6 as acting as ongoing sources of infection. Breakdown dura- tion was a function of infection status and test performance. Potential explanations for persistent infection included sub- optimal performance of the bovine TB tuberculin test, delay in its application, or reintroduction of infection. Skin test sensitivity has been estimated at 75.0–95.5% [12]. If the sensitivity was substantially lower [24], failure to detect and remove infected animals would create potential for within- herd persistence and onward spread. Factors associated with breakdown recurrence in Ire- land, where detailed animal-level data were available [44], included slurry spreading, purchase of bulls and cattle, pres- ence of inconclusive reactors in the breakdown, and presence of badgers and nutritional status. Where only population- level surveillance data were available, factors associated with recurrence included herd size, reactor number, and recent herd bovine TB history [44–46]. In the DEFRA SE3230 research project (The Prob- lem TB Herd: characterisation, prediction and resolution) breakdown conﬁrmation status was by far the strongest risk factor for persistence (OR = 12.6). They used an improved case deﬁnition and concluded that this strong association may best be explained by the tendency to deploy severe interpretation of the tuberculin test in herds with conﬁrmed status and the possibility that true prevalence was underestimated [43]. Their model could predict earlier those herds most likely to sustain persistent infection. Resources and earlier intervention could be directed at those herds. The model predicted that stopping animal movements onto the farm during the breakdown and moving salt licks indoors were associated with a small decreased risk. It is also plausible that a number of the unconﬁrmed herds were not actually infected. Analysis of the GB CCS2005 epidemiological data identiﬁed that despite increased testing during and after breakdowns ∼21% of breakdowns recurred within 12 months. 60% of these recurrences was disclosed at the 6- month followup, suggestive of within-herd persistence. 38% recurred within 24 months [45]. Factors associated with recurrence were reactor number and recent history of bovine TB in the herd, consistent with previous studies in Ireland and Northern Ireland [44, 46]. However, they found a lack of association with the conﬁrmation status of the initial breakdown. They conclude that their data support a higher prevalence of infection than observed, residual infection, or repeated reinfection. The main risk factors associated with recurrence in this study ranked as follows: use of “other housing types” (OR = 4.6), number of contiguous farms (OR = 3.2), and borrowing animals (OR = 2.1) [45]. Protective factors associated with decreased risk of recurrence included the presence of rough grass/moorland (OR = 0.3). These recurrent breakdowns may have been reinfected from a local source, such as, wildlife or from cattle movements into the herd. As well as consuming disproportionate resources, the existence of recurrent breakdowns suggests that such herds cannot reliably be cleared of infection and undermines stakeholder conﬁdence in the TB testing programme. They concluded that certain farm practices or characteristics may predispose to reinfection and that a combination of factors Veterinary Medicine International was associated with recurrence, rather than just one strong factor. Abernethy et al. [46] used comprehensive animal-level test and movement data to investigate the eﬀect of selected risk factors on recurrence of bovine TB in breakdown herds after derestriction in Northern Ireland. Factors associated with an increased risk included the number of reactors at the disclosing test, the number of reactors at follow-up tests, the number of follow up tests, the level of bovine TB in the district council area, herd size, the number of cattle purchased during the postoutbreak interval, and a history of bovine TB breakdown(s) within the previous two years. Despite variation between farming practices within the British Isles, reactor number and recent history of bovine TB were consistent risks for recurrence in Irish, Northern Irish, and GB studies [45]. Whether the breakdown was conﬁrmed or not was the major factor in the duration of breakdowns (persistence) in GB [43] but was not a factor in risk of recurrence and neither was herd size nor cattle movements [45]. This illustrates that the risk factors for diﬀerent types of breakdown (sporadic, persistent, recurrent, etc.) may well be diﬀerent. Either way, this could increase transmission potential to local wildlife or to local or more distant cattle herds through cattle contacts and movements, during periods when movement restrictions are not applied. The relative contribution of persistence versus reintroduction to recurrence is unknown [45] and although their wildlife data were relatively weak, no association was detected between badger presence and recurrence at the 6 month follow-up herd test. To increase the detection of exposed/infected cattle within herd, there have been suggestions to increase the between-test interval and the duration of herd restriction. Olea-Popelka et al. [47] attempted to estimate the levels of badger exposure for cattle and to test the hypothesis that increased badger exposure does not increase the risk of bovine TB in selected Irish herds. They used data from the Four Areas Trial badger cull in Kilkenny (1996–1999). The speciﬁc location of cattle within each farm, and the length of time that cattle spent in each farm ﬁeld during the grazing season, and in the barnyard during winter, was used to build an exposure coeﬃcient to quantify the amount of badger exposure that cattle encountered either on pasture or in the barn. The study design was a matched case-control study in which the control herds were selected using incidence density sampling. During the 4-year study period, 543 badgers were removed and of those 96 badgers were bovine TB positive and 96 herd breakdowns occurred. There was a signiﬁcant association between case herds and having a higher badger sett exposure coeﬃcient during 1996– 1998, but no signiﬁcant association between case herds and having a higher exposure coeﬃcient based on the number of badgers, or the number of bovine TB-positive badgers, during September 1997–December 1999 was found. It would be valuable to take the same approach to quantifying within- herd cattle contacts in housing and at pasture. Porphyre et al. [48] investigated risk factors for bovine TB on New Zealand cattle farms and their relationship with possum control strategies. Study design was a retrospec- tive cohort based on data obtained from the TB testing