Model of Selective and Non-Selective Management of Badgers (Meles meles) to Control Bovine Tuberculosis in Badgers and Cattle

Mathematical modeling at the livestock-wildlife interface: scoping review of drivers of disease transmission between species

Modeling of infectious diseases at the livestock-wildlife interface is a unique subset of mathematical modeling with many innate challenges. To ascertain the characteristics of the models used in these scenarios, a scoping review of the scientific literature was conducted. Fifty-six studies qualified for inclusion. Only 14 diseases at this interface have benefited from the utility of mathematical modeling, despite a far greater number of shared diseases. The most represented species combinations were cattle and badgers (for bovine tuberculosis, 14), and pigs and wild boar [for African (8) and classical (3) swine fever, and foot-and-mouth and disease (1)]. Assessing control strategies was the overwhelming primary research objective (27), with most studies examining control strategies applied to wildlife hosts and the effect on domestic hosts (10) or both wild and domestic hosts (5). In spatially-explicit models, while livestock species can often be represented through explicit and identifiable location data (such as farm, herd, or pasture locations), wildlife locations are often inferred using habitat suitability as a proxy. Though there are innate assumptions that may not be fully accurate when using habitat suitability to represent wildlife presence, especially for wildlife the parsimony principle plays a large role in modeling diseases at this interface, where parameters are difficult to document or require a high level of data for inference. Explaining observed transmission dynamics was another common model objective, though the relative contribution of involved species to epizootic propagation was only ascertained in a few models. More direct evidence of disease spill-over, as can be obtained through genomic approaches based on pathogen sequences, could be a useful complement to further inform such modeling. As computational and programmatic capabilities advance, the resolution of the models and data used in these models will likely be able to increase as well, with a potential goal being the linking of modern complex ecological models with the depth of dynamics responsible for pathogen transmission. Controlling diseases at this interface is a critical step toward improving both livestock and wildlife health, and mechanistic models are becoming increasingly used to explore the strategies needed to confront these diseases.

Analysis of a multi-type resurgence of Mycobacterium bovis in cattle and badgers in Southwest France, 2007-2019

Although control measures to tackle bovine tuberculosis (bTB) in cattle have been successful in many parts of Europe, this disease has not been eradicated in areas where Mycobacterium bovis circulates in multi-host systems. Here we analyzed the resurgence of 11 M. bovis genotypes (defined based on spoligotyping and MIRU-VNTR) detected in 141 farms between 2007 and 2019, in an area of Southwestern France where wildlife infection was also detected from 2012 in 65 badgers. We used a spatially-explicit model to reconstruct the simultaneous diffusion of the 11 genotypes in cattle farms and badger populations. Effective reproduction number R was estimated to be 1.34 in 2007-2011 indicating a self-sustained M. bovis transmission by a maintenance community although within-species Rs were both < 1, indicating that neither cattle nor badger populations acted as separate reservoir hosts. From 2012, control measures were implemented, and we observed a decrease of R below 1. Spatial contrasts of the basic reproduction ratio suggested that local field conditions may favor (or penalize) local spread of bTB upon introduction into a new farm. Calculation of generation time distributions showed that the spread of M. bovis has been more rapid from cattle farms (0.5-0.7 year) than from badger groups (1.3-2.4 years). Although eradication of bTB appears possible in the study area (since R < 1), the model suggests it is a long-term prospect, because of the prolonged persistence of infection in badger groups (2.9-5.7 years). Supplementary tools and efforts to better control bTB infection in badgers (including vaccination for instance) appear necessary.

Impact of test, vaccinate or remove protocol on home ranges and nightly movements of badgers a medium density population

In the British Isles, the European badger (Meles meles) is thought to be the primary wildlife reservoir of bovine tuberculosis (bTB), an endemic disease in cattle. Test, vaccinate or remove ('TVR') of bTB test-positive badgers, has been suggested to be a potentially useful protocol to reduce bTB incidence in cattle. However, the practice of removing or culling badgers is controversial both for ethical reasons and because there is no consistent observed effect on bTB levels in cattle. While removing badgers reduces population density, it may also result in disruption of their social behaviour, increase their ranging, and lead to greater intra- and inter-species bTB transmission. This effect has been recorded in high badger density areas, such as in southwest England. However, little is known about how TVR affects the behaviour and movement of badgers within a medium density population, such as those that occur in Northern Ireland (NI), which the current study aimed to examine. During 2014-2017, badger ranging behaviours were examined prior to and during a TVR protocol in NI. Nightly distances travelled by 38 individuals were determined using Global Positioning System (GPS) measurements of animal tracks and GPS-enhanced dead-reckoned tracks. The latter was calculated using GPS, tri-axial accelerometer and tri-axial magnetometer data loggers attached to animals. Home range and core home range size were measured using 95% and 50% autocorrelated kernel density estimates, respectively, based on location fixes. TVR was not associated with measured increases in either distances travelled per night (mean = 3.31 ± 2.64 km) or home range size (95% mean = 1.56 ± 0.62 km², 50% mean = 0.39 ± 0.62 km²) over the four years of study. However, following trapping, mean distances travelled per night increased by up to 44% eight days post capture. Findings differ from those observed in higher density badger populations in England, in which badger ranging increased following culling. Whilst we did not assess behaviours of individual badgers, possible reasons why no differences in home range size were observed include higher inherent 'social fluidity' in Irish populations whereby movements are less restricted by habitat saturation and/or that the numbers removed did not reach a threshold that might induce increases in ranging behaviour. Nevertheless, short-term behavioural disruption from trapping was observed, which led to significant increases in the movements of individual animals within their home range. Whether or not TVR may alter badger behaviours remains to be seen, but it would be better to utilise solutions such as oral vaccination of badgers and/or cattle as well as increased biosecurity to limit bTB transmission, which may be less likely to cause interference and thereby reduce the likelihood of bTB transmission.

Effect of selective removal of badgers (Meles meles) on ranging behaviour during a 'Test and Vaccinate or Remove' intervention in Northern Ireland

The role of the Eurasian badger (Meles meles) as a wildlife host has complicated the management of bovine tuberculosis (bTB) in cattle. Badger ranging behaviour has previously been found to be altered by culling of badgers and has been suggested to increase the transmission of bTB either among badgers or between badgers and cattle. In 2014, a five-year bTB intervention research project in a 100 km² area in Northern Ireland was initiated involving selective removal of dual path platform (DPP) VetTB (immunoassay) test positive badgers and vaccination followed by release of DPP test negative badgers (‘Test and Vaccinate or Remove’). Home range sizes, based on position data obtained from global positioning system collared badgers, were compared between the first year of the project, where no DPP test positive badgers were removed, and follow-up years 2–4 when DPP test positive badgers were removed. A total of 105 individual badgers were followed over 21 200 collar tracking nights. Using multivariable analyses, neither annual nor monthly home ranges differed significantly in size between years, suggesting they were not significantly altered by the bTB intervention that was applied in the study area.

Test and vaccinate or remove: Methodology and preliminary results from a badger intervention research project

BACKGROUND

In the British Isles, it is generally accepted that the Eurasian badger (Meles meles) plays a role in the maintenance of bovine tuberculosis (bTB) in cattle. Non-selective culling is the main intervention method deployed in controlling bTB in badgers along with smaller scale Bacillus Calmette-Guérin (BCG) vaccination areas. This paper describes the use of selective badger culling combined with vaccination in a research intervention trial.

METHODS

In Northern Ireland, a 100 km² area was subjected to a test and vaccinate or remove (TVR) badger intervention over a 5-year period. Badgers were individually identified and tested on an annual basis. Physical characteristics and clinical samples were obtained from each unique badger capture event.

RESULTS

A total of 824 badgers were trapped with 1520 capture/sampling events. There were no cage-related injuries to the majority of badgers (97%). A low level of badger removal was required (4.1%-16.4% annually), while 1412 BCG vaccinations were administered. A statistically significant downward trend in the proportion of test positive badgers was observed.

CONCLUSION

This is the first project to clearly demonstrate the feasibility of cage side testing of badgers. The results provide valuable data on the logistics and resources required to undertake a TVR approach to control Mycobacterium bovis in badgers.

Simulating the next steps in badger control for bovine tuberculosis in England

Industry-led culling of badgers has occurred in England to reduce the incidence of bovine tuberculosis in cattle for a number of years. Badger vaccination is also possible, and a move away from culling was "highly desirable" in a recent report to the UK government. Here we used an established simulation model to examine badger control option in a post-cull environment in England. These options included no control, various intermittent culling, badger vaccination and use of a vaccine combined with fertility control. The initial simulated cull led to a dramatic reduction in the number of infected badgers present, which increased slowly if there was no further badger management. All three approaches led to a further reduction in the number of infected badgers, with little to choose between the strategies. We do note that of the management strategies only vaccination on its own leads to a recovery of the badger population, but also an increase in the number of badgers that need to be vaccinated. We conclude that vaccination post-cull, appears to be particularly effective, compared to vaccination when the host population is at carrying capacity.

A Bayesian analysis of a Test and Vaccinate or Remove study to control bovine tuberculosis in badgers (Meles meles)

A novel five year Test and Vaccinate or Remove (TVR) wildlife research intervention project in badgers (Meles meles) commenced in 2014 in a 100km2 area of Northern Ireland. It aimed to increase the evidence base around badgers and bovine TB and help create well-informed and evidence-based strategies to address the issue of cattle-to-cattle spread and spread between cattle and badgers. It involved real-time trap-side testing of captured badgers and vaccinating those that tested negative for bTB (BadgerBCG-BCG Danish 1331) and removal of those that tested bTB positive using the Dual-Path Platform VetTB test (DPP) for cervids (Chembio Diagnostic Systems, Medford, NY USA). Four diagnostic tests were utilised within the study interferon gamma release assay (IGRA), culture (clinical samples and post mortem), DPP using both whole blood and DPP using serum. BCG Sofia (SL222) was used in the final two years because of supply issues with BadgerBCG. Objectives for this study were to evaluate the performance of the DPP in field conditions and whether any trend was apparent in infection prevalence over the study period. A Bayesian latent class model of diagnostic test evaluation in the absence of a gold standard was applied to the data. Temporal variation in the sensitivity of DPP and interferon gamma release assay (IGRA) due to the impact of control measures was investigated using logistic regression and individual variability was assessed. Bayesian latent class analysis estimated DPP with serum to have a sensitivity of 0.58 (95% CrI: 0.40-0.76) and specificity of 0.97 (95% CrI: 0.95-0.98). The DPP with whole blood showed a higher sensitivity (0.69 (95% CrI: 0.48-0.88)) but similar specificity (0.98 (95% Crl: 0.96-0.99)). The change from BCG Danish to BCG Sofia significantly impacted on DPP serum test characteristics. In addition, there was weak evidence of increasing sensitivity of IGRA over time and differences in DPP test sensitivity between adults and cubs. An exponential decline model was an appropriate representation of the infection prevalence over the 5 years, with a starting prevalence of 14% (95% CrI: 0.10-0.20), and an annual reduction of 39.1% (95% CrI: 26.5-50.9). The resulting estimate of infection prevalence in year 5 of the study was 1.9% (95% CrI: 0.8-3.8). These results provide field evidence of a statistically significant reduction in badger TB prevalence supporting a TVR approach to badger intervention. They give confidence in the reliability and reproducibility in the DPP Whole Blood as a real time trap-side diagnostic test for badgers, and describe the effect of vaccination and reduced infection prevalence on test characteristics.

When to kill a cull: factors affecting the success of culling wildlife for disease control

Culling wildlife to control disease can lead to both decreases and increases in disease levels, with apparently conflicting responses observed, even for the same wildlife-disease system. There is therefore a pressing need to understand how culling design and implementation influence culling's potential to achieve disease control. We address this gap in understanding using a spatial metapopulation model representing wildlife living in distinct groups with density-dependent dispersal and framed on the badger-bovine tuberculosis (bTB) system. We show that if population reduction is too low, or too few groups are targeted, a 'perturbation effect' is observed, whereby culling leads to increased movement and disease spread. We also demonstrate the importance of culling across appropriate time scales, with otherwise successful control strategies leading to increased disease if they are not implemented for long enough. These results potentially explain a number of observations of the dynamics of both successful and unsuccessful attempts to control TB in badgers including the Randomized Badger Culling Trial in the UK, and we highlight their policy implications. Additionally, for parametrizations reflecting a broad range of wildlife-disease systems, we characterize 'Goldilocks zones', where, for a restricted combination of culling intensity, coverage and duration, the disease can be reduced without driving hosts to extinction.

Evaluation of the Dual Path Platform (DPP) VetTB assay for the detection of Mycobacterium bovis infection in badgers

Bovine tuberculosis (bTB), caused by Mycobacterium bovis, represents a major animal health issue. In the United Kingdom and the Republic of Ireland, European badgers (Meles meles) have been shown to act as a reservoir of M. bovis infection, hindering the eradication of bTB in livestock. The availability of suitable diagnostic assays, particularly those that may be applied in a "trap-side" setting, would facilitate the implementation of a wider range of disease control strategies. Here we evaluate the Dual Path Platform (DPP) VetTB assay, a lateral-flow type test for detecting antibodies to M. bovis antigens (MPB83 and ESAT-6/CFP-10). Both serum and whole blood were evaluated as diagnostic samples. Additionally, two methods were evaluated for interpretation of test results (qualitative interpretation by eye and quantitative measurement using an optical reader). The antibody response to MPB83 detected by the DPP VetTB assay increased significantly following experimental M. bovis infection of badgers, whilst the response to ESAT-6/CFP-10 showed no significant change. In sera from TB-free captive and naturally M. bovis infected wild badgers the MPB83 response exhibited a sensitivity of 55 % by eye and quantitative reader (95 % CI: 40-71 and 38-71, respectively), with slightly lower specificity when read by eye (93 % compared to 98 %; 95 % CI: 85-100 and 90-100, respectively). In whole blood, the DPP VetTB assay MPB83 response exhibited a sensitivity of 65 % (95 % CI: 50-80) when interpreted by eye and 53 % (95 % CI: 36-69) using quantitative values, whilst the specificity was 94 % and 98 % respectively (95 % CI: 88-100 and 90-100). Comparison with contemporaneous diagnostic test results from putatively naturally infected and TB-free badgers demonstrated varying levels of agreement. Using sera from naturally M. bovis infected and TB-free badgers, with post mortem confirmation of disease status, the DPP VetTB assay exhibited a sensitivity of 60 % (95 % CI: 41-77) when interpreted using quantitative values (specificity 95 %; 95 % CI: 76-100), and 67 % (95 % CI: 50-84) when read by eye (specificity 95 %; 95 % CI: 86-100). Further work is required to robustly characterize the DPP VetTB assay's performance in a wider selection of samples, and in the practical and epidemiological contexts in which it may be applied.

Is moving from targeted culling to BCG-vaccination of badgers (Meles meles) associated with an unacceptable increased incidence of cattle herd tuberculosis in the Republic of Ireland? A practical non-inferiority wildlife intervention study in the Republic of Ireland (2011-2017)

Bovine tuberculosis (BTB) remains as a costly disease of cattle-herds in the Republic of Ireland (ROI). This persistence is partially attributable to the presence of M. bovis infection in a wildlife reservoir, the European badger (Meles meles). Thus, both area-wide and limited-area targeted-badger-culling have been part of the ROI-BTB control/eradication program to help reduce the future incidence of a cattle-herd BTB breakdown (i.e. a "new herd-level occurrence of BTB"). However, neither badger-culling practice can be sustained as a major component in the ongoing BTB eradication program in the ROI. Vaccination of badgers with Bacille Calmette-Guérin (BCG) has been proposed as an alternative to badger culling. Thus, in 2011, a five-year non-inferiority study was implemented in seven counties in the ROI. This study was designed to compare and contrast the cattle-herd-BTB-incidence in areas where intramuscular badger vaccination would be implemented versus the cattle-herd-BTB-incidence in the remaining area of the same county where targeted-badger-culling was maintained as the standard treatment response to probable badger-sourced BTB breakdowns. Our outcome of interest was a new cattle-herd-BTB-episode (breakdown) with a total of >2 standard skin-test (SICTT) reactors detected during the episode. Treatments (badger vaccination or targeted badger culling) were cluster allocated based on where the majority of the herd owner's land was located. To assess the impact of the two treatments, we compared the incidence-risk, of our defined outcome, for cattle herds in the area under vaccination to the outcome incidence-risk for cattle herds in the remainder of the same county after 4 and 5 years of having implemented badger vaccination. A random-effects logit model with adjustment for clustering by treatment, and statistical control of herd-type, herd-size and five-year prior-BTB-episode history was used for our analyses. Although not included in the logistic model, a relative badger density metric based on the annual number of badgers captured-per-sett-night of capturing effort was developed for each treatment area; this metric indicated that relative badger density was approximately 40 % higher in vaccination areas than in the targeted badger-culling areas during our study. Overall, our study results indicated that vaccination was not inferior to targeted badger-culling in four counties and badger vaccination was deemed to produce ambivalent results in one (County Cork North) of the seven study sites in the ROI. A post-study investigation, in County Galway, where vaccination was deemed inferior to target culling, revealed that widespread purchases of cattle from a nearby cattle mart, by herd owners in the vaccination-area, was associated with the increased herd and vaccination-area risk of BTB. No single "biasing hypothesis" was evident for the apparent vaccine inferiority in the second study site (County Monaghan) where vaccination was deemed inferior to targeted culling; hence no further investigations were conducted.

The risk of foot-and-mouth disease becoming endemic in a wildlife host is driven by spatial extent rather than density

In the past 20 years, free living populations of feral wild boar have re-established in several locations across the UK. One of the largest populations is in the Forest of Dean where numbers have been steadily increasing since monitoring began in 2008, with estimates from 2016 reporting a population of more than 1500. Feral wild boar have significant ecological and environmental impacts and may present a serious epidemiological risk to neighbouring livestock as they are a vector for a number of important livestock diseases. This includes foot-and-mouth disease (FMD) which is currently absent from the UK. We developed an individual-based spatially explicit modelling approach to simulate feral wild boar populations in the Forest of Dean (England, UK) and use it to explore whether current or future populations might be sufficient to produce long-lived outbreaks of FMD in this potential wildlife reservoir. Our findings suggest that if you exclude the spread from feral wild boar to other susceptible species, the current population of boar is insufficient to maintain FMD, with 95% of unmanaged simulations indicating disease burn-out within a year (not involving boar management specifically for disease). However, if boar are allowed to spread beyond their current range into the adjacent landscape, they might maintain a self-sustaining reservoir of infection for the disease.

Modeling as a Decision Support Tool for Bovine TB Control Programs in Wildlife

Computer modeling has a long history of association with epidemiology, and has improved our understanding of the theory of disease dynamics and provided insights into wildlife disease management. A summary of badger bovine TB models and their role in decision making is presented, from a simple initial SEI model, to SEIR (inclusion of a recovered category) and SEI₁I₂ (inclusion of two stages of disease progression) variants, and subsequent spatially-explicit individual-based models used to assess historical badger management strategies. The integration of cattle into TB models allowed comparison of the predicted impacts of different badger management strategies on cattle herd breakdown rates, and provided an economic dimension to the outputs. Estimates of R₀ for bovine TB in cattle and badgers are little higher than unity implying that the disease should be relatively easy to control, which is at odds with practical experience. A cohort of recent models have suggested that combined strategies, involving management of both host species and including vaccination may be most effective. Future models of badger vaccination will need to accommodate the partial protection from infection and likely duration of immunity conferred by the currently available vaccine (BCG). Descriptions of how models could better represent the ecological and epidemiological complexities of the badger-cattle TB system are presented, along with a wider discussion of the utility of modeling for bovine TB management interventions. This includes consideration of the information required to maximize the utility of the next generation of models.