The effectiveness of parallel gamma-interferon testing in New Zealand's bovine tuberculosis eradication programme

Enhancing bovine tuberculosis screening at Dhaka city in Bangladesh: Integrating gamma interferon blood test as ancillary testing with tuberculin skin test

Tuberculin skin test (TST) is the standard method for screening of bovine tuberculosis (bTB). However, gamma interferon blood test has been introduced in the bTB control program as an ancillary testing with TST in many countries of the world. The objective of this study was to recommend this screening test as an ancillary testing with TST for field application in Bangladesh. In this study 577 cattle of different age, sex and breeds from twenty nine (29) cattle herds were examined to determine skin response against bTB through single intradermal comparative tuberculin test (SICTT) that comprised of positive (n = 81), inconclusive (n = 44) and negative (n = 452) animals. Of which 74 animals that included positive (n = 63), inconclusive (n = 8) and negative (n = 3) animals were taken under this study. Blood samples were collected in heparinized tube and stimulated overnight with bovine and avian purified protein derivatives (PPDs) for the secretion of gamma interferon, and measured via sandwich ELISA. Cohen's kappa statistics was performed for the evaluation of agreement between the two tests. The agreement obtained between two tests was fair (Kappa agreement, K = 24.0%, 95% CI = 16.9-30.5%, P = 0.037). Of positive (n = 63), inconclusive (n = 8) and negative (n = 3) status of animals at SICTT, 82.54% (n = 52), 62.50% (n = 5), and 33.33% (n = 1) were found to be bTB positive respectively through this ancillary test. This test notably corroborates to TST result. A considerable number of inconclusive TB status animals were found to be positive through this gamma interferon assay. Therefore, this test could be used as an ancillary test with TST to maximize the proportion of bTB estimation in the infected cattle herd for early detection of zoonotic tuberculosis in Bangladesh before transmission at the animal-human interface.

Gamma-interferon assay for the ancillary diagnosis of bovine tuberculosis in dairy cattle in urban and adjacent areas of Dhaka city, Bangladesh

Background and Aim

Bovine tuberculosis (bTB) is an infectious disease of cattle, mainly caused by Mycobacterium bovis. This study aimed to evaluate the efficacy of the interferon-gamma (IFN-γ) assay and single-intradermal comparative tuberculin test (SICTT) in detecting bTB.

Materials and Methods

In an earlier study, 150 positive, 83 inconclusive, and 480 negative animals from 24 cattle herds were screened using SICTT. From these groups, 125 positive, 17 inconclusive, and six negative animals were subsequently verified using the IFN-γ assay. Single-intradermal comparative tuberculin test outcomes were interpreted according to standard guidelines, whereas blood samples were collected and stimulated with purified protein derivatives. Sandwich enzyme-linked immunosorbent assay was used to measure secreted IFN-γ. Concordant and Bayesian latent class analyses were performed to evaluate test performance.

Results

Results from the IFN-γ assay revealed that 83.2%, 64.7%, and 16.67% of the animals were positive in the SICTT-positive, inconclusive, and negative animal categories, respectively. Sensitivity (SE) and specificity (SP) of SICTT were 83.9% (95% confidence interval [CI]: 77.4-90.1) and 95.7% (95% CI: 86.9-99.7), respectively. Sensitivity and SP for the IFN-γ assay were 78.9% (95% CI: 71.9-85.4) and 83.9% (65.9-95.9), respectively. The use of both tests in parallel increases the SE of bTB detection (~94%), compared with SICTT alone.

Conclusion

Use of the IFN-γ assay with SICTT in parallel, predominantly on cattle demonstrating an inconclusive SICTT outcome, boosts bTB detection rate in low resource settings.

Assessing the potential impact of applying a higher sensitivity test to selected cattle populations for the control of bovine tuberculosis in England

Bovine tuberculosis (bTB) continues to be the costliest, most complex animal health problem in England. The effectiveness of the test-and-slaughter policy is hampered by the imperfect sensitivity of the surveillance tests. Up to half of recurrent incidents within 24 months of a previous one could have been due to undetected infected cattle not being removed. Improving diagnostic testing with more sensitive tests, like the interferon (IFN)-gamma test, is one of the government's top priorities. However, blanket deployment of such tests could result in more false positive results (due to imperfect specificity), together with logistical and cost-efficiency challenges. A targeted application of such tests in higher prevalence scenarios, such as a subpopulation of high-risk herds, could mitigate against these challenges. We developed classification machine learning algorithms (using 80% of 2012-2019 bTB surveillance data as the training set) to evaluate the deployment of IFN-gamma testing in high-risk herds (i.e. those at risk of an incident in England) in two testing data sets: i) the remaining 20% of 2012-19 data, and ii) 2020 bTB surveillance data. The resulting model, classification tree analysis, with an area under a receiver operating characteristic (ROC) curve (AUC) > 95, showed a 73% sensitivity and a 97% specificity in the 2012-2019 test dataset. Used on 2020 data, it predicted eight percent (3 510 of 41 493) of eligible active herds as at-risk of a bTB incident, the majority of them (66% or 2 328 herds) experiencing at least one. Whilst all predicted at-risk herds could have preventive measures applied, the additional application of IFN-gamma test in parallel interpretation to the statutory skin test, if the risk materialises, would have resulted in 8 585 additional IFN-gamma reactors detected (a 217% increase over the 2 710 IFN-gamma reactors already detected by tests carried out). Only 18% (330 of 1 819) of incidents in predicted high-risk herds had the IFN-gamma test applied in 2020. We therefore conclude that this methodology provides a better way of directing the application of the IFN-gamma test towards the high-risk subgroup of herds. Classification tree analysis ensured the systematic identification of high-risk herds to consistently apply additional measures in a targeted way. This could increase the detection of infected cattle more efficiently, preventing recurrence and accelerating efforts to achieve eradication by 2038. This methodology has wider application, like targeting improved biosecurity measures in avian influenza at-risk farms to limit damage to the industry in future outbreaks.

Evaluation of the performance of the IFN-γ release assay in bovine tuberculosis free herds from five European countries

The diagnostic methods for granting and maintenance of the official tuberculosis-free (OTF) status and for intra-Community movement of cattle are the tuberculin skin tests (single or comparative) and the interferon-γ (IFN-γ) release assay (IGRA). However, until now, IGRAs have been primarily applied in infected farms in parallel to the skin test to maximize the number of infected animals detected. Therefore, an evaluation of the performance of IGRAs in OTF herds to assess whether if their specificity is equal to or higher than that of the skin tests is needed. For this, a panel of 4365 plasma samples coming from 84 OTF herds in six European regions (five countries) was assembled and analysed using two IGRA kits, the ID Screen^® Ruminant IFN-g (IDvet) and the Bovigam™ TB Kit (Bovigam). Results were evaluated using different cut-offs, and the impact of herd and animal-level factors on the probability of positivity was assessed using hierarchical Bayesian multivariable logistic regression models. The percentage of reactors ranged from 1.7 to 21.0% (IDvet: S/P ≥ 35%), and 2.1-26.3% (Bovigam: OD_bovis-OD_PBS ≥ 0.1 and OD_bovis-OD_avium ≥ 0.1) depending on the region, with Bovigam disclosing more reactors in all regions. The results suggest that specificity of IGRAs can be influenced by the production type, age and region of origin of the animals. Changes in the cut-offs could lead to specificity values above 98-99% in certain OTF populations, but no single cut-off yielding a sufficiently high specificity (equal or higher than that of skin tests) in all populations was identified. Therefore, an exploratory analysis of the baseline IFN-γ reactivity in OTF populations could help to assess the usefulness of this technique when applied for the purpose of maintaining OTF status.

Bovine Tuberculosis Antemortem Diagnostic Test Agreement and Disagreement in a Naturally Infected African Cattle Population

The interferon-gamma (IFN-γ) assay and single comparative cervical skin test (SCITT) are used to estimate bovine tuberculosis (bTB) prevalence globally. Prevalence estimates of bTB, caused by Mycobacterium bovis, are poorly quantified in many Sub-Saharan African (SSA) cattle populations. Furthermore, antemortem diagnostic performance can vary at different stages of bTB pathogenesis and in different cattle populations. In this study, we aim to explore the level of agreement and disagreement between the IFN-γ assay and SCITT test, along with the drivers for disagreement, in a naturally infected African cattle population. In, 2013, a pastoral cattle population was sampled using a stratified clustered cross-sectional study in Cameroon. A total of 100 pastoral cattle herds in the North West Region (NWR) and the Vina Division (VIN) were sampled totalling 1,448 cattle. Individual animal data and herd-level data were collected, and animals were screened using both the IFN-γ assay and SCITT. Serological ELISAs were used to detect exposure to immunosuppressing co-infections. Agreement analyses were used to compare the performance between the two bTB diagnostic tests, and multivariable mixed-effects logistic regression models (MLR) were developed to investigate the two forms of IFN-γ assay and SCITT binary disagreement. Best agreement using the Cohen's κ statistic, between the SCITT (>2 mm) and the IFN-γ assay implied a ‘fair-moderate' agreement for the NWR [κ = 0.42 (95%CI: 0.31–0.53)] and ‘poor-moderate' for the VIN [κ = 0.33 (95% CI: 0.18–0.47)]. The main test disagreement was the animals testing positive on the IFN-γ assay and negative by the SCITT. From MLR modeling, adults (adults OR: 7.57; older adults OR = 7.21), females (OR = 0.50), bovine leucosis (OR = 2.30), and paratuberculosis positivity (OR = 6.54) were associated with IFN-γ-positive/SCITT-negative disagreement. Subsets to investigate diagnostic test disagreement for being SCITT-positive and IFN-γ-negative also identified that adults (adults OR = 15.74; older adults OR = 9.18) were associated with IFN-γ-negative/SCITT-positive disagreement. We demonstrate that individual or combined use of the IFN-γ assay and SCITT can lead to a large variation in bTB prevalence estimates. Considering that animal level factors were associated with disagreement between the IFN-γ assay and SCITT in this study, future work should further investigate their impact on diagnostic test performance to develop the approaches to improve SSA prevalence estimates.

Review on Bovine Tuberculosis: An Emerging Disease Associated with Multidrug-Resistant Mycobacterium Species

Bovine tuberculosis is a serious infectious disease affecting a wide range of domesticated and wild animals, representing a worldwide economic and public health burden. The disease is caused by Mycobacteriumbovis and infrequently by other pathogenic mycobacteria. The problem of bovine tuberculosis is complicated when the infection is associated with multidrug and extensively drug resistant M. bovis. Many techniques are used for early diagnosis of bovine tuberculosis, either being antemortem or postmortem, each with its diagnostic merits as well as limitations. Antemortem techniques depend either on cellular or on humoral immune responses, while postmortem diagnosis depends on adequate visual inspection, palpation, and subsequent diagnostic procedures such as bacterial isolation, characteristic histopathology, and PCR to reach the final diagnosis. Recently, sequencing and bioinformatics tools have gained increasing importance for the diagnosis of bovine tuberculosis, including, but not limited to typing, detection of mutations, phylogenetic analysis, molecular epidemiology, and interactions occurring within the causative mycobacteria. Consequently, the current review includes consideration of bovine tuberculosis as a disease, conventional and recent diagnostic methods, and the emergence of MDR-Mycobacterium species.

Prevalence of bovine tuberculosis in dairy cattle in China during 2010-2019: A systematic review and meta-analysis

BACKGROUND

Bovine tuberculosis (bTB), caused by members of the Mycobacterium tuberculosis complex bacteria, mainly Mycobacterium bovis (M. bovis), is a major threat to public health and economic development. There has been no systematic epidemiological assessment concerning bTB in dairy cattle in China.

METHODOLOGY/PRINCIPAL FINDINGS

Literature related to bTB in China was retrieved from China National Knowledge Infrastructure (CNKI), PubMed, ScienceDirect, VIP Chinese Journals Database, and Wan Fang Database to build the first meta-analysis for estimating the prevalence and infection moderators of bTB in dairy cattle in China. A total of 100 relevant studies published from 2010 to 2019 were included. We estimated the overall prevalence of bTB was 2.4% (95% CI: 2.1-2.8) during this decade. In the sampling year subgroup, the prevalence was lowest in 2017 or later at 0.8% (95% CI: 0.3-1.5). The lowest prevalence was 0.7% (95% CI: 0.5-1.0) in Northwestern China. The lowest prevalence was 2.1% (95% CI: 1.8-2.5) using SIT test. Heifer cows had the highest prevalence, which was 27.1% (95% CI: 9.7-49.2). The prevalence in scale farming was 3.7% (95% CI: 3.1-4.3), significantly higher than that in free-range farming (1.7%, 95% CI: 1.1-2.4). The prevalence of bTB was highest in summer at 4.0% (95% CI: 1.7-7.0). In addition, the influence of different geographical factors (altitude, longitude, latitude, precipitation, temperature, humidity) on the prevalence was analyzed.

CONCLUSIONS/SIGNIFICANCE

The results showed that bTB was widespread in China but has been gradually reduced through concerted national intervention. It is suggested that different countries should formulate corresponding prevention and control measures according to the epidemic situation in its cattle industry. Enhanced monitoring of warm and humid areas may play an important role in reducing the incidence of bTB. In addition, when large-scale breeding is promoted, attention should be paid to standardizing breeding management and improving animal welfare to reduce the prevalence of bTB in cattle.

Risk factors associated with testing positive for tuberculosis in high-yielding Holstein cows

Tuberculosis remains a significant infectious disease of farm animals and humans worldwide. The objective of this study was to assess various risk factors associated with testing positive for bovine tuberculosis (bTB) in high-yielding Holstein cows in an intensive dry-lot dairy operation. In a retrospective observational study, 9312 records from Holstein cows from a large dairy herd in northern Mexico were used. The incidence rate of lactating cows reactor to bTB was 7.3 cases/100 cow years (95% CI = 6.7-7.9%). Multiple logistic regression models indicated that cows with total milk yield during the first lactation >10,200 kg were 1.3 times (95% confidence intervals (CI) for odds ratio (OR) = 1.2-1.6) more likely to be detected as bTB reactors than cows with total milk yield <10,200 kg (8.2 vs. 6.4%; P < 0.01). The risk of being reactor to bTB increased with x̅ total milk yield/lactation >11,500 kg (OR=2.7, 95% CI = 2.3-3.3; 10.4 vs. 4.2%) compared with cows with x̅ total milk yield/lactation <11,500 kg. Cows with >10,200 kg 305-day milk yield in the last lactation was associated (P < 0.01) with 1.3 times higher odds (95% CI for OR = 1.1-1.5) of being classed as reactors than cows with <10,200 kg. Cows with peak milk yield >48 kg were 1.9 times (95% CI for OR = 1.6-2.2) more likely to be reactor to bTB than cows with peak milk yield <48 kg (9.2 vs. 5.1%; P < 0.01). Cows with either puerperal metritis (OR = 0.07, 95% CI = 0.5-0.9) or carrying twins (OR = 0.05, 95% CI = 0.01-0.19) had a protective role for being reactor to bTB. This study showed that increased milk production was associated with a higher risk of becoming positive to tuberculin skin test in high-yielding Holstein cows.

Intra-vitam Diagnosis of Tuberculosis in Pigs: Concordance Between Interferon-Gamma Release Assay and Comparative Tuberculin Skin Test

The role of pigs in the maintenance of bovine tuberculosis caused by Mycobacterium bovis has been demonstrated in many settings; however, the current control programs usually do not state any intra-vitam diagnostic procedure in this species, as for the cattle. Carcass inspection has shown to be insufficient to detect infection in swine; thus, the assessment of intradermal tuberculin test and interferon-gamma release assay (IGRA) in this species is mandatory. The current study compares the performances of the single intradermal comparative cervical tuberculin (SICCT) test and IGRA. A total of 628 Nebrodi Black pigs raised in free-roaming farms were subjected to the two tests simultaneously. Besides, 124 animals underwent postmortem examination for the detection of tuberculous lesions and isolation of mycobacteria from target organs. The two tests showed a concordance of 94.42% with a Cohen's k coefficient of 0.786 and McNemar chi-square of 4.83 (P = 0.03). Slightly lower levels of concordance (90.32%) between SICCT and IGRA were obtained in the group of 124 animals, with a Cohen's k = 0.797 and McNemar chi-squared value of 0.69 with a non-significant P = 0.41. Moreover, the results showed how IGRA tends to result positive in higher rates, mostly when non-tuberculous mycobacteria (NTM) were isolated, suggesting a possible impairment of specificity in the event of coinfections in the swine. In conclusion, the results obtained support the possibility of the strategic use of IGRA or SICCT in combination or alternatively one to the other, particularly IGRA which showed lower specificity but has evident advantages over SICCT.

Introduction and Application of the Interferon-γ Assay in the National Bovine Tuberculosis Control Program in South Korea

Bovine tuberculosis is a chronic disease impacting both public health and the livestock industry. The interferon (IFN)-γ assay has been introduced as an ancillary test for diagnosing bovine tuberculosis to overcome limitations of the skin test. The objective of this study was to assess the IFN-γ assay in terms of diagnostics and as a nationwide surveillance program in South Korea. From 2012 to 2013, cattle (n = 120) with bovine tuberculosis and cattle (n = 426) from bovine tuberculosis free herds were subjected to the IFN-γ assay to evaluate the sensitivity and specificity of the assay, respectively, depending on various cut-offs (0-3.5). When optical density of the cut-off was 0.1, the sensitivity and specificity were found to be 81.7% (74.7-88.6) and 99.5% (98.9-100.0), respectively. After introducing the IFN-γ assay as part of the national control program, the IFN-γ assay and single caudal fold skin test data were collected from 47 regional veterinary services to compare the results of these two tests. Overall, the agreement between the IFN-γ assay and the single caudal fold skin test (n = 492,068) was 98.2%, and Cohen's kappa value for the two methods was 0.47. Serial and parallel use of the IFN-γ assay and skin test for the bovine tuberculosis control program were compared using samples (n = 91) from cattle confirmed as bovine tuberculosis positive in laboratories from 2014 to 2016. Parallel screening for bTB showed much higher sensitivity (86/91, 94.5%) than the following screening approaches: serial (47.2%, 43/91), single screening using CFT (63.7%, 58/91), or the IFN-γ assay (78.0%, 71/91). These results indicate that the IFN-γ assay and single caudal fold skin test are complementary to each other; therefore, parallel use of these two tests is considered a useful approach to reduce the prevalence of bovine tuberculosis in South Korea.

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.

Antigen-specific interferon-gamma release is decreased following the single intradermal comparative cervical skin test in African buffaloes (Syncerus caffer)

Effective disease management of wildlife relies on the strategic application of ante-mortem diagnostic tests for early identification and removal of M. bovis-infected animals. To improve diagnostic performance, interferon-gamma release assays (IGRAs) are often used in conjunction with the tuberculin skin test (TST). Since buffaloes are major maintenance hosts of M. bovis, optimal application of bovine TB diagnostic tests are especially important. We aimed to determine whether the timing of blood collection relative to the TST has an influence on IFN-γ production and diagnostic outcome in African buffaloes. Release of IFN-γ in response to bovine purified protein derivative (PPD), avian PPD and PC-HP^® and PC-EC^® peptides was measured by Bovigam^® and an in-house IGRA in a group of Bovigam^®-positive and - negative buffaloes at the time the TST was performed and three days later. There was significantly lower IFN-γ release in response to these antigens post-TST in Bovigam^®-positive buffaloes, but no significant changes in Bovigam^®-negative buffaloes. Also, a significantly greater proportion of buffaloes were Bovigam^®-positive prior to the TST than three days later. We therefore recommend that blood samples for use in IGRAs be collected prior to or at the time the TST is performed to facilitate the correct identification of greater numbers of IGRA-positive buffaloes.

The History of In Vivo Tuberculin Testing in Bovines: Tuberculosis, a "One Health" Issue

Tuberculosis (TB) is more than 3 million years old thriving in multiple species. Ancestral Mycobacterium tuberculosis gave rise to multiple strains including Mycobacterium bovis now distributed worldwide with zoonotic transmission happening in both directions between animals and humans. M. bovis in milk caused problems with a significant number of deaths in children under 5 years of age due largely to extrapulmonary TB. This risk was effectively mitigated with widespread milk pasteurization during the twentieth century, and fewer young children were lost to TB. Koch developed tuberculin in 1890 and recognizing the possibility of using tuberculin to detect infected animals the first tests were quickly developed. Bovine TB (bTB) control/eradication programmes followed in the late nineteenth century/early twentieth century. Many scientists collaborated and contributed to the development of tuberculin tests, to refining and optimizing the production and standardization of tuberculin and to determining test sensitivity and specificity using various methodologies and injection sites. The WHO, OIE, and EU have set legal standards for tuberculin production, potency assay performance, and intradermal tests for bovines. Now, those using tuberculin tests for bTB control/eradication programmes rarely, see TB as a disease. Notwithstanding the launch of the first-ever roadmap to combat zoonotic TB, many wonder if bTB is actually a problem? Is there a better way of dealing with bTB? Might alternative skin test sites make the test “better” and easier to perform? Are all tuberculins used for testing equally good? Why have alternative “better” tests not been developed? This review was prompted by these types of questions. This article attempts to succinctly summarize the data in the literature from the late nineteenth century to date to show why TB, and zoonotic TB specifically, was and still is important as a “One Health” concern, and that the necessity to reduce the burden of zoonotic TB, to save lives and secure livelihoods is far too important to await the possible future development of novel diagnostic assays for livestock before renewing efforts to eliminate it. Consequently, it is highly probable that the tuberculin skin test will remain the screening test of choice for farmed livestock for the considerable future.

Successful Application of the Gamma-Interferon Assay in a Bovine Tuberculosis Eradication Program: The French Bullfighting Herd Experience

In the French Camargue region, where bovine tuberculosis had been enzootic for several years in bullfighting cattle herds, the gamma-interferon (IFN) assay was used since 2003 in parallel with the intradermal test in order to increase overall disease detection sensitivity in infected herds. This study presents the results of a field-evaluation of the assay during a 10-year period (2004–2014) of disease control and surveillance program and explores the particular pattern of IFN assay results in bullfight herds in comparison to cattle from other regions of France. The low sensitivity [59.2% (50.6; 67.3)] of IFN assay using the tuberculin stimulation could be related to the poor gamma-IFN production from bullfight cattle blood cells which is significantly lower than in animals of conventional breeds. The characteristics of the assay were progressively adapted to the epidemiological situation and the desired strategic applications. Data analysis with a receiver operating characteristic curve based on a simple S/P value algorithm allowed for the determination of a new cutoff adapted for a global screening, giving a high specificity of 99.9% results and a high accuracy of the assay. Having regularly risen to above 5% since 2005, with a peak around 10% in 2010, the annual incidence dropped to under 1% in 2014. The positive predictive value relative to the bacteriological confirmation evolved during the years, from 33% in 2009 to 12% during the last screening period, a normal trend in a context of decreasing prevalence. The estimated rate of false-positive reactions during screening campaigns was 0.67%, confirming the high specificity of the test, measured in bTB negative herds, in this epidemiological context. The proportion of false-positive reactions decreased with the age and was higher in males than in females. Although these results indicate that the IFN assay is accurate in the field, it also emphasizes great differences between interferon quantities produced by bullfight cattle blood samples compared to those of classical bovine breeds, which underlines the necessity to adapt the algorithms and combinations of the assay according to local epidemiological contexts.

Potential for rapid antibody detection to identify tuberculous cattle with non-reactive tuberculin skin test results

BACKGROUND

Bovine tuberculosis (TB) control programs generally rely on the tuberculin skin test (TST) for ante-mortem detection of Mycobacterium bovis-infected cattle.

RESULTS

Present findings demonstrate that a rapid antibody test based on Dual-Path Platform (DPP^®) technology, when applied 1-3 weeks after TST, detected 9 of 11 and 34 of 52 TST non-reactive yet M. bovis-infected cattle from the US and GB, respectively. The specificity of the assay ranged from 98.9% (n = 92, US) to 96.0% (n = 50, GB) with samples from TB-free herds. Multi-antigen print immunoassay (MAPIA) revealed the presence of antibodies to multiple antigens of M. bovis in sera from TST non-reactors diagnosed with TB.

CONCLUSIONS

Thus, use of serologic assays in series with TST can identify a significant number of TST non-reactive tuberculous cattle for more efficient removal from TB-affected herds.

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.