Recent spatial changes in bovine tuberculosis in the Republic of Ireland

Engaging With Farmers to Explore Correlates of Bovine Tuberculosis Risk in an Internationally Important Heritage Landscape: The Burren, in the West of Ireland

Bovith recene tuberculosis (bTB) continues to be a pathogen of concern in several countries globally. Analysis of areas that have higher incidences of bTB outbreaks has demonstrated how risk is not equally distributed, and local data collection, analysis and participatory engagement is required to develop tailored approaches. The Burren, an internationally important heritage landscape, has been an area of higher bTB incidence for many years in Ireland, and owing to its unique geology and farming heritage a survey was developed to engage with local farmers to gain greater insight into farming practices and bTB control to inform tailored approaches. The survey gathered data on the farm and animal management approaches being used within the Burren, including local farming techniques like the use of “winterage” (grazing exposed limestone dominated uplands). Thematic analysis of free text responses was undertaken. Quantitative data were then explored using statistical models to assess associations with recent (<3 years) self-reported bTB breakdown risk. There was a high number of responses demonstrating a high degree of willingness to engage on the issue. Thematic analysis suggested that wildlife and its management (culling and vaccination), testing quality, and its impact on the bTB scheme, and pessimism around eradication were important themes. Statistical analysis suggested that increasing bTB risk was primarily related to increasing herd-size and the percentage of herd owner's land inaccessible to those attempting to locate badger setts. There was less evidence for associations relating to the amount of time, or which season (i.e., summer), farmers utilized “winterage”. The results of the study will feed back directly to local bTB management plans and further stakeholder engagement and is an exemplar for local tailoring of national control measures in situations of high incidences of bTB outbreaks in particular areas.

Spatio-temporal models of bovine tuberculosis in the Irish cattle population, 2012-2019

INTRODUCTION

Bovine tuberculosis (bTB) is an important zoonotic disease which has serious and sometimes fatal effects on both human and non-human animals. In many countries it is endemic in the cattle population and has a considerable economic impact through losses in productivity and impacts on trade. The incidence rate in Ireland varies by herd and location and it is hoped that statistical disease-mapping models accounting for both spatio-temporal correlation and covariates might contribute towards explaining this variation.

METHODS

Ireland was divided into equally sized hexagons for computational efficiency (n = 997). Different spatio-temporal random-effects models (e.g. negative binomial Besag-York-Mollié) were explored, using comprehensive data from the national bTB eradication programme to examine the association between covariates and the number of bTB cattle. Leveraging a Bayesian framework, model parameter estimates were obtained using the integrated nested Laplace approximation (INLA) approach. Exceedance probabilities were calculated to identify spatial clusters of cases.

RESULTS

Models accounting for spatial correlation significantly improved model fit in comparison to non-spatial versions where independence between regions was assumed. In our final model at hexagon level, the number of cattle (IR = 1.142, CrI: 1.108 - 1.177 per 1000), the capture of badgers (IR = 5.951, CrI: 4.482 - 7.912), percentage of forest cover (IR = 1.031, CrI: 1.020 - 1.042) and number of farm fragments (IR = 1.012, CrI: 1.009 - 1.015 per 10 fragments) were all associated with an increased incidence of bTB. Habitat suitability for badgers, percentage of dairy herds and the number of cattle movements into the herd were not. As an epidemiological tool and to suggest future work, an interactive online dashboard was developed to monitor disease progression and disseminate results to the general public.

CONCLUSION

Accounting for spatial correlation is an important consideration in disease mapping applications and is often ignored in statistical models examining bTB risk factors. Over time, the same regions in Ireland generally show highest incidences of bTB and allocation of more resources to these areas may be needed to combat the disease. This study highlights national bTB incidence rates. Shifting from national level analysis to smaller geographical regions may help identify localised high-risk areas.

The challenge of estimating wildlife populations at scale: the case of the European badger (Meles meles) in Ireland

Estimating population size in space and time is essential for applied ecology and wildlife management purposes; however, making accurate and precise estimates at large scales is highly challenging. An example is the European badger (Meles meles), a widespread and abundant mammal in Ireland. Due to their role in the epidemiology of bovine tuberculosis, the species has been culled in agriculturally dominant landscapes with the intention of reducing spillback infection to local cattle populations. Despite several studies using different approaches having estimated badger populations at different time points and scales, there remains considerable uncertainty regarding the current population and its future trajectory. To explore this uncertainty, we use published data and expert opinion to estimate a snapshot of probable badger population size using a Monte Carlo approach, incorporating variation in three key components: social group numbers, group size, and culling efficacy. Using this approach, we estimate what the badger population in Ireland would be with/without culling, assuming a steady-state population at carrying capacity, and discuss the limitations of our current understanding. The mean estimate for the badger population size was 63,188 (5–95th percentile, 48,037–79,315). Population estimates were sensitive to the assumption of mean group size across landscape type. Assuming a cessation of culling (in favour of vaccination, for example) in agricultural areas, the mean estimated population size was 92,096 (5–95th percentile, 67,188–118,881). Despite significant research being conducted on badgers, estimates on population size at a national level in Ireland are only approximate, which is reflected in the large uncertainty in the estimates from this study and inconsistencies between recording of data parameters in previous studies. Focusing on carefully estimating group size, factors impacting its variation, in addition to understanding the dynamics of repopulation post-culling, could be a fruitful component to concentrate on to improve the precision of future estimates.

Further description of bovine tuberculosis trends in the United Kingdom and the Republic of Ireland, 2003-2015

Extending on earlier work, trends in bovine tuberculosis (bTB) from 2003 to 2015 are described for the countries of the UK and the Republic of Ireland using standardised definitions and measures. Based on measures of animal and herd incidence, there remains a stable situation of extremely low prevalence in Scotland and the Low Risk Area of England, and a higher but ongoing reduction in prevalence in the Republic of Ireland. In Northern Ireland, there has been a rising bTB trend during 2010–2015, although not to levels experienced during 2002–2004. In the High Risk Area and Edge Area of England during 2010–2015, the rising bTB trends have continued but with some recent evidence of stabilisation. In Wales, prevalence has fallen subsequent to a peak in 2008. The paper considers country-level differences in the light of key policy changes, which are presented in detail. This work is unique, and will assist policymakers when critically evaluating policy options for effective control and eradication. Ongoing updates of this analysis would be useful, providing an evidence base for country-level comparison of bTB trends into the future. The use of multivariable analytical methods should be considered, but will rely on substantial sharing of raw data across the five countries.

Trends and Predictors of Large Tuberculosis Episodes in Cattle Herds in Ireland

Persistence of bovine tuberculosis (bTB) in cattle is an important feature of Mycobacterium bovis infection, presenting either as herd recurrence or local persistence. One risk factor associated with the risk of recurrent episodes is the severity of a previous bTB episode (severity reflecting the number of bTB reactors identified during the episode). In this study, we have sought to identify predictors that can distinguish between small (less severe) and large (more severe) bTB episodes, and to describe nationally the severity of bTB episodes over time. The study included descriptive statistics of the proportion of episodes by severity from 2004 to 2015 and a case-control study. The case-control study population included all herds with at least one episode beginning in 2014 or 2015, with at least two full herd tests during the episode and a minimum herd-size of 60 animals. Case herds included study herds with at least 13 reactors whereas control herds had between 2 to 4 (inclusive) reactors during the first 2 tests of the episode. A logistic regression model was developed to identify risk factors associated with a large episode. Although there has been a general trend towards less severe herd bTB episodes in Ireland over time (2004–2015), the proportion of large episodes has remained relatively consistent. From the case-control study, the main predictors of a large episode were the year the episode started, increasing herd-size, previous exposure to bTB, increasing bTB incidence in the local area, an animal with a bTB lesion and a bTB episode in an associated herd. Herds that introduced more animals were more likely to have a smaller bTB episode, reflecting the reduced risk of within-herd transmission when an episode was due to an introduced infected bTB animal. Some of the risk factors identified in this study such as reactors in previous bTB episodes, herds with an associated herd undergoing a bTB episode, herds in high incidence areas etc. may help to target future policy measures to specific herds or animals for additional surveillance measures. This information has important policy implications.

Understanding and managing bTB risk: perspectives from Ireland

There is substantial variation in herd risk for bovine tuberculosis (bTB) in Ireland, with most herds playing little to no role in the ongoing endemic. In infected areas, bTB persistence (affecting one or a group of herds) is a key feature of the infection. In this paper, we present our current understanding and management of bTB risk in Ireland, based on a detailed review of research and policy. There is close interaction between science and policy in Ireland, seeking both to understand and effectively manage bTB risk. Detailed research on bTB persistence is presented, including current understanding of the relative importance of different infection sources, which can include residual infection in cattle and/or re-infection, either from local sources or following cattle introduction. In recent years, there have been three primary drivers for policy change, including scientific advances, ongoing improvements to programme supports, and ongoing programme review. In this review, three key future programme challenges are identified. Although good progress is being made, eradication has not yet been achieved. Firstly, a key question concerns the additional effort that will be required, to move towards final eradication. Secondly, a percentage of non-infected animals are falsely positive to current testing methods. This is an ongoing challenge, given the imperfect specificity of test methods but will become more so, as the positive predictive value falls with reducing bTB prevalence. Finally, there is a need to re-engage with the farming community, so that they play a much greater role in programme ownership.