Characteristics of Mycobacterium bovis infected herds tested with the interferon-gamma assay.

The IFN-γ (interferon gamma) assay is used in Ireland as an ancillary diagnostic test to the single intradermal comparative tuberculin test (SICTT) to maximise the detection of Mycobacterium bovis infected animals (bTB) in cattle herds. Understanding the relationships between herd and animal risk factors and IFN-γ test results is critical to enable the development and evaluation of policy measures on how best to use the test. In this study, we set out to characterise Irish herds with IFN-γ test positive animals in terms of herd size, number of SICTT reactors and number of IFN-γ positive tests, and to evaluate the IFN-γ test in terms of the test cut-off values. The results showed that larger herds with more SICTT reactors were likely to have more IFN-γ positives in the herd, and herds with an IFN-γ test positive animal that was also positive for bTB lesions at post-mortem had higher numbers of IFN-γ positive animals in the herd. Raising the cut-off values for the IFN-γ test only marginally decreased the combined sensitivity of the IFN-γ and the SICTT for diagnosis of bTB lesioned animals. The analysis has provided valuable information on the performance of the IFN-γ test as it is used under current bTB infection levels in Ireland.

Eradicating BVD, reviewing Irish programme data and model predictions to support prospective decision making.

Bovine Viral Diarrhoea is an infectious production disease of major importance in many cattle sectors of the world. The infection is predominantly transmitted by animal contact. Postnatal infections are transient, leading to immunologically protected cattle. However, for a certain window of pregnancy, in utero infection of the foetus results in persistently infected (PI) calves being the major risk of BVD spread, but also an efficient target for controlling the infection. There are two acknowledged strategies to identify PI animals for removal: tissue tag testing (direct; also known as the Swiss model) and serological screening (indirect by interpreting the serological status of the herd; the Scandinavian model). Both strategies are effective in reducing PI prevalence and herd incidence. During the first four years of the Irish national BVD eradication programme (2013-16), it has been mandatory for all newborn calves to be tested using tissue tag testing. During this period, PI incidence has substantially declined. In recent times, there has been interest among stakeholders in a change to an indirect testing strategy, with potential benefit to the overall programme, particularly with respect to cost to farmers. Advice was sought on the usefulness of implementing the necessary changes. Here we review available data from the national eradication programme and strategy performance predictions from an expert system model to quantify expected benefits of the strategy change from strategic, budgetary and implementation points of view. Key findings from our work include (i) drawbacks associated with changes to programme implementation, in particular the loss of epidemiological information to allow real-time monitoring of eradication progress or to reliably predict time to eradication, (ii) the fact that only 25% of the herds in the Irish cattle sector (14% beef, 78% dairy herds) would benefit financially from a change to serosurveillance, with half of these participants benefiting by less than EUR 75 per annum at herd level or an average of EUR 1.22 per cow, and (iii) opportunities to enhance the effectiveness of the current programme, particularly in terms of time to eradication, through enforced compliance with PI removal as currently outlined in programme recommendations. The assembled information provides scientific arguments, contributing to an informed debate of the pros and cons of a change in eradication strategy in Ireland.

The performance of the interferon gamma assay when used as a diagnostic or quality assurance test in Mycobacterium bovis infected herds.

There are two different contexts in the Irish bTB eradication programme in which the interferon-gamma assay (IFN-γ) is applied. Firstly, the IFN-γ assay is applied routinely to high risk cohorts in herds with four or more reactors to the SICTT. The IFN-γ test is then carried out on blood samples submitted to the laboratory within 8h of collection (diagnostic testing). Secondly, the use of the IFN-γ assay has recently been extended to test SICTT reactors as part of a general quality assurance (QA) scheme to monitor the performance of the SICTT. Blood samples from reactors are tested one day after blood collection (QA testing). In this study, we analysed the relative performance of the SICTT and IFN-γ when used in parallel as an 8h diagnostic test and as a 24h QA test on SICTT reactors. A total of 17,725 IFN-γ tests were included in the analysis (11,658 diagnostic tests and 6067 QA tests). Of the samples submitted for diagnostic testing, the proportion positive to IFN-γ decreased with the severity of interpretation of the SICTT result. Of the standard reactors that were tested with IFN-γ in the QA programme, 92.2% were positive to the IFN-γ test. Among animals that were SICTT -ve/IFN-γ +ve, 18.9% were positive at post-mortem compared to 11.8% of those that were SICTT +ve (standard reactor)/IFN-γ -ve. These results highlight the risk associated with retaining SICTT -ve/IFN-γ +ve animals, and suggest that prompt removal of these animals is necessary to reduce the potential for future transmission.

Temporal trends in the retention of BVD+ calves and associated animal and herd-level risk factors during the compulsory eradication programme in Ireland.

The national BVD eradication programme in Ireland started on a voluntary basis in 2012, becoming compulsory in 2013. The programme relies on accurate identification and prompt removal of BVD+ calves. However, a minority of herd owners have chosen to retain BVD+ animals (defined as still being alive more than seven weeks after the date of the initial test), typically with a view to fattening them to obtain some salvage value. During each year of the programme, additional measures have been introduced and implemented to encourage prompt removal of BVD+ animals. The objective of this study was to describe temporal trends in the retention of BVD+ calves and associated animal and herd-level risk factors during the first three years of the compulsory eradication programme in Ireland. The study population included all BVD+ calves born in Ireland in 2013-2015. A parametric survival model was developed to model the time from the initial BVD test until the animal was slaughtered/died on farm or until 31 December 2015 (whichever was earlier). A total of 29,504 BVD+ animals, from 13,917 herds, were included in the study. The proportion of BVD+ animals that were removed from the herd within 7 weeks of the initial test date increased from 43.7% in 2013 to 70.3% in 2015. BVD+ animals born in 2015 had a much lower survival time (median=33days) compared to the 2013 birth cohort (median=62days), with a year on year reduction in survival of BVD+ calves. In the initial parametric survival models, all interactions with herd type were significant. Therefore, separate models were developed for beef and dairy herds. Overall the results of the survival models were similar, with birth year, BVD+ status, herd size, county of birth and birth month consistently identified as risk factors independent of herd type (beef or dairy) or the numbers of BVD+ animals (single or multiple) in the herd. In addition, the presence of a registered mobile telephone number was identified as a risk factor in all models except for dairy herds with a single BVD+, while the sex of the BVD+ calf was only identified as a risk factor in this model. Significant progress has been made in addressing the issue of retention of BVD+ calves, however, there is a need for further improvement. A number of risk factors associated with retention have been identified suggesting areas where future efforts can be addressed.

A retrospective epidemiological analysis of risk factors for a primary necropsy diagnosis of bovine respiratory disease.

Bovine respiratory disease (BRD) is a multifactorial disease and the primary cause of both bovine morbidity and mortality in Ireland. The risk factors associated with a primary necropsy diagnosis of BRD among cattle in the traditional (non-feedlot) husbandry systems prevalent in Ireland have not been investigated previously. The aim of this case-control study was to investigate those risk factors among cattle of all ages over an 8 year period. A total of 3,090 BRD cases and 5,236 controls were matched by submitting veterinary practitioner. Univariable and multivariable analyses were performed to examine the association of selected animallevel, herd-level and environmental risk factors with case or control status using a conditional logistical regression model. Male cattle aged more than 31 days were significantly more likely to record a primary necropsy diagnosis of BRD than female cattle. Older cattle of both sexes were at increased odds of a BRD necropsy diagnosis than younger calves with the exception of female cattle aged greater than 165 days. The risk of a primary necropsy diagnosis of BRD increased with increasing herd size and decreased with increasing time in days since the last animal movement into the submitting herd. There were significantly reduced odds of a primary necropsy diagnosis of BRD in the summer (June to August) when compared with the autumn (September to November). These findings identify significant risk factors for a necropsy diagnosis of BRD under non-feedlot-type husbandry conditions.

Quantifying the risk of spread of bovine viral diarrhoea virus (BVDV) between contiguous herds in Ireland.

The control of bovine viral diarrhoea virus (BVDV) mainly focuses on the identification and restriction of persistently infected (PI) animals. However, other transmission pathways can also result in new breakdowns, including the movement of animals pregnant with PI calves (Trojan animals) and the spread of infection between contiguous farms. Contiguous spread is likely an important problem in the BVD eradication programme in Ireland, given the spatial distribution of residual infection, and the highly fragmented nature of land holdings on many Irish farms. In this study, we seek to quantify the risk of BVD spread between contiguous herds in Ireland. Multivariable logistic models were used to estimate the risk of a herd having BVD positive calves in January to June 2014 (the study period) when contiguous to a herd that had at least one BVD positive calf born in 2013. The models included risk factors relating to the study herd and to neighbouring herds. Separate multivariable models were built for each of four "PI-neighbour" factors relating to the presence of BVD+ animals and/or the presence of offspring of PI breeding animals. In total, 58,483 study herds were enrolled. The final model contained the province, the log of the number of calf births born during the study period, the number of cattle purchased between January 2013 and January 2014, and with a two-way interaction between the number of animals of unknown BVD status in the study herd and the PI-neighbour risk factor. When the number of PI-neighbour herds was used as the PI-neighbour risk factor, the odds ratio (OR) associated with the number of PI-neighbour herds ranged from 1.07 to 3.02, depending on the number of unknown animals present. To further explore the risk associated with PI-neighbour factors, the models were repeated using a subset of the study herds (n=7440) that contained no animals of unknown status. The best fitting model including "any PI-neighbour" as the PI-neighbour factor and also contained the log of the number of calf births born during the study period and the number of cattle purchased. The OR associated with "any PI-neighbour" was 1.92 (95% C.I. 1.37-2.70). This study provides the first quantitative information on the risks posed by the presence of BVD+ animals in neighbouring herds and also highlights the importance of clarifying the BVD status of animals that have not yet been tested in the context of the Irish eradication programme.

Risk factors for cattle presenting with a confirmed bTB lesion at slaughter, from herds with no evidence of within-herd transmission.

There has been a national bovine tuberculosis (bTB) eradication programme (BTBEP) in Ireland for many years. All cattle herds are tested at least annually using the Single Intradermal Comparative Tuberculin Test (SICTT). Further, abattoir surveillance is conducted on all animals at the time of slaughter. In the Irish BTBEP, a substantial number of confirmed bTB lesions are detected in non-reactor animals, to SICTT, from Officially Tuberculosis Free (OTF) herds at slaughter. In this study we investigate risk factors for non-reactor animals from OTF herds presenting with a confirmed bTB lesion at slaughter, but with no evidence of within-herd transmission. A case-control study was conducted, with animal as the unit of interest. The case animals were all SICTT non-reactor animals slaughtered in 2012, with a confirmed bTB lesion identified during routine abattoir surveillance and with no evidence of within-herd transmission. Control animals were selected from all SICTT non-reactor animals slaughtered in 2012 from OTF herds where no bTB lesion was found. Four controls matched by age (±1 year) and location (county) were randomly selected for each case. A conditional logistic regression model was developed for univariable and multivariable analysis. The final multivariable model included: number of movements, herd type, herd-size, inconclusive reactor status at any previous test, abattoir and time spent in a herd restricted for bTB. The odds of being a case increased with the number of times an animal had moved herds. Animals from suckler herds were significantly more likely to be a case compared to those from beef herds. The odds of being a case decreased with herd-size and increased as the time spent in a restricted herd increased. There were three key conclusions from this study. Firstly, the main risk factors for animals presenting with a confirmed bTB lesion at slaughter were: previous bTB exposure history, previous inconclusive reactor result at the SICTT, the number of herd movements and herd type/size. Secondly, there was very limited evidence that these animals could have been detected any earlier. Finally, there is a need to reconsider the importance of abattoir surveillance during the latter stages of an eradication campaign. As herd prevalence declines, an increasing proportion of herd restrictions will be triggered by a single bTB-lesioned animal, with no evidence of within-herd transmission.

Influence of the retention of PI calves identified in 2012 during the voluntary phase of the Irish national bovine viral diarrhoea virus (BVDV) eradication programme on herd-level outcomes in 2013.

This study was undertaken to investigate the impact of the retention of calves born in one calving season and considered to be persistently infected (PI) with bovine viral diarrhoea virus (BVDV) on herd-level outcomes in the following calving season. A secondary aim was to investigate the relationship between retention and the number of BVD+ calves detected the following season. The study population included a subset of herds enrolled in the 2012 voluntary BVD eradication programme in Ireland, specifically those with a birth registered to more than 80% of the cows between 1st January and 15th July and BVDV test results available for at least 80% of these calves, during both 2012 and 2013. Calves were considered PI based on either an initial positive result without further testing (BVDPOS) or a positive result on confirmatory testing (BVDPI), collectively considered BVD+ calves. Herd-level outcomes included the BVD status of the herd, and the number of BVD+ calves born between 1st January and 15th July 2013 (the study period). There was a significant univariable association between herd BVD status in 2013 and a number of general herd factors, including location, herd type, size and number of introduced animals (overall and those pregnant at time of introduction), as well as with each of six different factors related to the retention of virus-positive calves: the number of BVD+ calves in 2012; the maximum time (days) any one BVD+ born in 2012 was retained up to September 2013; the mean time (days) BVD+ animal(s) born in 2012 were retained up to September 2013; the date (quarter/year) the last BVD+ left the herd; the presence/number of 2012-born BVD+ retained in the herd at 1st January 2013. Separate multivariable models were constructed for each retention variable. The best model fit (based on AIC) was obtained using the retention variable "date (quarter/year) last BVD+ calf left the herd", followed by "total time all BVD+ calves were retained in the herd", with (log) herd size also retained in the models. Significant differences were also found in the number of positive calves detected in positive herds in 2013 for all of the calf retention risk factors. These findings confirm an increased probability of finding a BVD+ animal in a herd following the retention of positive calves born in the previous calving season, highlighting the importance of their prompt removal.

The relative effectiveness of testers during field surveillance for bovine tuberculosis in unrestricted low-risk herds in Ireland.

In Ireland, new bovine tuberculosis (bTB) cases are detected using both field and abattoir surveillance. Field surveillance is conducted on all cattle annually using the single intradermal comparative tuberculin test (SICTT). Testing is reliant on the skills and experience of the tester and a broad range of factors may adversely affect test accuracy. There is considerable emphasis on quality control (QC) within the national programme and field inspection of testers has been conducted in Ireland for many years. Since 2008, inspection has been supplemented with quantitative performance reports, enabling testers to be evaluated and ranked using a range of performance indicators. The objectives of this study were first, to quantify the relative effectiveness of testers during field surveillance and, second, to assess whether there has been any change in the performance of testers between 2008 and 2011. Mixed logistic regression was used to assess the relative effectiveness of testers. The study population included all testers who carried out at least ten eligible tests in Ireland during 2008 or 2011. The outcome measure was a herd restriction at the eligible test. Results from the mixed model indicated that the variation by tester had significantly (p=0.039) decreased from 0.589 in 2008 to 0.426 in 2011, indicating an increase in consistency of testing, after accounting for other known risk factors. This study provides objective data on the variation in tester performance over time and the relative performance of testers during field surveillance in Ireland.

Survival time of calves with positive BVD virus results born during the voluntary phase of the Irish eradication programme.

A retrospective case-control study was undertaken to investigate the temporal pattern of, and factors associated with, the survival of BVD virus-positive calves, identified between January and July 2012 during the voluntary phase of the Irish national eradication programme. Potential statuses for case and control calves consisted of: alive in birth herd; slaughtered; sold; dead (due to culling or death from natural causes and termed 'involuntary removal'). An initial comparison of cases and controls found significant differences between the outcomes for cases and controls and also between cases in relation to herd type (beef, dairy, dual purpose), sex, age and test status (BVDPOS - no confirmatory test; BVDPI - positive on confirmatory test). Key differences included a higher level of case animals still alive in, or slaughtered from, beef herds, a greater proportion of BVDPI animals being retained relative to those with a BVDPOS status and a significantly lower slaughter weight (89 kg) for case animals relative to controls. Separate multivariable models were constructed for dairy and beef cases. In the final dairy model breed (Jersey or non-Jersey), county and BVD status were retained, with the last two found to be time-varying covariates with significant changes in hazard ratios (HR) over time. In the beef model, herd size, county and BVD status were retained, with the HR for the last two factors again varying significantly over time. With the exception of the addition of the number of BVD positive calves in the herd to the dairy model, the same factors were identified when models were restricted to the first 90 days following the birth of case animals. A greater knowledge and understanding of all of these factors will allow refinement of programme communications and incentives to encourage prompt removal of PI calves from all sectors of the Irish breeding herd during the compulsory phase of the national eradication programme.

Future risk of bovine tuberculosis recurrence among higher risk herds in Ireland.

Within the Irish national bovine tuberculosis (bTB) eradication programme, controls are tighter on higher risk herds, known as H-herds. These H-herds are defined as herds that have previously had a bTB restriction (also known as a bTB episode), with at least 2 animals positive to the single intradermal comparative tuberculin test (SICTT) or with a bTB lesion detected at slaughter. Such herds are considered at higher risk of recurrence following the end of the bTB episode. In this study, we examined if, and when, the future bTB risk of H-herds returned to a similar level comparable to herds with no history of bTB. In addition, the proportion of bTB episodes in 2012 that could be attributed to the recent introduction of an infected animal was also estimated, providing an update of earlier work. The study population consisted of all Irish herds that were not bTB restricted at the start of 2012 and with at least one whole-herd SICTT in 2012, with the herd being the unit of interest. The outcome measure was a bTB restriction, defined as any herd where at least 1 standard SICTT reactor or an animal with a bTB lesion at slaughter in 2012 was identified. A logistic regression model was used to model the probability of a herd being restricted in 2012. Herds that were previously restricted had significantly higher odds of being restricted in 2012 compared to herds that had not. Similarly, the odds of being restricted in 2012 decreased as the time since the previous restriction increased, but increased as the severity of the previous restriction increased. Odds of being restricted also increased with an increase (although not linear) in herd size, the number of animals greater than 1 year of age purchased in 2011, the county incidence rate and the proportion of cows in the herd. The recent introduction of an infected animal accounted for 7.4% (6.7-8.2) of herd restrictions. This study confirms the key role of past bTB history in determining the future risk of Irish herds, with the odds related to both the severity of and time since the previous restriction. It also illustrates the difficulty in clearly defining H-herds, noting that risk persists for extended periods following a bTB restriction, regardless of breakdown severity. There is a need for robust controls on H-herds for an extended period post de-restriction.

Recent spatial changes in bovine tuberculosis in the Republic of Ireland.

There has been a steady reduction in the levels of bovine tuberculosis (bTB) in Ireland for some years. This paper presents a number of methods employing geographical information systems to examine how this reduction is manifested in space and time. The focus of disease mapping has traditionally been placed on identifying areas with problems. There has been limited work done in visualising improvements. The authors describe recent spatial changes in the relative incidence of bTB in Ireland across an uniform hexagonal grid, comparing the mean of the annual animal level bTB incidence during the 10-year period from 1998 to 2007 to the years 2008 to 2012. The authors map bTB incidence trends for three successive five-year periods (1998-2002, 2003-2007 and 2008-2012). Finally, the authors investigate the change in extent and severity of interpolated bTB reactor density over time.

Herd-level factors associated with the presence of bovine viral diarrhoea virus in herds participating in the voluntary phase of the Irish national eradication programme.

A risk factor study was conducted to identify variables associated with initial positive or inconclusive results for bovine viral diarrhoea virus (BVDV) in ear punch samples collected from calves between 1st January and 15th July 2012 (the study period) as part of the voluntary phase of the Irish national BVD eradication programme based on testing of ear tag tissue samples from calves born in participating herds. Univariable analysis indicated significant associations with the following factors: herd type; the number of cows in the herd; the number of calves born in the study period; the number of calves tested in the study period; the number of cattle purchased in 2011, between 2009 and 2011 and between 2007 and 2011; the number of tested calves whose dams had been purchased within 9 months of their calving date; and the percentage of calf mortality within 28 days of birth. The percentage of the cows in each herd that was homebred, location (province) the number of separate land parcels used by each herd, the presence of an associated sheep enterprise and the purchase of cattle through marts were not found to be significant. An initial logistic regression model was developed to model the probability of a herd having one or more BVD virus-positive or inconclusive calves. When vaccination status was initially excluded, province, log of the numbers of cows in the herd, the number of cows purchased between 2009 and 2011, the number of tested calves whose dams had been purchased within 9 months of their calving date and calf mortality were significant. When vaccination status was included, using a subset of the data based on farmers responding to a survey on vaccination status, it was retained as a significant variable along with the same variables already listed, showing a significant 2-way interaction with the log of the number of cows. There was not a significant association between an initial positive or inconclusive result and the length of time for which herds had been vaccinating. The study provides a series of key communication messages relating to both the delivery of, and benefits from, the national eradication programme.

Comparison of bovine tuberculosis recurrence in Irish herds between 1998 and 2008.

During the last several decades in Ireland, there has been substantial scientific progress in our understanding and related policy changes in the bovine tuberculosis (bTB) eradication programme. A range of performance measurements are routinely available, each highlighting a steadily improving situation in Ireland. However, recent research has highlighted an on-going problem of residual infection, contributing to recurrent breakdowns. In light of this general improvement, but also cognisant of residual infection, a critical evaluation of changes in effectiveness of managing recurrence is particularly valuable. Therefore, the objective of the study was to compare the herd-level risk of recurrence of bTB in Ireland between 1998 and 2008. A retrospective cohort study was carried out, using a Cox proportional-hazards model, to compare the risk of restriction recurrence in herds derestricted during 1998 and 2008. These herds were observed for up to 3 years from the end of the 'index restriction'. At the univariable level, 46.4% and 34.8% of study herds derestricted in 1998 and 2008, respectively, had a subsequent breakdown during the study period (χ(2)=70.6, P<0.001). In the multivariable analysis, there has been a significant reduction in bTB recurrence in Ireland, with 2008-derestricted herds being 0.74 times (95% confidence interval: 0.68-0.81) as likely to be restricted during the subsequent study period compared with 1998-derestricted herds. In the final Cox model, the rate of a future breakdown increased with increasing herd size, increasing number of standard reactors in the index restriction, increasing percentage of newly restricted herds within the District Electoral Division (DED) and if the herd had a previous bTB episode in the previous 5 years. The risk varied across herd type. The results from the current study provide further reassurance of an improved national situation, both in terms of limiting the establishment of new infection (bTB incidence) and in effectively clearing infection once detected (recurrence following derestriction). Recurrence of bTB requires effective implementation of multiple control strategies, focusing on identifying and removing residually infected cattle, and limiting environmental sources of infection, which in Ireland primarily relates to badgers.

The effect of somatic cell count data adjustment and interpretation, as outlined in European Union legislation, on herd eligibility to supply raw milk for processing of dairy products.

Somatic cell count (SCC) limits are a key component of national and international regulation for milk quality. As yet, very limited work has been published on SCC regulatory standards, including on the effect of different approaches to SCC data adjustment and interpretation. This study examines the effect of SCC data adjustment and interpretation, as outlined in current European Union (EU) legislation, on herd eligibility to supply raw milk for processing of dairy products for human consumption, using Irish data for illustration. The study used Irish milk-recording data as a proxy for bulk tank SCC (BTSCC) data, to calculate an unadjusted monthly SCC value for each herd during each month of participation. Subsequently, 4 data adjustments were applied, as outlined in EU and national legislation: seasonal adjustment; 3-mo rolling geometric average, without accounting for a break in the supply; 3-mo rolling geometric average, after accounting for a break in the supply; and seasonal adjustment and 3-mo rolling geometric average combined, after accounting for a break in the supply. Analyses were conducted to examine the effect, during the period from 2004 to 2010, of data adjustment on the percentage of herds with herd SCC >400,000 cells/mL. In all, 4 interpretation scenarios, incorporating different data adjustment combinations, were used to estimate herd eligibility (compliant, under warning, or suspended, as defined by legislation) to supply raw milk for processing. The 4 methods of data adjustment each led to a sizable reduction (6.7, 5.0, 5.3, and 11.1 percentage points, respectively, compared with the unadjusted data) in the percentage of herds exceeding a herd SCC of 400,000 cells/mL. Herd eligibility varied by interpretation scenarios, in particular those incorporating seasonal adjustment. The study provides new perspectives on the effect of data adjustment on herd SCC and of interpretation scenarios on herd eligibility. The results provide an illustrative, rather than definitive, picture of this effect, as national authorities use BTSCC data when determining herd eligibility, whereas this study was conducted using milk-recording data as a proxy. Some aspects of the primary EU legislation are unclear, which may lead to differences in interpretation and application. The potential impact of data adjustment and milk purchaser pricing on farm-level mastitis control in Ireland is considered.

The impact of animal introductions during herd restrictions on future herd-level bovine tuberculosis risk.

In Ireland new cases of bovine tuberculosis (bTB) are detected using both field (with the single intradermal comparative tuberculin test (SICTT)) and abattoir surveillance. Once a new case has been detected, herd restrictions, including restrictions on animal movements into and out of the herd, are implemented until the herd has passed two consecutive clear tests. While a herd is restricted, there may be several reasons why it may be desirable to introduce new stock, such as enabling routine management practices to continue 'as near to normal'. In Ireland, introduction of animals during a bTB episode is permitted under specific conditions, with permission from the local veterinary office. The objectives of this study were (1) to provide an overview of movement events associated with each bTB episode, (2) to determine whether introduction of animals during a bTB episode is associated with increased future bTB risk and (3) to identify the practices relating to the introduction of animals that are the most risky. All herds that were not restricted at the start of 2006, but experienced a bTB episode during 2006 with 2 or more SICTT standard reactors (the eligible bTB episode) were included in the study. We calculated the number of extended eligible bTB episodes and subsequent bTB episodes that could be directly attributed to introduced animals. The main outcome of interest was the time from de-restriction of the eligible bTB episode to the start of a subsequent bTB episode or the date of the last test prior to the end of the study (31 December 2010). Cox proportional-hazard models were developed, each using a different introduction variable: introduced animals during an episode (yes/no), introduced animals prior to the first retest/first clear test, time from start of episode until first animals introduced and number of animals introduced during the episode. Only a small proportion of subsequent bTB episodes (1.8%) or extended eligible bTB episodes (2.7%) could be directly attributed to introduced animals. The results highlight an increased risk of a subsequent bTB episode among only a subset of herds that introduced animals during the eligible bTB episode. Specifically, herds that introduced animals early during the eligible bTB episode were at significantly greater future bTB risk than herds where animals were only introduced later. To illustrate, herds that introduced animals after the first retest did not have a significantly different risk compared to herds that did not introduce animals at all. In contrast, herds that did introduce animals prior to the first retest had 1.5 times higher risk of a subsequent bTB episode. Future practices concerning the introduction of animals during an episode now need to be reviewed.

Evaluation of single reactor bovine tuberculosis breakdowns based on analysis of reactors slaughtered at an Irish export meat plant.

The 'Singleton Protocol' was adopted by the Irish Department of Agriculture Fisheries and Food (DAFF) in 1996 to address the incomplete specificity of the single intra-dermal comparative tuberculin test (SICTT) used in Ireland for the detection of animals infected with bovine tuberculosis (bTB). The protocol allows the early restoration of disease-free status to herds with a single reactor breakdown, where the herd was not confirmed as infected with Mycobacterium bovis by epidemiological investigation, by postmortem examination or by further test. The current study examines the ability of the Singleton Protocol to identify false-positive reactors. It investigates the subsequent herd-reactor rate following single reactor removal and analyses the factors leading to a positive postmortem lesion outcome and a positive reactor retest result. Postmortem lesion results were obtained for 371 reactor animals from single reactor breakdowns that were killed at an export meat plant over a 19-month period. Epidemiological and test data for these animals and their herds were obtained from DAFF databases and analysed by univariate and multivariate statistical analysis. Singleton candidates had an 18.7 per cent lower lesion rate than single animal breakdowns not meeting the singleton criteria. No significant difference was found between Singletons and non singletons in the subsequent reactor retest results. Skin thickness at the SICTT is the most significant determinant of a positive lesion result. The area bTB history was shown to be a significant variable in producing a positive reactor retest result.

Impact of the national full herd depopulation policy on the recurrence of bovine tuberculosis in Irish herds, 2003 to 2005.

This study evaluated the impact of the Irish herd bovine tuberculosis (bTB) depopulation policy (depopulation, disinfection, contiguous testing and local badger removal where implicated) on the recurrence of bTB infection, by comparing the future risk in restocked herds following depopulation for either bTB or bovine spongiform encephalopathy (BSE) during 2003 to 2005. Each herd was assigned a 'previous bTB risk', based on bTB history during the five years before depopulation. Future bTB risk was estimated, using a multivariable Cox proportional hazard model for time-to-breakdown for each study herd, to identify risk factors associated with bTB. Future bTB risk varied significantly by reason for depopulation and previous bTB risk. Herds depopulated for bTB (by definition, at high bTB risk) were not significantly different from BSE herds with no or a low previous bTB risk. BSE herds with a high previous bTB risk were found to be at significantly greater future bTB risk. Herd bTB depopulation measures, as currently applied in Ireland, are shown to be effective in enabling herds to attain and retain bTB freedom following restocking. Based on the data presented, and consistent with current knowledge of the bTB epidemiology, local badger removal contributes to efforts to limit recurrence of bTB in Ireland.

Shorter-term risk of Mycobacterium bovis in Irish cattle following an inconclusive diagnosis to the single intradermal comparative tuberculin test.

In Ireland, new cases of bovine tuberculosis (bTB) are detected using both field and abattoir surveillance (More and Good, 2006). Field surveillance is conducted through annual testing of all cattle using the single intradermal comparative tuberculin test (SICTT). An animal may be deemed a 'standard inconclusive reactor' (SIR) to the SICTT if the bovine response is >2mm and between 1 and 4mm>the avian response. The herdowner then has three choices for the management of the SIR: option 1 is to have the animal retested after a minimum period of 42 days (an inconclusive reactor retest, IRR), option 2 is to slaughter the SIR and, provided the animal has no visible lesions, have a full herd test 42 days after the SIR leaves the herd, option 3 is to slaughter the SIR and have the lymph nodes examined using histology and/or culture for bTB. In the current study, we examine the bTB risk for SIRs both at slaughter prior to the IRR and at the IRR, and the future bTB risk of TIR animals (so-called 'transient SIRs'; SIR animals with a negative SICTT result at the subsequent IRR) that moved from the herd of disclosure within 6 months of the IRR. We also investigate factors associated with the future bTB status of SIRs at slaughter prior to the IRR and at the IRR. The study population included all SIRs identified in Ireland between 2005 and 2009 inclusive in a herd otherwise Officially TB free (OTF). Between 11.8% and 21.4% of SIRs slaughtered prior to the IRR were confirmed bTB positive at post mortem (using histology or culture if histology was not definitive), compared to 0.13-0.22% of SICTT -ve cohort animals. The post mortem bTB lesion rate of SIRs is lower than the lesion rate reported for reactor animals between 2005 and 2009 of between 34% and 39%, reflecting the doubtful infection status of these animals. Between 20.3% and 27.9% of herds were restricted at the IRR. The herd restriction rate amongst the national herd between 2005 and 2009 varied from 5.09% to 6.02%. TIRs that moved out of the disclosing herd within 6 months of the IRR were 12 times more likely to be bTB positive at the next test/slaughter compared to all animals in the national herd. The same increased risk did not apply to the SICTT -ve cohort animals that moved out of the same herds at the same time. Based on a range of measures, SIRs and TIRs are each at increased bTB risk into the future. Consequently, differential treatment of TIR animals would be justified.

Longer-term risk of Mycobacterium bovis in Irish cattle following an inconclusive diagnosis to the single intradermal comparative tuberculin test.

In Ireland, new bovine tuberculosis (bTB) cases are detected using both field and abattoir surveillance. During field surveillance, an animal may be deemed a 'standard inconclusive reactor' (SIR) to the single intradermal comparative tuberculin test (SICTT) if the bovine response is >2mm, and from 1 to 4mm greater than the avian response. Little is known about the future infection risk posed by SIR animals that pass a subsequent retest, so-called 'transient SIR' (TIR) animals. The objective of this study was to critically evaluate the future bTB status of TIR animals, by examining the future risk of bTB diagnosis over the 4 years following initial SIR diagnosis and clearance at the subsequent retest. The study included all TIRs that were identified as SIRs in 2005 in otherwise free herds at tests with no other reactors at that test and that were clear at the subsequent retest. The analysis was restricted to cows that were neither sold, other than direct to slaughter, nor exported from the herd during the follow up period (to the end of 2009). Five control cows were randomly selected from each study herd. A parametric survival model with shared frailties, to account for clustering within herds, was developed to model time from passing a retest to future bTB diagnosis. The final parametric survival model contained the variables: TIR status in 2005, inconclusive status during the follow-up period, location, herd restricted during the study, time since last restriction within the herd and age. The time ratio for the TIR status variable was significant (p<0.001) indicating that on average the time to diagnosis with bTB for TIRs was 78% shorter compared to the non-TIRs. The frailty term was significant (p<0.001) indicating that animals within some herds were more likely to become reactors compared to other herds. These results have important implications for national policy and future management of TIR animals. Further, private veterinary practitioners and their clients should be aware of the increased risk associated with TIRs.