Local cattle and badger populations affect the risk of confirmed tuberculosis in British cattle herds

Contribution of herd management, biosecurity, and environmental factors to the risk of bovine tuberculosis in a historically low prevalence region

Eradication of bovine tuberculosis (bTB) in certain historically low-prevalence regions remains elusive. A complete characterisation of the husbandry practices, biosecurity, and environment where farms are located is crucial to implement targeted in-farm risk mitigation protocols. Here, a detailed survey performed in 94 dairy cattle farms located in Navarra, a low-prevalence region of Spain between 2016 and 2020 was carried out. Data on 73 biosecurity, farm-, and environmental-level factors potentially associated with the risk of bTB occurrence were evaluated using an ordinal logistic regression model: farms were classified based on their prevalence index, a score linked to each farm to account for the severity and recurrence of bTB cases: 22.3% of the farms had a score of 1, 21.3% a score of 2, 26.6% a score of ≥ 3, and 29.8% were negative herds. A statistically significant association between a higher prevalence index and the frequency of badger sightings along with the lease of pastures to sheep during Winter was identified. Farms that detected badgers on a monthly to daily basis in the surroundings and those that leased pastures for sheep flocks during Winter were four [odds ratio, 95% CI (4.3; 1.1-17.5)] and three (3.1; 1.0-9.9) times more likely to have the highest prevalence index, respectively (predicted probabilityprevalence index≥3 = 0.7; 95% CI 0.3-0.9). Conversely, farms that used a vehicle to transport animals from holdings to pastures were less likely (0.1; <0.1-0.3) to present higher levels of prevalence index compared with farms that used none (on foot). Results suggested that the combined effect of farm- and environmental-level risk factors identified here may be hampering disease eradication in Navarra, highlighting the need to implement targeted protocols on farms and grazing plots. An increased awareness of monitoring sheep and wildlife in direct or indirect contact with cattle herds in historically low bTB prevalence areas should be raised.

Quantifying Land Fragmentation in Northern Irish Cattle Enterprises

Farmland fragmentation is considered to be a defining feature of Northern Ireland’s (NI) agricultural landscape, influencing agricultural efficiency, productivity, and the spread of livestock diseases. Despite this, the full extent of farmland fragmentation in cattle farms in NI is not well understood, and little is known of how farmland fragmentation either influences, or is influenced by, different animal production types. Here, we describe and quantify farmland fragmentation in cattle farms for all of NI, using GIS processing of land parcel data to associate individual parcels with data on the cattle business associated with the land. We found that 35% of farms consisted of five or more fragments, with dairy farms associated with greater levels of farmland fragmentation, fragment dispersal and contact with contiguous neighbours compared to other production types. The elevated levels of farmland fragmentation in dairy production compared to non-dairy, may be associated with the recent expansion of dairy farms by land acquisition, following the abolition of the milk quota system in 2015. The comparatively high levels of farmland fragmentation observed in NI cattle farms may also have important implications for agricultural productivity and epidemiology alike. Whilst highly connected pastures could facilitate the dissemination of disease, highly fragmented land could also hamper productivity via diseconomies of scale, such as preventing the increase of herd sizes or additionally, adding to farm costs by increasing the complexity of herd management.

Investigating Farm Fragmentation as a Risk Factor for Bovine Tuberculosis in Cattle Herds: A Matched Case-Control Study from Northern Ireland

Bovine tuberculosis remains a challenging endemic pathogen of cattle in many parts of the globe. Spatial clustering of Mycoacterium bovis molecular types in cattle suggests that local factors are the primary drivers of spread. Northern Ireland’s agricultural landscape is comprised of highly fragmented farms, distributed across spatially discontinuous land parcels, and these highly fragmented farms are thought to facilitate localised spread. We conducted a matched case control study to quantify the risks of bovine tuberculosis breakdown with farm area, farm fragmentation, fragment dispersal, and contact with neighbouring herds. Whilst our results show small but significant increases in breakdown risk associated with each factor, these relationships were strongly confounded with the number of contiguous neighbours with bovine tuberculosis. Our key finding was that every infected neighbour led to an increase in the odds of breakdown by 40% to 50%, and that highly fragmented farms were almost twice as likely to have a bTB positive neighbour compared to nonfragmented farms. Our results suggest that after controlling for herd size, herd type, spatial and temporal factors, farm fragmentation increasingly exposes herds to infection originating from first-order spatial neighbours. Given Northern Ireland’s particularly fragmented landscape, and reliance on short-term leases, our data support the hypothesis that between-herd contiguous spread is a particularly important component of the region’s bovine tuberculosis disease system.

Bovine tuberculosis prevalence and risk factors in selected districts of Bangladesh

A cross-sectional survey was conducted in selected districts of Bangladesh to estimate the prevalence of bovine tuberculosis (bTB), and to identify the risk factors for bTB. We included 1865 farmed cattle from 79 herds randomly selected from five districts. Herd and animal level data were collected using semi-structured interviews with cattle herd owners. The single intradermal comparative tuberculin test (SICTT) was used to estimate the prevalence of bTB. The risk factors were identified using mixed-effect multiple logistic regression analyses. The overall herd and animal level prevalences of bTB were estimated to be 45.6% (95% Confidence Interval [CI] = 34.3–57.2%) and 11.3 (95% CI = 9.9–12.8%), respectively, using the OIE recommended >4 mm cut-off. The true animal level prevalence of bTB was estimated to be 11.8 (95% Credible Interval = 2.1–20.3%). At the herd level, farm size, bTB history of the farm and type of husbandry were significantly associated with bTB status in univariable analysis. Similarly, age group, sex, pregnancy status and parity were significantly associated with bTB at cattle level. However, in multivariable analysis only herd size at the herd level and age group and pregnancy status at the cattle level were significant. Compared to a herd size of 1–10, the odds of bTB were 22.8 (95% CI: 5.2–100.9) and 45.6 times (95% CI: 5.0–417.7) greater in herd sizes of >20–50 and >50, respectively. The odds of bTB were 2.2 (95% CI: 1.0–4.5) and 2.5 times (95% CI: 1.1–5.4) higher in cattle aged >3–6 years and > 6 years, compared to cattle aged ≤1 year. Pregnancy increased the odds of bTB by 1.7 times (95% CI: 1.2–2.4) compared to non-pregnant cattle. Taken together, the results suggest high herd and animal level prevalence of bTB in these 5 districts, with the greatest risk of bTB in older and pregnant cattle within large herds (>20), and highlight an urgent need for continued surveillance and implementation of bTB control programs in Bangladesh.

Quantifying intraherd cattle movement metrics: Implications for disease transmission risk

There are a paucity of data quantifying on-farm management practices such as the frequency of intraherd cattle movements, use of consolidated or spatially fragmented grazing pastures, and duration of time cattle spend at grass with respect to biosecurity and disease transmission. Such movement dynamics are important when attempting to understand the maintenance of chronic infectious disease, such as bovine tuberculosis (bTB). We captured empirical data on daily cattle movements for a sample of eighteen farms throughout one complete grazing season (n = 18,988 grazing days) and assessed these attributes in relation to herd bTB risk. Dairy herds were stocked at significantly higher densities compared to beef production systems (6.6 animals/ha, 95 % confidence intervals (CI) 6.5-6.7 and 4.1 animals/ha, 95 %CI 4.1 - 4.1 respectively, p < 0.001). Most notably milking cows, were grazed at higher densities than other life stages (e.g. calves, heifers and bullocks) (p < 0.001) and experienced four times the number of movements between pastures. Beef cattle were more likely to be grazed across multiple (rather than single) fields (p < 0.001), with greater time spent on fragmented land away from the main/home farm (p < 0.001). None of the farm or herd attributes analysed (e.g. stocking density, frequency of movement, movement distances or land fragmentation) were associated with herd bovine tuberculosis (bTB) breakdowns during this study. However, there was a weak positive association between bTB breakdowns during the 3 years prior to the study and cattle movement distances (p = 0.05) and time spent on fragmented land (p = 0.08). After a bTB breakdown occurs, restrictions on animals moving out of these herds are implemented to control disease spread, yet we argue that more attention is needed on the role of intraherd grazing patterns in modelling disease transmission risk between herds.

Bovine Tuberculosis (Mycobacterium bovis) Outbreak Duration in Cattle Herds in Ireland: A Retrospective Observational Study

Bovine tuberculosis (bTB) outbreaks, caused by Mycobacterium bovis infection, are a costly animal health challenge. Understanding factors associated with the duration of outbreaks, known as breakdowns, could lead to better disease management policy development. We undertook a retrospective observational study (2012-2018) and employed Finite Mixture Models (FMM) to model the outcome parameter, and to investigate how factors were associated with duration for differing subpopulations identified. In addition to traditional risk factors (e.g., herd size, bTB history), we also explored farm geographic area, parcels/farm fragmentation, metrics of intensity via nitrogen loading, and whether herds were designated controlled beef finishing units (CBFU) as potential risk factors for increased duration. The final model fitted log-normal distributions, with two latent classes (k) which partitioned the population into a subpopulation around the central tendency of the distribution, and a second around the tails of the distribution. The latter subpopulation included longer breakdowns of policy interest. Increasing duration was positively associated with recent (<3 years) TB history and the number of reactors disclosed, (log) herd size, beef herd-type relative to other herd types, number of land parcels, area, being designated a CBFU ("feedlot") and having high annual inward cattle movements within the "tails" subpopulation. Breakdown length was negatively associated with the year of commencement of breakdown (i.e., a decreasing trend) and non-significantly with the organic nitrogen produced on the farm (N kg/hectare), a measure of stocking density. The latter finding may be due to confounding effects with herd size and area. Most variables contributed only moderately to explaining variation in breakdown duration, that is, they had moderate size effects on duration. Herd-size and CBFU had greater effect sizes on the outcome. The findings contribute to evidence-based policy formation in Ireland.

Environmental DNA: A promising factor for tuberculosis risk assessment in multi-host settings

Attaining and maintaining the Official Tuberculosis Free status continues to be a challenge when several domestic and wild hosts contribute to the maintenance of the Mycobacterium tuberculosis complex (MTC). Local tuberculosis hotspots are sometimes identified in cattle in low-prevalence regions. We have, therefore, studied one such hotspot in depth in order to produce an epidemiological diagnosis. Host population size and MTC prevalence were estimated in selected wildlife and in livestock, while on-cattle environmental DNA detection was additionally used as a proxy for risk of exposure at the farm (herd) level. Positive skin test reactors were found on16 of the 24 cattle farms studied in the period 2012–2016. Although all goats tested negative to the skin test during this period, MTC was confirmed in four sheep at slaughter, thus indicating an unknown prevalence of infection in this host species. With regard to wildlife, the prevalence of MTC infection based on culture was 8.8% in the case of wild boar (Sus scrofa), and the only road-killed badger (Meles meles) submitted for culture tested positive. Two criteria were employed to divide the cattle farms into higher or lower risk: tuberculosis testing results and environmental DNA detection. Environmental MTC DNA detection yielded significant differences regarding “use of regional pastures” and “proximity to woodland”. This study suggests that on-animal environmental DNA sampling may help when assessing contact risk as regards MTC in livestock at the herd level. This tool opens up new avenues of epidemiological research in complex multi-host settings.

Combining genomics and epidemiology to analyse bi-directional transmission of Mycobacterium bovis in a multi-host system

Quantifying pathogen transmission in multi-host systems is difficult, as exemplified in bovine tuberculosis (bTB) systems, but is crucial for control. The agent of bTB, Mycobacterium bovis, persists in cattle populations worldwide, often where potential wildlife reservoirs exist. However, the relative contribution of different host species to bTB persistence is generally unknown. In Britain, the role of badgers in infection persistence in cattle is highly contentious, despite decades of research and control efforts. We applied Bayesian phylogenetic and machine-learning approaches to bacterial genome data to quantify the roles of badgers and cattle in M. bovis infection dynamics in the presence of data biases. Our results suggest that transmission occurs more frequently from badgers to cattle than vice versa (10.4x in the most likely model) and that within-species transmission occurs at higher rates than between-species transmission for both. If representative, our results suggest that control operations should target both cattle and badgers.

Interspecific visitation of cattle and badgers to fomites: A transmission risk for bovine tuberculosis?

In Great Britain and Ireland, badgers (Meles meles) are a wildlife reservoir of Mycobacterium bovis and implicated in bovine tuberculosis transmission to domestic cattle. The route of disease transmission is unknown with direct, so-called "nose-to-nose," contact between hosts being extremely rare. Camera traps were deployed for 64,464 hr on 34 farms to quantify cattle and badger visitation rates in space and time at six farm locations. Badger presence never coincided with cattle presence at the same time, with badger and cattle detection at the same location but at different times being negatively correlated. Badgers were never recorded within farmyards during the present study. Badgers utilized cattle water troughs in fields, but detections were infrequent (equivalent to one badger observed drinking every 87 days). Cattle presence at badger-associated locations, for example, setts and latrines, were three times more frequent than badger presence at cattle-associated locations, for example, water troughs. Preventing cattle access to badger setts and latrines and restricting badger access to cattle water troughs may potentially reduce interspecific bTB transmission through reduced indirect contact.

Exploring the Fate of Cattle Herds With Inconclusive Reactors to the Tuberculin Skin Test

Bovine tuberculosis (TB) is an important animal health issue in many parts of the world. In England and Wales, the primary test to detect infected animals is the single intradermal comparative cervical tuberculin test, which compares immunological responses to bovine and avian tuberculins. Inconclusive test reactors (IRs) are animals that demonstrate a positive reaction to the bovine tuberculin only marginally greater than the avian reaction, so are not classified as reactors and immediately removed. In the absence of reactors in the herd, IRs are isolated, placed under movement restrictions and re-tested after 60 days. Other animals in these herds at the time of the IR result are not usually subject to movement restrictions. This could affect efforts to control TB if undetected infected cattle move out of those herds before the next TB test. To improve our understanding of the importance of IRs, this study aimed to assess whether median survival time and the hazard of a subsequent TB incident differs in herds with only IRs detected compared with negative-testing herds. Survival analysis and extended Cox regression were used, with herds entering the study on the date of the first whole herd test in 2012. An additional analysis was performed using an alternative entry date to try to remove the impact of IR retesting and is presented in the Supplementary Material. Survival analysis showed that the median survival time among IR only herds was half that observed for clear herds (2.1 years and 4.2 years respectively; p < 0.001). Extended Cox regression analysis showed that IR-only herds had 2.7 times the hazard of a subsequent incident compared with negative-testing herds in year one (hazard ratio: 2.69; 95% CI: 2.54, 2.84; p < 0.001), and that this difference in the hazard reduced by 63% per year. After 2.7 years the difference had disappeared. The supplementary analysis supported these findings showing that IR only herds still had a greater hazard of a subsequent incident after the IR re-test, but that the effect was reduced. This emphasizes the importance of careful decision making around the management of IR animals and indicates that re-testing alone may not be sufficient to reduce the risk posed by IR only herds in England and Wales.

Risk-based strategies for surveillance of tuberculosis infection in cattle for low-risk areas in England and Scotland

Disease surveillance can be made more effective by either improving disease detection, providing cost savings, or doing both. Currently, cattle herds in low-risk areas (LRAs) for bovine tuberculosis (bTB) in England are tested once every 4 years. In Scotland, the default herd testing frequency is also 4 years, but a risk-based system exempts some herds from testing altogether. To extend this approach to other areas, a bespoke understanding of at-risk herds and how risk-based surveillance can affect bTB detection is required. Here, we use a generalized linear mixed model to inform a Bayesian probabilistic model of freedom from infection and explore risk-based surveillance strategies in LRAs and Scotland. Our analyses show that in both areas the primary herd-level risk factors for bTB infection are the size of the herd and purchasing cattle from high-risk areas of Great Britain and/or Ireland. A risk-based approach can improve the current surveillance system by both increasing detection (9% and 7% fewer latent infections), and reducing testing burden (6% and 26% fewer animal tests) in LRAs and Scotland, respectively. Testing at-risk herds more frequently can also improve the level of detection by identifying more infected cases and reducing the hidden burden of the disease, and reduce surveillance effort by exempting low-risk herds from testing.

Localised Badger Culling Increases Risk of Herd Breakdown on Nearby, Not Focal, Land

Bovine tuberculosis is an important disease affecting the UK livestock industry. Controlling bovine tuberculosis (TB) is made more complex by the presence of a wildlife host, the Eurasian badger, Meles meles. Repeated large-scale badger culls implemented in the Randomised Badger Culling Trial (RBCT) were associated with decreased cattle risks inside the culling area, but also with increased cattle risks up to the 2km outside the culling area. Intermediate reductions in badger density, as achieved by localised reactive culling in the RBCT, significantly increased cattle TB. Using a matched-pairs case-control study design (n = 221 pairs of cattle herds), we investigated the spatial scale over which localised badger culling had its biggest impact. We found that reactive badger culling had a significant positive association with the risk of cattle TB at distances of 1-3km and 3-5km, and that no such association existed over shorter distances (<1km). These findings indicate that localised badger culls had significant negative effects, not on the land on which culling took place, but, perhaps more importantly, on adjoining lands and farms.

A review of risk factors for bovine tuberculosis infection in cattle in the UK and Ireland

Bovine tuberculosis (bTB) is an important disease of cattle caused by infection withMycobacterium bovis, a pathogen that may be extremely difficult to eradicate in the presence of a true wildlife reservoir. Our objective was to identify and review relevant literature and provide a succinct summary of current knowledge of risk factors for transmission of infection of cattle. Search strings were developed to identify publications from electronic databases to February 2015. Abstracts of 4255 papers identified were reviewed by three reviewers to determine whether the entire article was likely to contain relevant information. Risk factors could be broadly grouped as follows: animal (including nutrition and genetics), herd (including bTB and testing history), environment, wildlife and social factors. Many risk factors are inter-related and study designs often do not enable differentiation between cause and consequence of infection. Despite differences in study design and location, some risk factors are consistently identified, e.g. herd size, bTB history, presence of infected wildlife, whereas the evidence for others is less consistent and coherent, e.g. nutrition, local cattle movements. We have identified knowledge gaps where further research may result in an improved understanding of bTB transmission dynamics. The application of targeted, multifactorial disease control regimens that address a range of risk factors simultaneously is likely to be a key to effective, evidence-informed control strategies.

Herd-level bovine tuberculosis risk factors: assessing the role of low-level badger population disturbance

Bovine TB (bTB) is endemic in Irish cattle and has eluded eradication despite considerable expenditure, amid debate over the relative roles of badgers and cattle in disease transmission. Using a comprehensive dataset from Northern Ireland (>10,000 km²; 29,513 cattle herds), we investigated interactions between host populations in one of the first large-scale risk factor analyses for new herd breakdowns to combine data on both species. Cattle risk factors (movements, international imports, bTB history, neighbours with bTB) were more strongly associated with herd risk than area-level measures of badger social group density, habitat suitability or persecution (sett disturbance). Highest risks were in areas of high badger social group density and high rates of persecution, potentially representing both responsive persecution of badgers in high cattle risk areas and effects of persecution on cattle bTB risk through badger social group disruption. Average badger persecution was associated with reduced cattle bTB risk (compared with high persecution areas), so persecution may contribute towards sustaining bTB hotspots; findings with important implications for existing and planned disease control programmes.

Modeling tuberculosis dynamics, detection and control in cattle herds

Epidemiological models are key tools for designing and evaluating detection and control strategies against animal infectious diseases. In France, after decades of decrease of bovine tuberculosis (bTB) incidence, the disease keeps circulating. Increasing prevalence levels are observed in several areas, where the detection and control strategy could be adapted. The objective of this work was to design and calibrate a model of the within-herd transmission of bTB. The proposed model is a stochastic model operating in discrete-time. Three health states were distinguished: susceptible, latent and infected. Dairy and beef herd dynamics and bTB detection and control programs were explicitly represented. Approximate Bayesian computation was used to estimate three model parameters from field data: the transmission parameter when animals are inside (β_inside) and outside (β_outside) buildings, and the duration of the latent phase. An independent dataset was used for model validation. The estimated median was 0.43 [0.16–0.84] month⁻¹ for β_inside and 0.08 [0.01–0.32] month⁻¹ for β_outside. The median duration of the latent period was estimated 3.5 [2]–[8] months. The sensitivity analysis showed only minor influences of fixed parameter values on these posterior estimates. Validation based on an independent dataset showed that in more than 80% of herds, the observed proportion of animals with detected lesions was between the 2.5% and 97.5% percentiles of the simulated distribution. In the absence of control program and once bTB has become enzootic within a herd, the median effective reproductive ratio was estimated to be 2.2 in beef herds and 1.7 in dairy herds. These low estimates are consistent with field observations of a low prevalence level in French bTB-infected herds.

Spatial-temporal Variations of Bovine Tuberculosis Incidence in France between 1965 and 2000

We analysed the spatiotemporal variations of bovine tuberculosis (bTB) incidence between 1965 and 2000 in France at the department level (95 areas). Using a Bayesian space-time model, we studied the association between the evolution of bTB incidence and changes of cattle population structure and of herd management practices. Several spatiotemporal hierarchical Bayesian models were compared, and the deviance information criterion was used to select the best of them. Southern France remained a high-risk area over the analysed period, whereas central and western regions were low-risk areas. Besides the frequency of tuberculin skin testing (fixed according to bTB incidence in the preceding years), four factors were associated with an increased risk of bTB: the average herd density and size, the percentage of dairy cows in the cattle population, and the percentage of permanent grassland in cultivated surfaces area. These four factors are linked to the progressive professionalization and specialization of cattle farming, with the disappearance of family farms and of the intensification of breeding systems (especially in dairy farms after the application of the milk quota system in the 1980s). Both trends probably played a significant role in reducing the risk of bTB in France between 1965 and 2000, besides mandatory detection and control procedures.

Estimating epidemiological parameters for bovine tuberculosis in British cattle using a Bayesian partial-likelihood approach

Fitting models with Bayesian likelihood-based parameter inference is becoming increasingly important in infectious disease epidemiology. Detailed datasets present the opportunity to identify subsets of these data that capture important characteristics of the underlying epidemiology. One such dataset describes the epidemic of bovine tuberculosis (bTB) in British cattle, which is also an important exemplar of a disease with a wildlife reservoir (the Eurasian badger). Here, we evaluate a set of nested dynamic models of bTB transmission, including individual- and herd-level transmission heterogeneity and assuming minimal prior knowledge of the transmission and diagnostic test parameters. We performed a likelihood-based bootstrapping operation on the model to infer parameters based only on the recorded numbers of cattle testing positive for bTB at the start of each herd outbreak considering high- and low-risk areas separately. Models without herd heterogeneity are preferred in both areas though there is some evidence for super-spreading cattle. Similar to previous studies, we found low test sensitivities and high within-herd basic reproduction numbers (R₀), suggesting that there may be many unobserved infections in cattle, even though the current testing regime is sufficient to control within-herd epidemics in most cases. Compared with other, more data-heavy approaches, the summary data used in our approach are easily collected, making our approach attractive for other systems.

Spatial relationships between Eurasian badgers (Meles meles) and cattle infected with Mycobacterium bovis in Northern Spain

Recent studies suggest that badgers may be a potential reservoir of Mycobacterium bovis infection for cattle in Northern Spain. The objective of this study was to investigate potential epidemiological links between cattle and badgers. Culture and molecular typing data were available for cattle culled during the national tuberculosis (TB) eradication campaigns between 2008 and 2012, as well as from 171 necropsied badgers and 60 live animals trapped and examined over the same time period. Mycobacterium tuberculosis complex strains were isolated from pooled tissues of 14 (8.2%) necropsied badgers, of which 11 were identified as M. bovis: six different spoligotypes of M. bovis were subsequently identified. In two geographical locations where these isolates were shared between cattle and badgers, infected cattle herds and badgers lived in close contact. Although it remains unclear if badgers are a maintenance or spill-over host of M. bovis in this setting, it would appear prudent to have precautionary measures in place to reduce contact between cattle and badgers.

The effect of badger culling on breakdown prolongation and recurrence of bovine tuberculosis in cattle herds in Great Britain

Bovine tuberculosis is endemic in cattle herds in Great Britain, with a substantial economic impact. A reservoir of Mycobacterium bovis within the Eurasian badger (Meles meles) population is thought to have hindered disease control. Cattle herd incidents, termed breakdowns, that are either ‘prolonged’ (lasting ≥240 days) or ‘recurrent’ (with another breakdown within a specified time period) may be important foci for onward spread of infection. They drain veterinary resources and can be demoralising for farmers. Randomised Badger Culling Trial (RBCT) data were re-analysed to examine the effects of two culling strategies on breakdown prolongation and recurrence, during and after culling, using a Bayesian hierarchical model. Separate effect estimates were obtained for the ‘core’ trial areas (where culling occurred) and the ‘buffer’ zones (up to 2 km outside of the core areas). For breakdowns that started during the culling period, ‘reactive’ (localised) culling was associated with marginally increased odds of prolongation, with an odds ratio (OR) of 1.7 (95% credible interval [CI] 1.1–2.4) within the core areas. This effect was not present after the culling ceased. There was no notable effect of ‘proactive’ culling on prolongation. In contrast, reactive culling had no effect on breakdown recurrence, though there was evidence of a reduced risk of recurrence in proactive core areas during the culling period (ORs and 95% CIs: 0.82 (0.64–1.0) and 0.69 (0.54–0.86) for 24- and 36-month recurrence respectively). Again these effects were not present after the culling ceased. There seemed to be no effect of culling on breakdown prolongation or recurrence in the buffer zones. These results suggest that the RBCT badger culling strategies are unlikely to reduce either the prolongation or recurrence of breakdowns in the long term, and that reactive strategies (such as employed during the RBCT) are, if anything, likely to impact detrimentally on breakdown persistence.

Estimating risk over time using data from targeted surveillance systems: application to bovine tuberculosis in Great Britain

For infections that are typically asymptomatic, targeted surveillance systems (whereby individuals at increased risk are tested more frequently) will detect infections earlier on average than systems with random testing or in systems where all individuals are tested at the same intervals. However, estimating temporal trends in infection risk using data from such targeted surveillance systems can be challenging. This is similarly a problem for targeted surveillance to detect faults of individual industrial components. The incidence of bovine tuberculosis (TB) in British cattle has been generally increasing in the last thirty years. Cattle herds are routinely tested for evidence of exposure to the aetiological bacteria Mycobacterium bovis, in a targeted surveillance programme in which the testing interval is determined by past local TB incidence and local veterinary discretion. The UK Department for Environment, Food and Rural Affairs (Defra) report the monthly percentage of tests on officially TB-free (OTF) herds resulting in a confirmed positive test for M. bovis (i.e. the percentage of tested herds with OTF status withdrawn), which contains substantial fluctuations (three years apart) within the increasing trend. As the number of herds tested changes over time, this cyclic trend is difficult to interpret. Here we evaluate an alternative to the Defra method in which we distribute each incident event across the period at risk to infer the underlying trends in infection incidence using a stochastic model of cattle herd incidence and testing frequencies fitted to data on the monthly number of herds tested and number of these with OTF status withdrawn in 2003-2010. We show that for an increasing underlying incidence trend, the current Defra approach can produce artefactual fluctuations whereas the alternative method described provides more accurate descriptions of the underlying risks over time.

Factors affecting European badger (Meles meles) capture numbers in one county in Ireland

Understanding factors affecting the number of badgers captured at and around badger setts (burrows) is of considerable applied importance. These factors could be used to estimate probable badger densities for bovine tuberculosis (bTB) control and also for monitoring badger populations from a conservation perspective. Furthermore, badger management and vaccination programs would benefit by increasing the probability of efficiently capturing the target badger populations. Within this context, it was investigated whether badger capture numbers can be estimated from field signs and previous capture histories. Badger capture records (initial and repeated capture numbers at a sett) from a large-scale removal program (405 km(2), 643 setts) were used. Univariable count models indicated that there were a number of significant potential predictors of badger numbers, during initial capture attempts. Using a multivariable zero-inflated Poisson (ZIP) model of initial captures we found that badger capture numbers were significantly affected by sett type, season, year, and the number of sett entrances in active use. Badger capture numbers were also affected by the total previous catch during repeated capture events and by the number of previous capture attempts. There was a significant negative trend in badger captures across events. Measures of the ability of these models to estimate badger captures suggested that the models might be useful in estimating badger numbers across a population; however the confidence intervals associated with these predictions were large.

Herd-level risk factors for bovine tuberculosis: a literature review

Bovine tuberculosis (TB), caused by Mycobacterium bovis, is one of the most challenging endemic diseases currently facing government, the veterinary profession, and the farming industry in the United Kingdom and Ireland and in several other countries. The disease has a notoriously complex epidemiology; the scientific evidence supports both cattle-cattle and wildlife-cattle transmission routes. To produce more effective ways of reducing such transmission, it is important to understand those risk factors which influence the presence or absence of bovine TB in cattle herds. Here we review the literature on herd-level risk factor studies. Whilst risk factors operate at different scales and may vary across regions, epidemiological studies have identified a number of risk factors associated with bovine TB herd breakdowns, including the purchase of cattle, the occurrence of bovine TB in contiguous herds, and/or the surrounding area as well as herd size. Other factors identified in some studies include farm and herd management practices, such as, the spreading of slurry, the use of certain housing types, farms having multiple premises, and the use of silage clamps. In general, the most consistently identified risk factors are biologically plausible and consistent with known transmission routes involving cattle-cattle and wildlife-cattle pathways.

Risk factors for bovine Tuberculosis at the national level in Great Britain

Background

The continuing expansion of high incidence areas of bovine Tuberculosis (bTB) in Great Britain (GB) raises a number of questions concerning the determinants of infection at the herd level that are driving spread of the disease. Here, we develop risk factor models to quantify the importance of herd sizes, cattle imports from Ireland, history of bTB, badgers and cattle restocking in determining bTB incidence. We compare the significance of these different risk factors in high and low incidence areas (as determined by parish testing intervals).

Results

Large herds and fattening herds are more likely to breakdown in all areas. In areas with lower perceived risk (longer testing intervals), the risk of breaking down is largely determined by the number of animals that a herd buys in from high incidence areas. In contrast, in higher perceived risk areas (shorter testing intervals), the risk of breakdown is defined by the history of disease and the probability of badger occurrence. Despite differences in the management of bTB across different countries of GB (England, Wales and Scotland), we found no significant differences in bTB risk at the national level after these other factors had been taken into account.

Conclusions

This paper demonstrates that different types of farm are at risk of breakdown and that the most important risk factors vary according to bTB incidence in an area. The results suggest that significant gains in bTB control could be made by targeting herds in low incidence areas that import the greatest number of cattle from high incidence areas.

Geographical association between the genotype of bovine tuberculosis in found dead badgers and in cattle herds

In a survey, 457 badgers that had been found dead in Wales were postmortem-examined, and samples were examined by histology and by extended culture (for up to 12 weeks). Mycobacterium bovis was cultured from 55 badgers (12.0 per cent), and the histology typical of M bovis infection was seen in a further six (1.3 per cent). The prevalence in badgers in each of 10 geographical areas varied between 0 and 26 per cent (P<0.001), and was associated with the incidence of confirmed M bovis infection in cattle herds in the same areas (P<0.01). In northern Wales, bTB was rare in both hosts. An infected badger was 12.3 times more likely to be within 5 km of a confirmed cattle bTB breakdown than an uninfected badger. The M bovis isolates from badgers belonged to one of four genotypes defined by spoligotype and variable number tandem repeat type. These genotypes were also found in 290 concurrent confirmed herd breakdowns, and tended to be similar to the genotypes in badgers in the same geographical areas. When badgers and cattle no more than 30 km apart were compared, the genotype diversity was greater in cattle than in badgers (P=0.016), suggesting that the movement of cattle plays a greater part in the spatial distribution of M bovis than the movement of badgers.

Modelled impacts of badger culling on cattle TB in a real area with geographic boundaries

As a part of bovine tuberculosis eradication strategy, the Welsh Government has proposed a badger cull in a defined area in and around North Pembrokeshire, and has published information on herd densities and incidence levels within and close to the area. In the present study, three sets of previously published data relating the impact of badger culling inside and around previous culling areas to distances from culling area boundaries have been used to model possible impacts of the proposed cull, taking account of three possible scenarios in which geographic boundaries reduce, to varying extents, adverse effects caused by increased badger movements. For the scenarios considered, the results predict average changes in confirmed herd incidences (CHIs) in the range -15.7 (-29.1 to 1.6 per cent) to -25.3 per cent (-52.2 to 46.1 per cent) over a period of 10 years, comprising average changes in the culling area in the range -26.1 (-34.8 to -14.8 per cent) to -32.6 per cent (-59.6 to 47.6 per cent), and average changes on adjoining land in the range 4.5 (-21.8 to 39.8 per cent) to 7.8 per cent (-16.1 to 38.5 per cent). The overall impacts equate to average reductions in the number of CHIs of between 122 (37 to 187) and 158 (-254 to 304).

Localized reactive badger culling increases risk of bovine tuberculosis in nearby cattle herds

Human and livestock diseases can be difficult to control where infection persists in wildlife populations. Control of bovine tuberculosis (bTB) in British cattle is complicated by the maintenance of Mycobacterium bovis (the causative agent of bTB) in badgers, acting as reservoirs of infection. Although over 20 000 badgers were culled to control bTB between 1975 and 1997, the incidence of bTB in cattle has substantially increased in parts of Great Britain in recent decades. Our case-control study, involving 1208 cattle herds, provides further evidence of the detrimental effect of localized reactive badger culling in response to the disclosure of a confirmed bTB herd breakdown in cattle. The presence of any reactive badger culling activity and increased numbers of badgers culled in the vicinity of a herd were associated with significantly increased bTB risk, even after adjusting for other important local risk factors. Such findings may partly explain why some earlier localized approaches to bTB control were ineffective.