Responses to diagnostic tests for bovine tuberculosis in dairy and non-dairy cattle naturally exposed to Mycobacterium bovis in Great Britain.

Field surveillance of British cattle using the single intradermal comparative cervical tuberculin (SICCT) test shows a higher incidence rate of bovine tuberculosis (bTB) in dairy compared to beef herds, but a lower probability of post-mortem examination confirmed (PMC) Mycobacterium bovis infection in dairy herds. A cross-sectional study was conducted to compare animal level differences in bTB detection between dairy and non-dairy cattle in Great Britain. During the period from 2002 to 2005, 200 (41% dairy) reactors in the SICCT test (standard interpretation) were randomly selected, and 200 in-contact cattle (43% dairy) were purposively selected from bTB-infected herds. Interferon (IFN)-γ responses in blood to bovine and avian purified protein derivative (PPD), and early secretory antigen target 6 kDa and culture filtrate protein 10 (ESAT-6/CFP10), were measured. The post-mortem examination included gross pathological examination, mycobacterial culture and histopathology. The proportions of cattle positive to ESAT6/CFP10 were 26% (95% confidence interval, CI, 15-39%) in dairy reactors and 62% (95% CI 51-72%) in non-dairy reactors (P <0.001). PMC risk was 34% (95% CI 24-45%) in dairy reactors and 69% (95% CI 60-78%) in non-dairy reactors (P <0.001). The odds ratio for PMC risk in dairy reactors compared to non-dairy reactors, after controlling for bTB prevalence, herd size and SICCT test response, was 0.27 (95% CI 0.14-0.53; P <0.001). In surveillance data, adjusted animal level PMC risks were lower for dairy reactors than for beef reactors aged >2 years (P <0.001).

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

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

Farm characteristics and farmer perceptions associated with bovine tuberculosis incidents in areas of emerging endemic spread.

While much is known about the risk factors for bovine tuberculosis (bTB) in herds located in high incidence areas, the drivers of bTB spread in areas of emerging endemicity are less well established. Epidemiological analysis and intensive social research identified natural and social risk factors that may prevent or encourage the spread of disease. These were investigated using a case-control study design to survey farmers in areas defined as recently having become endemic for bTB (from or after 2006). Telephone surveys were conducted for 113 farms with a recent history of a bTB incident where their officially tuberculosis free status had been withdrawn (OTFW) (cases) and 224 controls with no history of a bTB incident, matched on location, production type and the rate of endemic bTB spread. Farmers were questioned about a range of farm management strategies, farm characteristics, herd health, wildlife and biosecurity measures with a focus on farmer attitudes and behaviours such as farmers' perception of endemicity and feelings of control, openness and social cohesion. Data generated in the telephone surveys was supplemented with existing herd-level data and analysed using conditional logistic regression. Overall, herd size (OR 1.07), purchasing an animal at a cattle market compared to purchasing outside of markets (OR 2.6), the number of contiguous bTB incidents (2.30) and the number of inconclusive reactors detected in the 2 years prior to the case incident (OR 1.95) significantly increased the odds of a bTB incident. Beef herds using a field parcel more than 3.2km away from the main farm and dairy herds reporting Johne's disease in the previous 12 months were 3.0 and 4.7 times more likely to have a recent history of a bTB incident, respectively. Beef herds reporting maize growing near, but not on, their farm were less likely to be case herds. Operating a closed farm in the two years prior to the case breakdown did not reduce the odds of a bTB incident. Farmers that had recently experienced a bTB incident were more likely to have implemented badger biosecurity in the previous year, but no more likely than control farms to have implemented cattle biosecurity. Case farmers felt significantly less likely to be influenced by government, vets or other farmers compared to those with no history of bTB. This suggests that alternative methods of engaging with farmers who have recently had a breakdown may need to be developed.

Exploration of the power of routine surveillance data to assess the impacts of industry-led badger culling on bovine tuberculosis incidence in cattle herds.

In the UK, badgers (Meles meles) are a well-known reservoir of infection, and there has been lively debate about whether badger culling should play a role within the British Government's strategy to control and eventually eradicate tuberculosis (TB) in cattle. The key source of information on the potential for badger culling to reduce cattle TB in high-cattle-TB-incidence areas remains the Randomised Badger Culling Trial (RBCT). In late 2013, two pilot areas were subjected to industry-led badger culls. These culls differed importantly from RBCT culling in that free-ranging as well as cage-trapped badgers were shot, and culling took place over a longer time period. Their impacts will be harder to evaluate because culling was not randomised between comparable areas for subsequent comparisons of culling versus no culling. However, the authors present calculations that explore the power of routine surveillance data to assess the impacts of industry-led badger culling on cattle TB incidence. The rollout of industry-led culling as a component of a national cattle TB control policy would be controversial. The best possible estimates of the effects of such culling on confirmed cattle TB incidence should be made available to inform all stakeholders and policy-makers.

Specificity of the comparative skin test for bovine tuberculosis in Great Britain.

A novel method for estimating specificity of the Single Intradermal Comparative Cervical Tuberculin (SICCT) test for bovine tuberculosis (bTB) using surveillance tests results is reported. The specificity of the SICCT test at three cut-offs was estimated from the dates, locations and skinfold measurements of all routine tests carried out in Officially TB Free (OTF) cattle herds in Great Britain (GB) between 2002 and 2008, according to their separation (by distance and time) from known infected (OTF-withdrawn) herds. The proportion of animals that tested positive was constant (P>0.20) when the distance between tested herds and nearest infected herd exceeded 8 km. For standard cut-off, calculated specificity was 99.98 per cent (95 per cent confidence interval ±0.004 per cent), equating to one false positive result per 5000 uninfected animals tested. For severe cut-off it was 99.91 per cent (±0.013 per cent) and for ultrasevere cut-off (selecting all reactors and inconclusive reactors) it was 99.87 per cent (±0.017 per cent). The estimated positive predictive value of the test averaged 91 per cent and varied by regional prevalence. This study provides further evidence of the high specificity of the SICCT test under GB conditions, suggests that over 90 per cent of cattle currently culled using this test in GB were infected, and endorses slaughter of at least these cattle for bTB control.

A novel approach to mapping and calculating the rate of spread of endemic bovine tuberculosis in England and Wales.

A mathematical method for estimating the endemic status of bovine tuberculosis (bTB) in cattle in England and Wales has been developed. 6.25km(2) hexagonal cells were used as the base resolution. Maps were produced for overlapping two-year periods spanning 2001/03 to 2009/11. Distance from a farm to the ten nearest 'Officially Tuberculosis Free status - Withdrawn' incidents within the same time period was measured. Endemic areas were defined as those hexagons containing farms where the 3rd nearest incident occurred within 7km. Temporal spread of endemic bTB was estimated by creating a contour map displaying the spread of endemic bTB over the two-year periods, and using boundary displacement to calculate the rate of spread across each hexagon. A rate was obtained for ∼2300 cells and varied between 0.04km and 15.9km per year (median=3.3km per year). This work will enable further analysis of the factors associated with this expansion.

Bovine TB infection status in cattle in Great Britain in 2013.

This report, provided by the APHA, summarises the infection status of bovine TB in cattle in Great Britain from January 1 to December 31, 2013 and describes some of the temporal trends observed over a longer period. It updates the previous annual summary for 2012, also published in Veterinary Record (VR, June 14, 2014, vol 174, pp 600-604).

Bovine TB infection status in cattle in Great Britain in 2012.

This report, provided by the AHVLA, summarises the infection status of bovine tuberculosis (TB) in cattle in Great Britain from January 1 to December 31, 2012 and describes some of the temporal trends observed over a longer period. The AHVLA intends to produce similar reports for future years to provide a concise summary of how the situation is developing.

Tuberculin manufacturing source and breakdown incidence rate of bovine tuberculosis in British cattle, 2005-2009.

The single intradermal comparative cervical tuberculin (SICCT) test is the primary test used for surveillance for bovine tuberculosis (bTB) in cattle in Great Britain (GB). The tuberculin used can, with other factors, influence test accuracy. In this analysis, the detection of infected cattle in GB 2005-2009 was compared between SICCT tests using tuberculins manufacturered by different manufacturers. Higher rates of reactors (adjusted rate 209 vs 186 per 100,000 tests, P = 0.003) and herd bTB incidents (adjusted total breakdown rate 5.1 vs 4.5 per 100 herd-years at risk, P < 0.001) were detected using tuberculin manufactured at Weybridge compared with Lelystad. However, confirmation of infection in reactors by postmortem evidence was higher with Lelystad tuberculin (adjusted percent 44.1 vs 47.1, P = 0.018). The findings, overall, suggest slightly higher test sensitivity and lower test specificity associated with Weybridge tuberculin compared with Lelystad. Assuming effective adjustment for confounding, the overall impact of tuberculin manufacturing source (2007-2009), was calculated to range somewhere between 315 false positive breakdowns, and 1086 bTB breakdowns missed (624 confirmed) as a result of using Weybridge and Lelystad tuberculin, respectively. However, animals that tested negative to the SICCT were not slaughtered at the time of the tests, so definitive conclusions are not possible.

Bovine tuberculosis trends in the UK and the Republic of Ireland, 1995-2010.

Selected demographic features and trends in bovine tuberculosis (BTB) from 1995 to 2010 are described for the countries of the UK and the Republic of Ireland, using standardised definitions and measures. All countries experienced a reduction in the number of cattle and herds and in the proportion of dairy herds, while average herd size increased. In general, the trends indicate a stable situation of very low BTB prevalence in Scotland and, over most of the period, a rising prevalence in England and Wales. The prevalence in the Republic of Ireland declined while Northern Ireland experienced both a rise and fall. Differences in demography, BTB programme structure and test results were noted, particularly between the island of Ireland and Great Britain. Further investigation of these differences may provide valuable insights into risk factors for BTB and optimisation of existing BTB programmes.

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

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

Recurrence of bovine tuberculosis breakdowns in Great Britain: risk factors and prediction.

Bovine tuberculosis (bTB) is an important economic disease worldwide with implications for both animal and human health. In Great Britain the number of herds that test positive for bTB, termed "breakdowns", has increased over the last two decades. Despite more intensive testing during a breakdown, around 23% of breakdowns recur within 12 months of the previous breakdown ending, and around 38% within 24 months. These "recurrent" breakdowns may be important for onward transmission of infection. Detailed case-control data were analysed to identify factors associated with recurrence within 12 months. The model predicted 83% of all recurrent breakdowns, with a positive predictive value (PPV) of 44%. A further model, restricted to data currently available nationally, was not sufficient to predict recurrence reliably; at a sensitivity of 72-76%, the PPV was 33-36%, when validated on independent data. Factors identified to be associated with recurrence are consistent with previous studies; namely, the number of reactors, a recent bTB history in the herd and a lack of association with the confirmation status of the initial breakdown. These variables are indicative of a higher level of infection or residual infection, and could be useful in the future development of predictive models for bTB recurrence.

Predicting prolonged bovine tuberculosis breakdowns in Great Britain as an aid to control.

Bovine tuberculosis (bTB) is an important notifiable disease in cattle in Great Britain (GB), and is subject to statutory control measures. Despite this, disease incidence has increased since the mid-1980s, and around 30% of herd breakdowns continue for more than 240 days. This is twice the shortest possible time for confirmed breakdowns to test clear from infection (≈120 days), and four times the shortest possible time for unconfirmed breakdowns (≈60 days). These "prolonged" breakdowns consume substantial resources and may act as an ongoing source of infection. It is not clear why some breakdowns become prolonged. Existing detailed case-control data have been re-analysed to determine risk factors for breakdowns lasting longer than 240 days, the strongest of which was the confirmation status of the breakdown: OR 12.6 (95%CI: 6.7-25.4). A further model restricted to data available early on in a breakdown for all breakdowns nationally, can predict 82-84% of prolonged breakdowns with a positive predictive value of 44-49% when validated using existing national datasets over a 4-year period. Identification of prolonged breakdowns at an earlier stage could help to target bTB controls in GB.

Cattle-to-cattle transmission of Mycobacterium bovis.

This review, illustrated with data on the characteristics of herds infected with Mycobacterium bovis (TB) in Great Britain (GB), attempts to identify the role of cattle-to-cattle transmission (CCT) of TB. CCT plays a part in the entry of infection into herds, through purchased infected animals or contiguous spread, although CCT can have a relatively small role in comparison with an established wildlife source. Experimental studies have shown that it is possible for CCT to occur within herds. In 1999, more than one reactor was found in over two-thirds of confirmed TB incidents in Great Britain. The details of transmission from an infectious animal to a susceptible animal are described: proximate, which depends on factors such as closeness of contact and ventilation, or indirect, which also depends on survival outside the host. Herd size is a risk factor for the incidence of TB, both in herds and in individual cattle. Control of TB is considered in relation to the skin test: failure of the test to remove all infected animals from incidents is possible, but probably of less significance than failure to prevent reinfection from sources external to the herd. It is concluded that CCT may have significance in determining the total number of reactors. Safeguarding herds from other sources of TB is likely to reduce CCT as a side effect.