8 Table 1: The most consistently identiﬁed herd-level risk factors for bovine TB. Herd-level risk factors most consistently identiﬁed∗ Cattle movement (estimated to contribute <20% in some GB and Irish studies) Occurrence of TB on contiguous premises and/or level of TB in surrounding areas (infection pressure) Herd size ∗ It is important to note that epidemiological studies may diﬀer in the variables examined, the exact measures used (in relation to the association with badgers etc.), and the study size and power. Therefore, not all risk factors would be expected to be identiﬁed equally across all studies. Table 2: Other herd-level risks identiﬁed in some studies. Other herd-level risk factors identiﬁed in some studies∗ Contact with contiguous cattle Indicators of badger density/activity Sourcing cattle from herds with TB history Providing cattle feed inside housing Use of multiple premises Housing type Herd type Farmland habitat Fertiliser usage Mineral deﬁciencies (selenium) Use of silage clamps Rotational grazing ∗ It is important to note that epidemiological studies may diﬀer in the variables examined, the exact measures used (in relation to the association with badgers etc.), and the study size and power. Therefore, not all risk factors would be expected to be identiﬁed equally across all studies. other case-control studies of bovine TB in cattle had yielded widely diﬀering recommendations [1]. Taken together, these studies illustrate the complexity of the host/pathogen/environment interactions or “episystem” in bovine TB [56, 57] and the variation in study design and outcome. It may not be possible to reliably identify particular risk factors which could be widely adopted and predicted to lead to reduced transmission of disease to and from cattle. More insight may be achieved when risk factors are classiﬁed locally into management, wildlife, and environment factors, [1] and it should be appreciated that environmental features are rarely controllable by the herd keeper. One primary risk factor is cattle density, which increases the probability of transmission via aerosol between infectious and susceptible animals [35]. Regarding management factors, results suggest that cattle movements, herd contacts, use of fertilizer, hous- ing, and feeding practices may impact on risk, although study ﬁndings identify association and not necessarily causation. Nevertheless there is suﬃcient evidence that by applying the broad principles of biosecurity it should be possible to reduce the risk of cattle becoming infected by other animals, including wildlife. Account should be taken of cattle Veterinary Medicine International movement on and oﬀ the premises, minimising contact with other cattle and between cattle and wildlife and taking greater care with animal housing and feeding practices. In particular, studies, such as, the TB99 and CCS2005 analyses of the GB RBCT data indicate that there is no universal solution for farm management to reduce the risk of a herd breakdown. Occasionally a clear cause and eﬀect relationship can be demonstrated by epidemiological studies, but in most cases the situation is more complex and the research tells us what factors are important concerning a speciﬁc question or a theoretical level of risk associated with a particular event, behavior, or contact. While many of the general risk factors for the introduction and spread of bovine TB have been identiﬁed, less is known about the practical measures that farmers can take to minimize their risk and the possible impact of common biosecurity practices. 8. Methodology We searched systematically on-line resources (PubMed, Web of Science) to ﬁnd appropriate peer-reviewed literature and relied on previously identiﬁed key publications. Literature was accessed until October 2011, inclusive. We purposefully selected publications that were judged most relevant for the review, with a preference for high-quality systematic reviews. Whilst publications in the last 10 years were favoured we did not exclude highly regarded older publications. Searches were conducted using combinations of the following key words: “bovine”, “tuberculosis”, and “risk”. In addition, open-access DEFRA R&D project web-pages were searched. DEFRA web-pages on bovine TB, Phillips and others [58], and the Final Report of the Independent Scientiﬁc Group [1], “Bovine TB: the Scientiﬁc Evidence” were refer- enced throughout. Acknowledgment The authors gratefully acknowledge funding from the Department of Agriculture and Rural Development for Northern Ireland. References [1] J. F. Bourne, “Bovine TB: The Scientiﬁc Evidence. A science base for a sustainable policy to control TB in cattle. Final report of the Independent Scientiﬁc Group on Cattle TB,” DEFRA Publications, London, UK, 2007, http://archive.Defra .gov.uk/foodfarm/farmanimal/atoz/tb/isg/report/ﬁnal report .pdf. [2] OIE, 2009, http://www.oie.int/ﬁleadmin/Home/eng/Health standards/tahm/2.04.07 BOVINE TB.pdf. [3] J. M. Pollock and S. D. Neill, “Mycobacterium bovis infection and tuberculosis in cattle,” The Veterinary Journal, vol. 163, no. 2, pp. 115–127, 2002. [4] F. Mathews, D. W. Macdonald, G. M. Taylor et al., “Bovine tuberculosis (Mycobacterium bovis) in British farmland wildlife: the importance to agriculture,” Proceedings of the Royal Society B, vol. 273, no. 1584, pp. 357–365, 2006.