The impact of BCG strains and repeat vaccinations on immunodiagnostic tests in Eurasian badgers (Meles meles)

Bacille Calmette-Guerin (BCG) is a potential tool in the control of Mycobacterium bovis in European badgers (Meles meles). A five year Test and Vaccinate or Remove (TVR) research intervention project commenced in 2014 using two BCG strains (BCG Copenhagen 1331 (Years 1-3/ BadgerBCG) and BCG Sofia SL2222 (Years 4-5). Badgers were recaptured around 9 weeks after the Year 5 vaccination and then again a year later. The Dual-Path Platform (DPP) Vet TB assay was used to detect serological evidence of M. bovis infection. Of the 48 badgers, 47 had increased Line 1 readings (MPB83 antigen) between the Year 5 vaccination and subsequent recapture. The number of BCG Sofia vaccinations influenced whether a badger tested positive to the recapture DPP VetTB assay Line 1 (p < 0.001) while the number of BadgerBCG vaccinations did not significantly affect recapture Line 1 results (p = 0.59). Line 1 relative light units (RLU) were more pronounced in tests run with sera than whole blood. The results from an in_house MPB83 ELISA results indicated that the WB DPP VetTB assay may not detect lower MPB83 IgG levels as well as the serum DPP VetTB assay. Changes in interferon gamma assay (IFN-γ) results were seen in 2019 with significantly increased CFP-10 and PPDB readings. Unlike BadgerBCG, BCG Sofia induces an immune response to MPB83 (the immune dominant antigen in M. bovis badger infection) that then affects the use of immunodiagnostic tests. The use of the DPP VetTB assay in recaptured BCG Sofia vaccinated badgers within the same trapping season is precluded and caution should be used in badgers vaccinated with BCG Sofia in previous years. The results suggest that the DPP VetTB assay can be used with confidence in badgers vaccinated with BadgerBCG as a single or repeated doses.

Seroconversion against antigen MPB83 in badgers (Meles meles) vaccinated with multiple doses of BCG strain Sofia

Serological diagnosis of Mycobacterium bovis infection in badgers (Meles meles) has relied primarily on antibody recognition of MPB83, a sero-dominant antigen of M. bovis. Most vaccine studies in badgers to date have used the Bacille Calmette-Guerin (BCG) Danish strain, a low producer of MPB83. Due to a supply shortage of the BCG Danish strain, the BCG Sofia SL222 strain has been considered as an alternative vaccine. This strain is a high producer of MPB83 raising the possibility that vaccinated animals will test sero-positive in diagnostic assays that use this antigen. In this study we vaccinated a group of eleven badgers with BCG Sofia SL222 by injection via the intramuscular route and a booster vaccine dose was similarly delivered at 12 weeks and 64 weeks. Primary vaccination did not result in measured detection of antibodies against MPB83 in any badger during the first twelve weeks using serum or whole blood tested by the Dual Path Platform (DPP) VetTB, however, MPB83 antibodies were detected in a semi-quantitative ELISA assay. Following delivery of booster BCG at 12 weeks and 64 weeks, antibody responses against MPB83 were recorded in badgers using whole blood and serum on DPP VetTB and by ELISA. At all time points, vaccination was also associated with the in vitro production of gamma interferon (IFN-γ) following stimulation of lymphocytes with bovine and avian tuberculin (PPD) but not with MPB83 or M. bovis specific antigen CFP-10. The results indicate that serological diagnosis of tuberculosis using tests that target MPB83 may be compromised if badgers are repeatedly vaccinated with BCG Sofia.

Development of a lateral flow immunochromatography test for the rapid detection of bovine tuberculosis

Detection of specific antibodies would be a useful test strategy for bovine tuberculosis (bTB) as a complement to the single skin test. We developed a lateral flow immunochromatography (LFIC) test for rapid bTB detection based on the use of a conjugate of gold nanoparticles with a recombinant G protein. After evaluating 3 Mycobacterium bovis (MB) antigens: ESAT-6, CFP-10 and MPB83 for the control line, we selected MPB83 given it was the most specific. The performance of the test was analyzed with 820 bovine sera, 40 sera corresponding to healthy animals, 5 sera from animals infected with Mycobacterium avium subsp. paratuberculosis (MAP) and 775 sera of animals from herds with bTB. All these sera were also submitted to a validated bTB-ELISA using whole-cell antigen from MB. From the 775 sera of animals from herds with bTB, 87 sera were positive by the bTB-ELISA, 45 were positive by LFIC and only 5 animals were positives by skin test (TST). To confirm bTB infection in the group of TST (-), bTB-ELISA (+) and LFIC (+) animals, we performed postmortem examination in 15 randomly selected animals. Macroscopically, these 15 animals had numerous small and large yellow-white granulomas, characteristic of bTB, and the infection was subsequently confirmed by PCR in these tissues with lesions (gold standard). No false positive test result was detected with the developed LFIC either with the sera from healthy animals or from animals infected with MAP demonstrating that it can be a useful technique for the rapid identification of animals infected with bTB.

Strong antibody responses to Mycobacterium bovis infection in domestic pigs and potential for reliable serodiagnostics

Mycobacterium bovis (M. bovis), the main cause of animal tuberculosis (TB), can infect a wide variety of domestic and wild animal species, including suids. Suids may serve as reservoir hosts or disease sentinels in different scenarios. Accurate detection of M. bovis infection in pigs is important for TB control programs. Although previous studies have shown the value of serological assays for screening animal populations, the diagnostic accuracy was considered suboptimal. In this study, we used Dual Path Platform (DPP) technology and multi-antigen print immunoassay (MAPIA) to characterize antigen recognition profiles and temporal antibody responses. Four M. bovis experimentally infected pigs developed an early antibody response to antigen MPB83, with a peak in IgG levels starting around 4-6 weeks post-inoculation, although none of the pigs developed antibodies to fusion protein CFP10/ESAT6 within 16 weeks of the experiment. Three of four experimentally infected pigs developed antibody responses before detectable antigen-specific interferon gamma responses. Naturally infected pigs with gross lesions containing viable M. bovis showed IgM (19/40 infected animals) and IgG (39/40) antibody responses to both MPB70/MPB83 (39/40) and CFP10/ESAT6 (34/40). Using MPB70/MPB83 antigen alone to measure IgG antibody levels by DPP assay, an estimated test sensitivity was 97.5 % (95 % CI: 85.3-99.9 %). None of the 57 negative control samples had detectable IgM or IgG antibodies to either of the two test antigens in DPP assay, suggesting an estimated specificity of 100 % (95 % CI: 92.1-100.0 %) in pigs. MAPIA showed robust IgG reactivity to multiple protein antigens of M. bovis in the naturally infected pigs. The results demonstrate that serological assays which detect IgG antibodies to MPB83 have high sensitivity and specificity for accurate detection of M. bovis infection in pigs. Further investigations should be done to validate anti-MPB70/MPB83 antibodies as a reliable serodiagnostic biomarker for TB diagnosis in pigs.

Evaluation of five serologic assays for bovine tuberculosis surveillance in domestic free-range pigs from southern Spain

In countries where bovine tuberculosis (bTB) is still prevalent the contact among different animal species in extensive systems contributes to the circulation of Mycobacterium bovis (M. bovis) and other members of the Mycobacterium tuberculosis complex (MTC). Thus, free-range pigs can develop subclinical infections and may contribute to disease spread to bovine and wildlife. Serodiagnosis has been proposed as a screening tool for detecting infected pig herds; however, the value of this method to obtain an accurate diagnosis in this species is still not clear. In this study, sensitivity (Se) and specificity (Sp) estimates of four ELISAs and a lateral flow immunochromatographic antibody assay based on different M. bovis antigens, including MPB70 and MPB83 proteins, were evaluated in naturally infected domestic free-range pigs. For this purpose, submandibular lymph nodes and blood samples from 217 pigs from both TB-infected and historically negative farms were sampled at slaughterhouse and analysed by gross examination, histopathology, bacteriological culture and qPCR. Se and Sp estimates of the 5 evaluated assays ranged from 66.1% to 78% (CI₉₅ from 52.6 to 87.7%) and from 98.9% to 100% (CI₉₅ from 93.8 to 100%), respectively. Results of our study suggest that all the evaluated assays could be used as a first screening tool to conduct bTB surveillance in domestic pigs at population level; however, animals from seropositive herds should later be surveyed by other methods in order to reduce false negative results.

Serological Follow-up of Tuberculosis in a Wild Boar Population in Contact with Infected Cattle

There is an increasing concern in several European countries over the role that tuberculosis (TB)-infected wild boar may play in the progress of bovine TB eradication campaigns. In 2004, as a consequence of the detection of a TB focus in wild boar from a National Game Reserve (NGR) located in southern Catalonia, a surveillance programme based on post-mortem inspection for detection of macroscopic TB-like lesions (TBLL) was initiated in the affected area. The source of infection for wild boar was linked to a tuberculous cattle herd located in the same area. Besides, the results of the surveillance programme in wild boar were used for the validation of an indirect enzyme-linked immunosorbent assay (ELISA) specific for Mycobacterium tuberculosis complex (MTBC) IgG antibodies. Using this ELISA, a seven-year serological study of MTBC in wild boar from the NGR was conducted in 173 animals (93 adults, 44 juveniles-yearlings and 36 piglets) culled between 2004 and 2010. ELISA results and presence of TBLL showed excellent agreement for adult and juvenile wild boar (Kappa index = 0.85; 95% CI: 0.76-0.95). Of the thirty-eight adults, yearlings and juveniles classified as positives by the ELISA, 34 (89%) showed TBLL at necropsy. In contrast, none of the ELISA-positive wild boar piglets (n = 20) showed TBLL, suggesting the detection of early antibody responses to the infection. Overall, this study contributes to the knowledge of wild boar humoral responses to MTBC. The results also highlight the usefulness of this serological test for wild boar TB surveillance.