysis was used to examine factors associated with the risk of HBD.By the end of the study period, November 2004, 50% of study herds had experienced a HBD with bTB at least once. Farms that were restocked for less than 1 year after FMD had a reduced risk of HBD (P<0.01) compared with continuously stocked farms in the same year. This reduced risk did not persist after 1 year of restocking. Feeding vitamin and mineral lick supplements compared with not feeding these supplements also reduced the risk of HBD. Factors associated with an increased risk of HBD were storing manure and slurry indoors or in a closed container, spreading manure all year round, herds with dairy cattle compared with herds without dairy cattle, increasing herd size, purchase of cattle from markets, location of the farm in the proactive area of the RBCT compared with survey only and location of farms in Somerset and North Devon.The lower risk of HBD in the first year after restocking but not the second or third year suggests that removal of all cattle might have lowered the infectious load of M. bovis on these premises for a period of time but that this did not persist once cattle were reintroduced. Purchase of cattle from markets suggests that there was a risk of introduction or re-introduction of bTB from these cattle. Method of storage or lack of storage of slurry might aid persistence of M. bovis in the environment if M. bovis survives in slurry in some circumstances.