Patterns of delayed detection and persistence of bovine tuberculosis in confirmed and unconfirmed herd breakdowns in cattle and cattle herds in Great Britain

Approximately 1500/6000 cattle farms that were depopulated during the foot and mouth epidemic in GB in 2001 had been repopulated and subjected to two unrestricted (herd considered free from bovine tuberculosis (bTB)) herd tests. Factors associated with herd breakdown(s) (HBD) and individual cattle reactor status at the second test were investigated. There were 96 HBD in total, with a 3-fold increased risk of HBD in herds that had had a HBD at the first test after restocking. Two mixed effect models were used to investigate factors associated with 324/246,060 reactor cattle at the second bTB test; 228 reactors were at confirmed HBD and 96 at unconfirmed HBD; 253 (79%) reactors at the second test were present and test negative at the first test. In confirmed HBD, the odds of cattle reacting were higher if the restocked farm had a history of bTB before 2001 and if the source and restocked farms were high frequency tested (HFT) farms (routine bTB tests at ≥1 per 2 years). Reacting cattle were more likely to have been born on the restocked farm before the first test after FMD and less likely to have been purchased from a low frequency tested (LFT) farm (routine bTB tests at 3-4 year intervals) after the first test compared with a baseline of cattle purchased from a LFT farm before the first test. Unconfirmed HBD at the second test was more likely when the first test was a confirmed HBD and when there was a history of bTB in the restocked farm. In contrast to confirmed HBD, cattle purchased from a LFT farm after the first test were at increased risk of reacting at an unconfirmed HBD at the second test. We conclude that a farm history of bTB suggests persistence of bTB on the farm. Confirmed tests indicate exposure to bTB for some time indicated by the increased risk from HFT source and restocked farms and a farm history of bTB. The risks for reactors are related to the farm and herd and duration of exposure to these risks. Therefore, the spread of bTB to na?ve herds would be reduced if farmers only introduced cattle known not to have been in herds and on farms exposed to bTB. Management of bTB on farms with bTB is complicated because there is undisclosed infection in cattle and environmental contamination.     

An ecological and comparative perspective on the control of bovine tuberculosis in Great Britain and the Republic of Ireland

Disease ecology involves a systematic approach to understanding the interactions and evolution of host-pathogen systems at the population level, and is essential for developing a comprehensive understanding of the reasons for disease persistence and the most likely means of control. This systems or ecological approach is being increasingly recognised as a progressive method in disease control and is exploited in diverse fields ranging from obesity management in humans to the prevention of infectious disease in animal populations. In this review we discuss bovine tuberculosis (bTB) in Great Britain (GB) within a disease ecology context, and suggest how a comparative ecological perspective helps to reconcile apparent conflicts with the evidence on the effectiveness of badger culling to assist in the control of bTB in GB and the Republic of Ireland (ROI). Our examination shows that failure of past measures to control bTB and the disparity in outcomes of badger culling experiments are the result of a complex relationship amongst the agent, host and environment, i.e. the episystem, of bTB. Here, we stress the role of distinctive bTB episystems and badger culling trial design in the ambiguity and resulting controversy associated with badger culling in GB and ROI. We argue this episystem perspective on bTB control measures in cattle and badger populations provides a useful and informative perspective on the design and implementation of future bTB management in GB, particularly at a time when both scientific and lay communities are concerned about the ongoing epidemic, the cost of current control measures and the execution of future control procedures.     

Breakdown severity during a bovine tuberculosis episode as a predictor of future herd breakdowns in Ireland

A retrospective cohort study of Irish cattle herds investigated whether the severity of a herd’s bovine tuberculosis (BTB) breakdown was a predictor of the hazard of a future BTB breakdown in that herd. Data on 10,926 herds not having had BTB in 1995 (the “non-exposed” group) were obtained using a 10% random sample from all herds without BTB in 1995. Data on 6757 herds that had a new BTB breakdown in 1995 (the “exposed” group) were obtained and categorized into five increasing exposure-severity classes based on the total number of standard reactors (to the single intra-dermal comparative cervical tuberculin test) detected during the breakdown. Exposed herds were deemed to be free of BTB after they passed a 6-month check test; non-exposed herds were deemed free as of the date of the first negative herd-test in 1995.

In the 5 years after 1995, 18% of the non-exposed herds had a BTB breakdown, whereas 31% of the exposed herds had a subsequent breakdown. Relative to the hazard for non-exposed herds, the hazard for the first future singleton standard reactor breakdown, was 1.6-times higher for exposed herds with only 1 standard reactor in 1995, and 1.8-times higher for those exposed herds with 4–8 standard reactors during the 1995 episode. When the outcome for future breakdowns was 2 or more standard reactors, the hazard ratios ranged from 1.6 for exposed herds with only 1 standard reactor in 1995 up to 2.9 in exposed herds with 8 or more standard reactors during the 1995 episode. The latter hazard ratio varied over time, decreasing to 1.7 after 3 years of risk. The hazard of a future BTB breakdown increased directly with number of cattle in the herd, a positive history of previous BTB in the herd, and the local herd prevalence of BTB. The presence of confirmed BTB lesions in reactor cattle was not predictive of the future breakdown hazard when the effects of other factors were controlled.     

Changing trends in infectious bovine rhinotracheitis in Great Britain

The incidence and severity of infectious bovine rhinotracheitis in Great Britain increased markedly in the mid to late 1970s. Data available in MAFF records have been used to analyse the changing trends in disease incidence and serological prevalence, with particular emphasis on the most recent decade.     

The improved specificity of bovine brucellosis testing in Great Britain

The brucellosis surveillance scheme in Great Britain includes the serological testing of approximately 1 million bovine samples per year. These are screened by iELISA, positives going forward for confirmatory testing by CFT and SAT. Samples positive by confirmatory testing prompt substantial field investigations and interventions, but the animals involved are usually uninfected. Described below are a series of modifications to the screening method, which have resulted in a 10-fold reduction in false positive results whilst maintaining sensitivity. The key modifications include the introduction of blocking agents, a change in serum test dilution and the introduction of a control that directly defines the positive/negative cut-off. These simple modifications have had a large impact in reducing the cost of the surveillance programme due to reductions in confirmatory test requirements and a knock on effect of reducing costly field intervention.     

A review of tuberculosis science and policy in Great Britain

Bovine tuberculosis is one of the most complex animal health problems that the farming industry in Great Britain faces today. In leading and facilitating the changes to policy required to reverse the long-term upward trend in the disease, Government is heavily reliant on evidence emerging from its wide-ranging bovine tuberculosis research programme. The paper outlines development of policy in Great Britain and its relationship to research findings.     

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.     

Clinical findings in 78 suspected cases of bovine spongiform encephalopathy in Great Britain

The clinical findings in 59 cows with bovine spongiform encephalopathy (BSE) were compared with those in 19 cattle that were submitted as BSE suspects but not confirmed by immunohistochemistry. Both groups were also compared with a control group of 20 healthy cows. Abnormalities in behaviour, temperament, mental status and activity, neurogenic disorders of gait and hyperreactivity to touch were frequently observed in the cattle with BSE. Not every animal with BSE displayed clinical signs in all these categories, and the severity of the signs was not always useful for differentiating them from the BSE suspects that were not confirmed by pathology. The neurological examination was better than passive observations for the clinical diagnosis of BSE. Tests of the animals' responses to sudden auditory, visual and tactile stimuli were very useful for distinguishing cases of BSE from unconfirmed BSE suspects if the cases did not display signs in all the categories.     

Descriptive spatial analysis of the epidemic of bovine spongiform encephalopathy in Great Britain to June 1997

This was a spatial analysis of the epidemic of bovine spongiform encephalopathy (BSE) in Great Britain, based on agricultural census data collected between 1986 and 1996 and BSE case data collected up to June 1997. Kernel smoothing techniques were used to plot the distribution of BSE-positive cattle holdings per 100 holdings per square kilometre and the distribution of confirmed BSE cases per 100 head of cattle per square kilometre. In the early stages of the epidemic reported BSE cases were scattered widely throughout Great Britain, with no clearly identifiable focus. By June 1997, a statistically significant cluster of BSE-positive holdings was identifiable in the eastern part of the South west region of England. During the epidemic the highest densities of confirmed BSE cases per 100 cattle per square kilometre occurred in the greater part of the South west region of England and within Dyfed in the south west of Wales. In Wales, a small number of holdings experienced large numbers of confirmed BSE cases. In the South west region of England a large number of holdings experienced small numbers of confirmed cases. By June 1997, the distribution of BSE-positive holdings across Great Britain was largely determined by factors that influenced the amount of recycled infectious material they were exposed to.     

Public attitudes towards badger culling to control bovine tuberculosis in cattle

In 1999, a questionnaire survey was conducted to evaluate public preferences towards badger culling to control bovine tuberculosis in cattle. Three alternative treatments were considered: (1) widespread culling, (2) the current experimental trials, and (3) no culling. One hundred residents from Glastonbury and York were interviewed in person and asked to give preference ratings to each of the three treatments. The single most preferred treatment was no culling, and the least preferred was the widespread cull. Respondents who favoured either the widespread cull or the experimental trials tended to be more knowledgeable about the problem and cited the level of tuberculosis in cattle as the primary factor guiding their preferences. Respondents who favoured the no culling option tended to be less knowledgeable, and cited the conservation and welfare impacts on badger populations as the most important factors. Analysis of the distribution of preference scores suggested that although it was not necessarily the most preferred treatment the experimental trial may be a relatively acceptable alternative.     

A cost-benefit analysis of culling badgers to control bovine tuberculosis

Bovine tuberculosis (TB) is an important economic problem. The incidence of TB in cattle herds has steadily risen in the UK, and badgers are strongly implicated in spreading disease. Since the mid-1970s the UK government has adopted a number of badger culling strategies to attempt to reduce infection in cattle. In this report, an established model has been used to simulate TB in badgers, transmission to cattle, and control by badger culling. Costs were supplied by the UK Government's Department for Environment Food and Rural Affairs (Defra) for badger trapping and gassing. Regardless of culling intensity or area simulated, an overall reduction in the herd breakdown rate was seen. With a high culling efficacy and no social perturbation, the mean Net Present Value of a few simulated culling strategies in an "ideal world" was positive, meaning the economic benefits outweighed the costs. Further work is required before these results could be considered definitive, as it is necessary to evaluate uncertainties and simulate less than perfect conditions.