2 “direct” aerosol transmission between animals in close proximity. Although considered to be of lesser importance, oral ingestion of mycobacteria from farm environments cannot currently be excluded [1]. Whilst it is important to view bovine TB as an infectious disease which requires preventive as well as control measures, M. bovis infection in cattle now rarely presents as clinical disease. More commonly it appears as apparently healthy animals responding to an immunological test based on tuberculin, an entirely diﬀerent scenario to that which existed when control programmes were ﬁrst introduced [20]. In countries with advanced test and control programmes (a comprehensive set of surveillance and control measures to address cattle-cattle transmission) bovine TB tends to be a low-incidence infectious disease with an apparently low transmission rate. Infection appears to be relatively poorly transmitted between cattle in most, but not all, circumstances. 2. Herd Testing and Management Monitoring of the cattle population for M. bovis infection depends on national programmes of herd tuberculin testing, supported by active abattoir surveillance [1, 12, 21]. The frequency of such testing is determined by the recent local incidence, ranging from annual testing (in Northern Ireland and other parts of the UK and Ireland) to four- year testing (in parts of England) [1, 21]. Limited tuberculin test sensitivity is also likely to have contributed to under- or over-estimation of the impact of several risk factors, such as, cattle contact and movement in some studies. To compound the above, multiple unreported local cattle movements and contacts are described between farms in several studies and are recognised as a factor in underestimating the role of contact and movements, particularly over short range. Early diagnosis and intervention to interrupt transmis- sion are the priority for control and the eﬀectiveness of testing and removal of infectious animals will impact on transmission and depends on: how early the infections are detected, how sensitive the test(s) actually are in practice and other variables, including interoperator characteristics, test interval, and/or time to derestriction. More severe tuberculin test interpretation and/or supplementary immunological tests (cell-mediated immunity and/or humoral immunity) may be applied in deﬁned circumstances. The herd is placed under movement restriction until all animals clear two short-interval tuberculin tests. Animals may also test “inconclusive” to the tuberculin test, a proportion of which test negative at the next short-interval test but have since been shown in an Irish study to be 12 times more likely to be TB positive at the next test than other animals [22]. Between 11.8% and 21.4% was conﬁrmed positive at the laboratory, compared to 34–39% for standard tuberculin reactors. In summary, inconclusive reactors are at increased future TB risk [22, 23], whether they clear the next test or not and this is now reﬂected in policy options. Bovine TB “reactors”, and on occasions the exposed cohort, should be removed quickly from aﬀected herds and contact tracing and testing should be implemented. Veterinary Medicine International Whole-herd depopulation may be indicated on rare occa- sions. The impact of such whole herd depopulation, for either bovine TB or bovine spongiform encephalopathy (BSE), on the recurrence of bovine TB was investigated in a recent Irish study [21]. Recurrence may reﬂect residual infection in cattle and/or reinfection from other sources, potentially including contiguous spread, acquired infection, infectious wildlife, and environmental contamination and may also be a consequence of imperfect test sensitivity [24]. It was concluded that future risk varied signiﬁcantly by previous TB risk and reason for depopulation and that whole herd depopulation was eﬀective in reducing future bovine TB risk. The BSE depopulated herd results were similar to a recent GB study [25] where whole herd depopulation for foot-and-mouth disease (FMD) was also not associated with reduced future bovine TB risk. Key to understanding bovine TB epidemiology is the relationship between infection and disease (TB) and the relationship between disease and transmission. Hence, it is important to consider those risk factors, such as, infectious contacts between animals and their movements, which theoretically facilitate such transmission. The identiﬁcation of risk factors and risk settings for infection and transmission is also intimately linked with those factors which aﬀect susceptibility. 3. Risk Factor Studies Infectious diseases in general arise from an interaction between the infectious agent, the host, and a range of covariables, which may include other infectious diseases and the environment. Risk factors (biological, behavioural, environmental, or genetic) are known to inﬂuence both transmission and susceptibility. They may operate at diﬀer- ent scales; regional-level, herd-level, and animal-level and may vary across regions due to factors, such as, diﬀering farm structures, farm management practices, bovine TB con- trol and eradication programmes, regional TB incidences, wildlife densities, and the relative importance of speciﬁc risk factors by area. The risk of a bovine TB episode is accepted to vary between herds with some herds experiencing multiple break- downs over time, whilst others appear to remain free of infection. Also, the nature of bovine TB breakdowns is not uniform; they can be classiﬁed as “sporadic”, “persistent”, “recurrent”, and so forth and the literature supports the view that diﬀerent risk factors are likely to apply, almost on a case-by-case basis. However, some risk factors have tended to emerge in several published studies and we have discussed these here. It is important to note that epidemiological studies may diﬀer in the variables examined, the exact measures used (in relation to the association with wildlife, etc.), and the study size and power. Therefore, not all risk factors would be expected to be identiﬁed equally across all studies. Those risk factors which tend to converge from disparate studies would support the currently hypothesized sources of infection and routes of transmission.