The importance of ‘neighbourhood’ in the persistence of bovine tuberculosis in Irish cattle herds

Local persistence of infection is a key feature of bovine tuberculosis (bTB) among cattle herds in the Republic of Ireland. The aim of this study was to determine the relative importance of ‘neighbourhood’, specifically farm-to-farm spread and spread from wildlife, in the persistence of bTB by investigating herds having a bTB episode in 2006. A case–control study was conducted on the association between the occurrence of a bTB episode in 2006 and the occurrence of bTB in previous years among neighbouring herd(s) within 1 km, while controlling for each herd’s bTB history and other risk factors. Neighbouring herds were grouped into three zones, based on distance, and bTB incidence measures summarised within each zone and by calendar year (2001–2005). The incidence of bTB was associated with an increased animal incidence in two subsets of neighbouring herds: (i) herds directly contiguous during the previous 2 years (attributable fraction = 0.20), and (ii) herds at a distance of >25 m in the previous year (attributable fraction = 0.19). Other predictors of bTB in a herd in 2006 included the occurrence of a bTB episode within that herd in any of the previous 5 years, herd size, and the number of animals purchased at age greater than 12 months. An infected wildlife source best explains the existence of a “neighbouring herd risk” for bTB at distances greater than 25 m. Further studies will be necessary to determine to what extent neighbouring herd risk within 25 m may be confounded by the same wildlife (badger) source