Inconclusive Reactors Research Articles

Evaluating the risk of bovine tuberculosis posed by standard inconclusive reactors identified at backward-traced herd tests in Northern Ireland that disclosed no reactors

Bovine tuberculosis is a notifiable disease in Northern Ireland with the national eradication programme of compulsory testing and slaughter of reactor animals costing approximately £40 million per year. Backward tracing, known as Backward Check Tests (BCTs), of reactor animals is used to identify previous herds where the bTB positive animal has resided. The aim of this study was to quantify the bovine tuberculosis (bTB) risk posed by inconclusive reactors (ICs) at BCTs at both the individual animal and the herd level. ICs to the Comparative Intradermal Tuberculin Test (CITT) at a BCT, in which no reactors were found, were matched with CITT negative animals, based on age, sex, test ID and follow up period, in Northern Ireland between 1st January 2008 and 31st December 2017 (inclusive). A retrospective matched cohort study design was used with the outcome of interest being the bTB status of each animal and subsequent bTB herd breakdowns. After adjusting for herd size, IC animals at a BCT had 16 times the odds (95% confidence interval: 7.75 to 38.28, p < 0.001) of becoming bTB positive compared to CITT negative animals. The percentage population attributable risk was 0.0001%. The majority 75% (n = 71) of ICs that became bTB positive were identified at the 42 day retest. Of those that were not disclosed at the 42 day retest (n = 24), almost a third (29%) had moved to an unrestricted herd. However, after adjusting for herd size and type, herds that had ICs only identified at a BCT did not have an increased odds of a subsequent bTB herd breakdown compared to herds that had a CITT negative BCT. Given the increased risk posed by ICs at a BCT, it may be justifiable to remove them from the herd immediately or place them under lifetime movement restrictions to the herd where they were detected. However, further action regarding the herd of origin does not appear to be justified.

Machine learning classification methods informing the management of inconclusive reactors at bovine tuberculosis surveillance tests in England

Bovine tuberculosis (bTB) remains one of the most complex, challenging, and costly animal health problems in England. Identifying and promptly removing all infected cattle from affected herds is key to its eradication strategy; the imperfect sensitivity of the diagnostic testing regime remaining a serious obstacle.The main diagnostic test for bTB in cattle in England, the Single Intradermal Comparative Cervical Tuberculin Test (SICCT: also known as the skin test), can produce inconclusive results below the reactor threshold. The immediate isolation of inconclusive reactor (IR) animals followed by a 60-day retest may not prevent lateral spread within the herd (if it is substandard, allowing transmission) or transmission to wildlife. Over half of IR-only herds that went on to have a positive skin test result (a bTB herd ‘incident’) in 2020, had it triggered by at least one IR not clearing their 60-day retest, instead of by another test within the previous 15 months.Machine learning classification algorithms (classification tree analysis and random forest), applied to England’s 2012–2020 IR-only surveillance herd tests, identified at-risk tests for an incident at the IRs’ 60-day retest. In this period, 4 739 out of 22 946 (21 %) IR-only surveillance tests disclosing 6 296 out of 42 685 total IRs, had an incident at retest (2 716 IRs became reactors and 3 580 IRs became two-time IRs). Both models showed an AUC above 80 % in the 2012–2019 dataset. Classification tree analysis was preferred due to its easy-to-interpret outputs, 70 % sensitivity, and 93 % specificity in the 20 % of 2019–2020 testing dataset.The paper aimed to identify IR-only surveillance tests at-risk of an incident at the 60-day retest to target them with appropriate measures to mitigate the IRs’ risk. Sixteen percent (341 out of 2 177) of IR-only herd tests were identified as high-risk in the 2020 dataset, with 265 (78 %) of these having at least one reactor or IR at retest. Severe-level reinterpretation of the high-risk IR-only disclosing tests identified in this dataset would turn 68 out of the 590 (12 %) IRs into reactors, generating 23 incidents, the majority (19 or 83 %) part of the 265 incidents that would have been declared at the retest.Classification tree analysis used to identify IR-only high-risk tests in herds eligible for severe interpretation would enhance the sensitivity of the test-and-slaughter regime, cornerstone of the bTB eradication programme in England, further mitigating the risk of disease spread posed by IRs.

Exploring the Risk Posed by Animals with an Inconclusive Reaction to the Bovine Tuberculosis Skin Test in England and Wales.

The single intradermal comparative cervical tuberculin (SICCT) test is the primary test for ante-mortem diagnosis of bovine tuberculosis (TB) in England and Wales. When an animal is first classified as an inconclusive reactor (IR) using this test, it is not subject to compulsory slaughter, but it must be isolated from the rest of the herd. To understand the risk posed by these animals, a case-control study was conducted to measure the association between IR status of animals and the odds of them becoming a reactor to the SICCT at a subsequent test. The study included all animals from herds in which only IR animals were found at the first whole herd test in 2012 and used data from subsequent tests up until the end of 2016. Separate mixed-effects logistic regression models were developed to examine the relationship between IR status and subsequent reactor status for each risk area of England and for Wales, adjusting for other explanatory variables. The odds of an animal becoming a subsequent reactor during the study period were greater for IR animals than for negative animals in the high-risk area (odds ratio (OR): 6.85 (5.98–7.86)) and edge area (OR: 8.79 (5.92–13.04)) of England and in Wales (OR: 6.87 (5.75–8.22)). In the low-risk area of England, the odds were 23 times greater, although the confidence interval around this estimate was larger due to the smaller sample size (11–48, p < 0.001). These findings support the need to explore differential controls for IR animals to reduce the spread of TB, and they highlight the importance of area-specific policies.

Exploring the Fate of Cattle Herds With Inconclusive Reactors to the Tuberculin Skin Test.

Bovine tuberculosis (TB) is an important animal health issue in many parts of the world. In England and Wales, the primary test to detect infected animals is the single intradermal comparative cervical tuberculin test, which compares immunological responses to bovine and avian tuberculins. Inconclusive test reactors (IRs) are animals that demonstrate a positive reaction to the bovine tuberculin only marginally greater than the avian reaction, so are not classified as reactors and immediately removed. In the absence of reactors in the herd, IRs are isolated, placed under movement restrictions and re-tested after 60 days. Other animals in these herds at the time of the IR result are not usually subject to movement restrictions. This could affect efforts to control TB if undetected infected cattle move out of those herds before the next TB test. To improve our understanding of the importance of IRs, this study aimed to assess whether median survival time and the hazard of a subsequent TB incident differs in herds with only IRs detected compared with negative-testing herds. Survival analysis and extended Cox regression were used, with herds entering the study on the date of the first whole herd test in 2012. An additional analysis was performed using an alternative entry date to try to remove the impact of IR retesting and is presented in the Supplementary Material. Survival analysis showed that the median survival time among IR only herds was half that observed for clear herds (2.1 years and 4.2 years respectively; p < 0.001). Extended Cox regression analysis showed that IR-only herds had 2.7 times the hazard of a subsequent incident compared with negative-testing herds in year one (hazard ratio: 2.69; 95% CI: 2.54, 2.84; p < 0.001), and that this difference in the hazard reduced by 63% per year. After 2.7 years the difference had disappeared. The supplementary analysis supported these findings showing that IR only herds still had a greater hazard of a subsequent incident after the IR re-test, but that the effect was reduced. This emphasizes the importance of careful decision making around the management of IR animals and indicates that re-testing alone may not be sufficient to reduce the risk posed by IR only herds in England and Wales.

Genomic regions underlying susceptibility to bovine tuberculosis in Holstein-Friesian cattle

BackgroundThe significant social and economic loss as a result of bovine tuberculosis (bTB) presents a continuous challenge to cattle industries in the UK and worldwide. However, host genetic variation in cattle susceptibility to bTB provides an opportunity to select for resistant animals and further understand the genetic mechanisms underlying disease dynamics.MethodsThe present study identified genomic regions associated with susceptibility to bTB using genome-wide association (GWA), regional heritability mapping (RHM) and chromosome association approaches. Phenotypes comprised de-regressed estimated breeding values of 804 Holstein-Friesian sires and pertained to three bTB indicator traits: i) positive reactors to the skin test with positive post-mortem examination results (phenotype 1); ii) positive reactors to the skin test regardless of post-mortem examination results (phenotype 2) and iii) as in (ii) plus non-reactors and inconclusive reactors to the skin tests with positive post-mortem examination results (phenotype 3). Genotypes based on the 50 K SNP DNA array were available and a total of 34,874 SNPs remained per animal after quality control.ResultsThe estimated polygenic heritability for susceptibility to bTB was 0.26, 0.37 and 0.34 for phenotypes 1, 2 and 3, respectively. GWA analysis identified a putative SNP on Bos taurus autosomes (BTA) 2 associated with phenotype 1, and another on BTA 23 associated with phenotype 2. Genomic regions encompassing these SNPs were found to harbour potentially relevant annotated genes. RHM confirmed the effect of these genomic regions and identified new regions on BTA 18 for phenotype 1 and BTA 3 for phenotypes 2 and 3. Heritabilities of the genomic regions ranged between 0.05 and 0.08 across the three phenotypes. Chromosome association analysis indicated a major role of BTA 23 on susceptibility to bTB.ConclusionGenomic regions and candidate genes identified in the present study provide an opportunity to further understand pathways critical to cattle susceptibility to bTB and enhance genetic improvement programmes aiming at controlling and eradicating the disease.

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.

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.

Field application of immunoassays for the detection of Mycobacterium bovis infection in the African buffalo (Syncerus caffer)

The African buffalo (Syncerus caffer) is considered the most important maintenance host of bovine tuberculosis (BTB) in wildlife in Southern Africa. The diagnosis of Mycobacterium bovis infection in this species mostly relies on the single intradermal comparative tuberculin test (SICTT). As an alternative, the BOVIGAM® 1G, an interferon-gamma (IFN-γ) release assay, is frequently used. The test performance of cell-mediated immunity (CMI-) and humoral immunity (HI-) based assays for the detection of M. bovis infections in buffaloes was compared to identify the test or test combination that provided the highest sensitivity in the study. Buffaloes were sampled during the annual BTB SICTT testing in the Hluhluwe-iMfolozi-Park (KwaZulu-Natal, South Africa) during June 2013. A total of 35 animals were subjected to the SICTT, 13 of these tested positive and one showed an inconclusive reaction. CMI-based assays (BOVIGAM® 1G (B1G) and BOVIGAM® 2G (B2G)) as well as a serological assay (IDEXX TB ELISA) were used to further investigate and compare immune responsiveness. Thirteen SICTT positive buffaloes and one inconclusive reactor were slaughtered and a post-mortem (PM) examination was conducted to confirm BTB. Lesions characteristic of BTB were found in 8/14 animals (57.1%). Test results of individual assays were compared with serial and parallel test interpretation and the sensitivity was calculated as a percentage of test positives out of the number of SICTT positive animals with granulomatous lesions (relative sensitivity). The B1G assay showed the highest individual sensitivity (100%; 8/8) followed by the B2G assay (75%; 6/8) and the IDEXX TB ELISA (37.5%; 3/8). Therefore, using in parallel interpretation, any combination with the B1G showed a sensitivity of 100% (8/8), whereas combinations with the B2G showed a 75% sensitivity (6/8). Out of the 21 SICTT negative animals, 7 animals showed responsiveness in the B2G or IDEXX TB ELISA. In conclusion, this study has shown that the BOVIGAM® IFN-γ assay had the highest test performance.

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...

Risk factors for bovine tuberculosis in low incidence regions related to the movements of cattle

BackgroundBovine tuberculosis (bTB) remains difficult to eradicate from low incidence regions partly due to the imperfect sensitivity and specificity of routine intradermal tuberculin testing. Herds with unconfirmed reactors that are incorrectly classified as bTB-negative may be at risk of spreading disease, while those that are incorrectly classified as bTB-positive may be subject to costly disease eradication measures. This analysis used data from Scotland in the period leading to Officially Tuberculosis Free recognition (1) to investigate the risks associated with the movements of cattle from herds with different bTB risk classifications and (2) to identify herd demographic characteristics that may aid in the interpretation of tuberculin testing results.ResultsFrom 2002 to 2009, for every herd with confirmed bTB positive cattle identified through routine herd testing, there was an average of 2.8 herds with at least one unconfirmed positive reactor and 18.9 herds with unconfirmed inconclusive reactors. Approximately 75% of confirmed bTB positive herds were detected through cattle with no known movements outside Scotland. At the animal level, cattle that were purchased from Scottish herds with unconfirmed positive reactors and a recent history importing cattle from endemic bTB regions were significantly more likely to react positively on routine intradermal tuberculin tests, while cattle purchased from Scottish herds with unconfirmed inconclusive reactors were significantly more likely to react inconclusively. Case-case comparisons revealed few demographic differences between herds with confirmed positive, unconfirmed positive, and unconfirmed inconclusive reactors, which highlights the difficulty in determining the true disease status of herds with unconfirmed tuberculin reactors. Overall, the risk of identifying reactors through routine surveillance decreased significantly over time, which may be partly attributable to changes in movement testing regulations and the volume of cattle imported from endemic regions.ConclusionsAlthough the most likely source of bTB infections in Scotland was cattle previously imported from endemic regions, we found indirect evidence of transmission within Scottish cattle farms and cannot rule out the possibility of low level transmission between farms. Further investigation is needed to determine whether targeting herds with unconfirmed reactors and a history of importing cattle from high risk regions would benefit control efforts.

Bacteriological and molecular detection of Mycobacterium bovis in cattle with inconclusive results to intradermal tuberculin tests

A dairy herd (77 cows) from Rio de Janeiro, Brazil, with a history of tuberculosis infection was tested by a comparative cervical test (CCT). Seventeen cows were reactive and seven were inconclusive (swelling ≥ 2.0 mm and ≤ 3.9 mm, respectively). All of these 24 cows were slaughtered and necropsied ; samples from lungs and lymph nodes were collected for multiplex polymerase chain reaction (PCR) and culturing. Infection was confirmed in 23/24 (95.8%) of the slaughtered animals (five by culturing, four by PCR, and 14 by both tests). All cows with inconclusive results at CCT were confirmed as infected. Although slaughter of inconclusive reactor cows is not mandatory in many countries, our study provided evidence to support the slaughter of these cows, at least during an outbreak.

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 >2 mm and between 1 and 4 mm > 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 >2 mm, and from 1 to 4 mm 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.

Avaliação do tratamento com isoniazida em bovinos infectados naturalmente pelo Mycobacterium bovis

Foi adotado o tratamento intermitente da tuberculose com isoniazida em 240 bovinos de um rebanho naturalmente infectado pelo M. bovis. No início do tratamento 36,6% dos animais foram reagentes positivos e 2,9% inconclusivos. Todos os animais do rebanho receberam isoniazida por via oral em doses de 25mg/kg de peso vivo, administrada três vezes por semana, às segundas, quartas e sextas-feiras, durante 10 meses, perfazendo 120 doses. A cura de 98,9% dos animais tratados foi verificada por meio da dessensibilização alérgica realizada pela tuberculinização e controle bacteriológico de 39 animais abatidos. O tratamento neste rebanho não resultou em seleção de cepas resistentes a isoniazida, comprovada pelo teste de sensibilidade a essa droga, realizado em cultura de M. bovis isolada de um animal não curado.

An improved enzyme linked immunosorbent assay (ELISA) for the detection of porcine serum antibodies against Mycoplasma hyopneumoniae

An ELISA for the detection of serum antibodies to Mycoplasma hyopneumoniae in pigs and based on a 43 kDa purified protein derived from the cytoplasmic membrane of M. hyopneumoniae strain J is described. This ELISA (MHPP ELISA) was compared with another recently described (Auspharm ELISA, Sheldrake and Romalis 1992) that is based on column-purified sonicated proteins of strain J. Using sample to negative ELISA ratios of 3 and 4 as cutoffs for inconclusive and positive reactors respectively (compared to 2 and 3 for the Auspharm ELISA), the two tests had high specificity (MHPP 99.6%; Auspharm 100%) in 280 SPF pigs. In 176 pigs from commercial herds with endemic M. hyopneumoniae, the MHPP ELISA showed a higher sensitivity than the Auspharm ELISA in both high lung score (LS ≥ 5) (85.5% vs. 69.9%) and low lung score (0 < LS < 5) (57.9% vs. 49%) pigs when the positive cutoff for each test was selected. The sensitivity when the inconclusive cutoff was selected was similar in both tests (85%; 85.7%) when low and high lung score pigs were pooled. Although the MHPP also gave more positive reactors in 36 pigs from M. hyopneumoniae-infected herds with no lung pathology at slaughter than the Auspharm ELISA (11 vs. 4), the total number of inconclusive and positive reactors in these pigs was similar for both tests (18 vs. 14). The MHPP ELISA gave significantly higher ELISA ratios in infected pigs (up to 17.9) than the Auspharm ELISA (up to 9), and earlier seroconversion in naturally-infected (6–8 weeks vs. 9–10 weeks) and experimentally-infected pigs (2–4 weeks vs. 4–6 weeks post infection).

Practical application of bovine tuberculin PPD in testing cattle in Great Britain

A national tuberculin testing trial was carried out in 8151 cattle in 132 herds due for normal herd testing and distributed throughout the 11 animal health administrative regions of England, Scotland and Wales. The numbers selected from each region were in approximately equal proportions to the total cattle populations of the regions and comprised a representative sample of the national herd. The standard intradermal comparative tuberculin test using Weybridge avian and human purified protein derivatives (PPD) was carried out on one side of the neck, and Rotterdam bovine PPD (1 X 0 mg per ml) was injected on the opposite side in the usual 'mammalian' site. Of the cattle tested, 9 X 4 per cent were classified as positive reactors to a single intradermal injection of the bovine tuberculin applying the official EEC interpretation for tuberculin tests. A single intradermal injection of human PPD would have resulted in a significantly higher proportion (10 X 4 per cent) of cattle being designated for removal. The rules of interpretation developed by Lesslie and Hebert (1975) for a comparative tuberculin test using avian and bovine PPDs, when applied to this sample, classified 0 X 80 per cent as inconclusive reactors compared with 0 X 58 per cent for the avian/human comparative test. The avian/bovine test also classified as positive reactors 0 X 12 per cent of the cattle tested, whilst the avian/human test classified as positive 0 X 05 per cent of cattle in routine herd tests during the three-year period 1972 to 1974.