Assessment of in vivo and in vitro tuberculosis diagnostic tests in Mycobacterium caprae naturally infected caprine flocks

Mycobacterium tuberculosis complex detection in rural goat herds in South Africa using Bayesian latent class analysis

Animal tuberculosis affects a wide range of domestic and wild animal species, including goats (Capra hircus). In South Africa, Mycobacterium tuberculosis complex (MTBC) testing and surveillance in domestic goats is not widely applied, potentially leading to under recognition of goats as a potential source of M. bovis spread to cattle as well as humans and wildlife. The aim of this study was to estimate diagnostic test performance for four assays and determine whether M. bovis infection was present in goats sharing communal pastures with M. bovis positive cattle in the Umkhanyakude district of Northern Zululand, KwaZulu Natal. In 2019, 137 M. bovis-exposed goats were screened for MTBC infection with four diagnostic tests: the in vivo single intradermal comparative cervical tuberculin test (SICCT), in vitro QuantiFERON®-TB Gold (QFT) bovine interferon-gamma release assay (IGRA), QFT bovine interferon gamma induced protein 10 (IP-10) release assay (IPRA), and nasal swabs tested with the Cepheid GeneXpert® MTB/RIF Ultra (GXU) assay for detection of MTBC DNA. A Bayesian latent class analysis was used to estimate MTBC prevalence and diagnostic test sensitivity and specificity. Among the 137 M. bovis-exposed goats, positive test results were identified in 15/136 (11.0%) goats by the SICCT; 4/128 (3.1%) goats by the IPRA; 2/128 (1.6%) goats by the IGRA; and 26/134 (19.4%) nasal swabs by the GXU. True prevalence was estimated by our model to be 1.1%, suggesting that goats in these communal herds are infected with MTBC at a low level. Estimated posterior means across the four evaluated assays ranged from 62.7% to 80.9% for diagnostic sensitivity and from 82.9% to 97.9% for diagnostic specificity, albeit estimates of the former (diagnostic sensitivity) were dependent on model assumptions. The application of a Bayesian latent class analysis and multiple ante-mortem test results may improve detection of MTBC, especially when prevalence is low. Our results provide a foundation for further investigation to confirm infection in communal goat herds and identify previously unrecognized sources of intra- and inter-species transmission of MTBC.

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.

Effect of the Inoculation Site of Bovine and Avian Purified Protein Derivatives (PPDs) on the Performance of the Intradermal Tuberculin Test in Goats From Tuberculosis-Free and Infected Herds

The single and comparative intradermal tuberculin (SIT and CIT) tests are used for the ante-mortem diagnosis of caprine tuberculosis (TB). The tuberculin injection site has been associated with a different performance of the test in cattle. In contrast to that required in cattle in Europe (cervical injection), it can be carried out in the scapular region in goats. Nevertheless, there are no previous data concerning the effect of the injection site on the performance of the test in goats. The aim of the present study was to evaluate the effect of two different inoculation sites (cervical and scapular) on the performance of the SIT/CIT tests. This was done by intradermally inoculating 309 goats from two infected herds and one TB-free herd with both avian and bovine PPDs in the mid-cervical and scapular regions. None of the animals from the TB-free herd had positive reactions, and the number of reactors was not significantly higher, regardless of the inoculation site, in the high and low prevalence herds. However, significantly higher increases in skin fold thickness were observed on the cervical site when compared to the scapular site after the avian and bovine PPD inoculations in the TB-free herd (p < 0.001) and after the bovine PPD injection in the high prevalence herd (p = 0.003). The presence of clinical signs was also more evident on the cervical site when using avian and bovine PPDs in the high prevalence herd (p < 0.01). In contrast, increases in higher skin fold thickness were observed on the scapular site when compared to the cervical site after the bovine and avian PPD inoculations were employed in the low prevalence herd (p < 0.01). These results suggest that the cervical injection of PPDs may improve the sensitivity of the intradermal tuberculin test in high TB prevalence caprine herds, mainly owing to the increased presence of local clinical signs and a better performance of the CIT test. Moreover, specificity was not affected when using standard interpretations, although further analyses in a great number of herds are required in order to confirm these findings.

Spoligotype-specific risk of finding lesions in tissues from cattle infected by Mycobacterium bovis

Background

Although the pathogenic effect of members of the Mycobacterium tuberculosis complex in susceptible hosts is well known, differences in clinical signs and pathological findings observed in infected animals have been reported, likely due to a combination of host and pathogen-related factors. Here, we investigated whether Mycobacterium bovis strains belonging to different spoligotypes were associated with a higher risk of occurrence of visible/more severe lesions in target organs (lungs and/or lymph nodes) from infected animals. A large collection of 8889 samples belonging to cattle were classified depending on the presence/absence of tuberculosis-like lesions and its degree of severity. All samples were subjected to culture irrespective of the presence of lesions, and isolates retrieved were identified and subjected to spoligotyping. The association between the presence/severity of the lesions and the isolation of strains from a given spoligotype was assessed using non-parametric tests and Bayesian mixed multivariable logistic regression models that accounted for origin (region and herd) effects.

Results

Results suggested a difference in severity in lesioned samples depending on the strain’s spoligotype. An association between specific spoligotypes and presence of lesions was observed, with a higher risk of finding lesions in animals infected with strains with spoligotypes SB0120, SB0295 and SB1142 compared with SB0121, and in those coming from certain regions in Spain.

Conclusions

Our results suggest that strains belonging to certain spoligotypes may be associated with a higher probability in the occurrence of gross/macroscopic lesions in infected cattle, although these observational findings should be confirmed in further studies that allow accounting for the effect of other possible confounders not considered here, and ultimately through experimental studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-02848-3.

Long-term efficacy of BCG vaccination in goat herds with a high prevalence of tuberculosis

Vaccination of goats against tuberculosis (TB) has been promoted as an ancillary tool for controlling the disease in infected livestock herds. A three-year trial to assess the efficacy of BCG vaccine was carried out in five goat herds. At the beginning of the trial (month 0), all animals were tested for TB using thee different diagnostic tests. Animals negative to all tests were vaccinated with BCG and all replacement goat kids were also systematically vaccinated throughout the trial. All animals were tested by Interferon-gamma release assay (IGRA) using vaccine compatible reagents at months 6, 12, 24, and 36. The risk factors for TB infection were also evaluated. At the end of the study, four out of five farms showed variable reductions of the initial prevalence (93.5%, 28.5%, 23.2%, and 14.3% respectively), and an overall incidence reduction of 50% was observed in BCG vaccinated goats, although adult vaccinated goats showed higher incidences than vaccinated goat kids. The unvaccinated positive animals remaining in herds and adult BCG vaccinated goats significantly enhanced the risk of infection in vaccinated animals. A systematic vaccination of goats with BCG, together with the removal of positive unvaccinated animals, may contribute to reducing the TB prevalence in goat herds.

Accuracy of tuberculosis diagnostic tests in small ruminants: A systematic review and meta-analysis

Tuberculosis (TB) in small ruminants is a neglected disease despite its major impact on goat and sheep production and the global public health. The awareness of the role of small ruminants in the epidemiology of animal TB has increased in the last two decades, however, there is a lack of standardization of procedures and robust quantitative estimates on the accuracy of diagnostic TB tests in the scientific literature. To address this knowledge gap, all the available information regarding the use of ante-mortem diagnostic techniques in small ruminants was collected and summarized through a systematic review process. Furthermore, a random-effects meta-analysis was conducted to separately estimate the sensitivity (Se) and specificity (Sp) of cell-based tests among the retrieved studies in goats. Studies included in the meta-analysis were also evaluated using the Quality Assessment of Diagnostic Accuracy Studies included in systematic reviews adapted for animal diagnostic tests (VETQUADAS). Median pooled Se estimates of the single intradermal tuberculin (SIT) test (ranged from 0.51 to 0.59), the comparative intradermal tuberculin (CIT) test (ranged from 0.30 to 0.50) and the interferon-gamma (IFN-γ) release assay (IGRA) (ranged from 0.66 to 0.72) were lower than that reported previously in cattle, regardless the interpretation criteria and the reporting of MAP infection or vaccination. However, the specificity was adequate for all the tests (ranged from 0.95 to 0.99), except for the SIT test in MAP vaccinated herds (ranged from 0.78 to 0.90). This study provides an overview of the accuracy of diagnostic tests for TB in goats, however, the considerable between-study heterogeneity found hampered the conclusive interpretation of the pooled Se and Sp estimates. Therefore, further studies in small ruminants are necessary to optimize the diagnostic Se, which could help to design effective control strategies, accelerate the eradication of TB in these species and harmonize test procedures.

Evaluation of P22 Antigenic Complex for the Immuno-Diagnosis of Tuberculosis in BCG Vaccinated and Unvaccinated Goats

Current eradication strategies of tuberculosis (TB) in goats mainly rely on the single intradermal tuberculin test (SIT) and single intradermal cervical comparative tuberculin tests (SICCTs). TB vaccination has been proposed as a cost-effective option in high-prevalence herds or countries where economic compensation for the slaughter of positive animals is not affordable. However, TB vaccination compromises the efficiency of tuberculin-based diagnostic tests. In this study, the performance of a new diagnostic platform, based on the P22 antigenic complex, was assessed for skin test (ST), interferon-gamma release assay (IGRA), and serology under different TB scenarios. The sensitivity (Se) of diagnostic tests was assessed in TB-infected goats from the same farm (herd A, N = 77). The specificity (Sp) was assessed in two TB-negative farms (both vaccinated against paratuberculosis): one TB unvaccinated (herd B, N = 77) and another vaccinated with bacille Calmette-Guérin (BCG) (herd C, N = 68). The single (s) P22-IGRA showed the highest Se among IGRA tests (91%), and the comparative (c) P22-ST showed the highest Sp (100% in herd B and 98% in herd C). Combined interpretation of techniques enabled the best diagnostic performances. Combining the SICCT + sP22-IGRA improved Se (97%) compared to SICCT + tuberculin-based IGRA (95%), with a reduction of Sp (95 and 100%, respectively). Besides, combination of P22-ELISA with cP22-ST or SICCT elicited a similar performance in the non-vaccination context (Se: 94 and 95%; Sp: 95 and 95%, respectively), but Sp was significantly higher for the combination with cP22-ST compared to SICCT in the TB vaccination context (95 and 79%, respectively). The combination of serological tests based on P22 and MPB83 showed higher complementarity and improved 13 percentage points the Se of P22-ELISA alone. These findings suggest that either cell-mediated or antibody-based diagnostic techniques, using the P22 antigen complex, can contribute to improve the immunodiagnostics of TB in goats under different TB control strategies.

Evaluation of single and comparative intradermal tuberculin tests for tuberculosis eradication in caprine flocks in Castilla y León (Spain)

Goats can act as reservoirs for tuberculosis (TB) infection. The main etiological agents of TB in goats are Mycobacterium caprae and Mycobacterium bovis and they infect also a wide range of domestic and wild animals and humans. Control programmes based mainly on the application of single and comparative intradermal tuberculin (SIT and SCIT respectively) tests are being implemented in certain regions of Spain with a high density of caprine flocks as Castilla y León, including goats with epidemiological relationship with cattle. The aim of this study was to evaluate the performance of the intradermal tests in naturally TB-infected caprine flocks from this region. The study was performed using data from 17,450 goats in 54 different flocks that were classified as TB-infected in the control programmes executed in 2010 and 2011. Data from 1237 goats from 7 dairy flocks depopulated after the first intradermal testing were used to estimate the sensitivity (Se) using bacteriology as the gold-standard. Overall Se of the SIT test using the severe interpretation was 43.9% (CI 95%, 40.4-47.4) and decreased to 38.8% (CI 95%, 35.5-42.3) using the standard interpretation. Overall Se of the SCIT test ranged between 21.3% (CI 95%, 17.6-25.4) and 7% (CI 95%, 4.9-9.8) depending of the interpretation criteria. A significant weak positive correlation was found between age and skin fold thickness (Spearman's test p<0.05). Results from this study yielded, in general, low Se values probably due the systematic detection and slaughter of reactors as a consequence of the eradication programme in previous years or the presence of factors that may interfere in the diagnosis. Therefore, these results suggest the necessity of including ancillary diagnostic tools and/or strict interpretation criteria to maximize detection of positive animals in infected settings.

Experimental aerosol Mycobacterium bovis model of infection in goats

The use of animal models is essential in testing the efficacy for novel therapies against tuberculosis (TB). Calves and non-human primates are examples of large animal models currently used to test TB vaccine efficacy but these animals are difficult and very expensive to house under high containment conditions. The goat may represent an effective but less expensive alternative for testing prototype vaccines against TB. Goats are susceptible to Mycobacterium bovis, Mycobacterium caprae and Mycobacterium tuberculosis infection. Aerosolized bacteria are the most common source of natural infection in humans and the primary site of natural infection is the respiratory tract. We developed a simple procedure for infecting goats with M. bovis by aerosol exposure. After 8 and 12 weeks of infection the goats were euthanized, post-mortem analysis was performed, and all exposed animals presented TB compatible lesions in the lung and associated lymph nodes. Selected lung lesions and respiratory lymph nodes were evaluated and cultured for bacteriological and histological analysis. The present work shows a reliable new animal model of aerosol infection to be used in the understanding of TB disease and development of new therapies.

Evaluation of the performance of cellular and serological diagnostic tests for the diagnosis of tuberculosis in an alpaca (Vicugna pacos) herd naturally infected with Mycobacterium bovis

Tuberculosis (TB) in llamas and alpacas has gained importance in recent years since they are imported into the European Union mainly for serving as pets and for production of natural fibre. The intradermal tuberculin test has been widely used for diagnosis of TB in these species showing lack of sensitivity (Se) although little information has been previously reported evaluating the effect on its performance of different PPD inoculation sites and time of readings. Moreover, different cost-effective serological assays have been developed in the recent years for TB diagnosis in camelids obtaining a variety of results and, for this reason, new assays still being developed. The main objectives of this study were: (1) to evaluate the performance of the intradermal tuberculin test using different inoculation sites (axillary, prescapular and cervical) and times of reading (72 and 120 h) and (2) to test a novel serological assay based on MPB83 antigen in a Mycobacterium bovis naturally infected alpaca herd in Spain. In regards to skin test, single intradermal tuberculin (SIT) test at the prescapular site and reading at 72 h showed the highest proportion of test-positive-culture positive animals among all culture positive animals (T+/C+), ranging from 53.8% (95% CI, 37.2-69.9) to 80% (95% CI, 44.4-97.5) using a more stringent interpretation than typically prescribed although, in general, low T+/C+ was achieved using both SIT and single comparative intradermal tuberculin (SCIT) tests alone. T+/C+ of the serological assay increased using samples collected 15-30 days after PPD injection [76.9% (95% CI, 60.7-88.9) - 100% (95% CI, 69.2-100)]. The best results of T+/C+ were obtained applying in parallel the most sensitive SIT test and serology using samples collected 15-30 days after PPD inoculation [90% (95% CI, 55.5-99.7)-100% (95% CI, 69.2-100)]. Therefore implementation of serology in parallel with the most sensitive skin test could maximize the detection of infected animals.

Effects of vaccination against paratuberculosis on tuberculosis in goats: diagnostic interferences and cross-protection

Background

Most countries carrying out campaigns of bovine tuberculosis (TB) eradication impose a ban on the use of mycobacterial vaccines in cattle. However, vaccination against paratuberculosis (PTB) in goats is often allowed even when its effect on TB diagnosis has not been fully evaluated. To address this issue, goat kids previously vaccinated against PTB were experimentally infected with TB.

Results

Evaluation of interferon-γ (IFN-γ) secretion induced by avian and bovine tuberculins (PPD) showed a predominant avian PPD-biased response in the vaccinated group from week 4 post-vaccination onward. Although 60% of the animals were bovine reactors at week 14, avian PPD-biased responses returned at week 16. After challenge with M. caprae, the IFN-γ responses radically changed to show predominant bovine PPD-biased responses from week 18 onward. In addition, cross-reactions with bovine PPD that had been observed in the vaccinated group at week 14 were reduced when using the M. tuberculosis complex-specific antigens ESAT-6/CFP-10 and Rv3615c as new DIVA (differentiation of infected and vaccinated animals) reagents, which further maintained sensitivity post-challenge. Ninety percent of the animals reacted positively to the tuberculin cervical comparative intradermal test performed at 12 weeks post-infection. Furthermore, post-mortem analysis showed reductions in tuberculous lesions and bacterial burden in some vaccinated animals, particularly expressed in terms of the degree of extrapulmonary dissemination of TB infection.

Conclusions

Our results suggest a degree of interference of PTB vaccination with current TB diagnostics that can be fully mitigated when using new DIVA reagents. A partial protective effect associated with vaccination was also observed in some vaccinated animals.

Experimental model of tuberculosis in the domestic goat after endobronchial infection with Mycobacterium caprae

Caprine tuberculosis (TB) has increased in recent years, highlighting the need to address the problem the infection poses in goats. Moreover, goats may represent a cheaper alternative for testing of prototype vaccines in large ruminants and humans. With this aim, a Mycobacterium caprae infection model has been developed in goats. Eleven 6-month-old goats were infected by the endobronchial route with 1.5 × 10(3) CFU, and two other goats were kept as noninfected controls. The animals were monitored for clinical and immunological parameters throughout the experiment. After 14 weeks, the goats were euthanized, and detailed postmortem analysis of lung lesions was performed by multidetector computed tomography (MDCT) and direct observation. The respiratory lymph nodes were also evaluated and cultured for bacteriological analysis. All infected animals were positive in a single intradermal comparative cervical tuberculin (SICCT) test at 12 weeks postinfection (p.i.). Gamma interferon (IFN-γ) antigen-specific responses were detected from 4 weeks p.i. until the end of the experiment. The humoral response to MPB83 was especially strong at 14 weeks p.i. (13 days after SICCT boost). All infected animals presented severe TB lesions in the lungs and associated lymph nodes. M. caprae was recovered from pulmonary lymph nodes in all inoculated goats. MDCT allowed a precise quantitative measure of TB lesions. Lesions in goats induced by M. caprae appeared to be more severe than those induced in cattle by M. bovis over a similar period of time. The present work proposes a reliable new experimental animal model for a better understanding of caprine tuberculosis and future development of vaccine trials in this and other species.