Bovine Tuberculosis: Diagnostic Advances and Wildlife Reservoirs

Introduction

Bovine tuberculosis (bTB) is a chronic infectious disease of cattle caused primarily by Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex (MTBC). The disease imposes substantial economic burdens on livestock industries worldwide through reduced productivity, carcass condemnation, trade restrictions, and costly test-and-slaughter programs. Recent meta-analyses have confirmed that bTB prevalence remains significant in many regions, with herd-level risk factors including large herd size, co-mingling with wildlife, and inadequate biosecurity measures [1]. Diagnosis of bTB relies on a combination of ante-mortem immunological assays and post-mortem molecular confirmation. Over the past decade, significant advances have been made in the sensitivity and specificity of these methods, while the role of wildlife reservoirs in perpetuating infection cycles has been increasingly recognized. This review examines the principal diagnostic platforms for bTB and the epidemiological importance of wildlife hosts, particularly the European badger (Meles meles) and various deer species.

Immunological Diagnostics: The Tuberculin Skin Test

The intradermal tuberculin test (ITT), also known as the tuberculin skin test (TST), remains the primary ante-mortem screening tool for bTB in cattle. The test measures a delayed-type hypersensitivity (DTH) response after intradermal injection of purified protein derivative (PPD) from M. bovis (bovine PPD) and, in comparative formats, avian PPD from M. avium. Swelling at the injection site is measured at 72 hours post-inoculation. The comparative test improves specificity by distinguishing responses due to M. avium complex or nontuberculous mycobacteria (NTM) [2]. The cut-off threshold for a positive reaction varies by national program; a skin thickness increase of 2 mm or more over the avian reaction is commonly used.

Despite its widespread deployment, the ITT has well-documented limitations. Sensitivity is approximately 80 to 90 percent in naturally infected cattle, but specificity can be compromised by cross-reactivity with NTM. A refresher training program for veterinary field technicians can improve the consistency of test application and interpretation, particularly in resource-limited settings [3]. The test also fails to detect early-stage infections (the "window period" of 3 to 6 weeks post-exposure) and may yield false negatives in anergic, severely diseased animals.

Interferon-Gamma Release Assays (IGRAs)

The interferon-gamma (IFN-γ) release assay offers a complementary approach that detects cell-mediated immune responses in whole blood stimulated with M. bovis-specific antigens such as ESAT-6 and CFP-10. These antigens are encoded within the region of difference 1 (RD1) of the MTBC genome and are absent from most NTM and M. bovis BCG vaccine strains. The assay quantifies IFN-γ production using a sandwich ELISA platform. Strain-dependent effects of ESAT-6 and CFP-10 on inflammasome activation have been demonstrated in bovine macrophages, which may influence host immune recognition and assay performance [4].

IGRAs have several advantages over the ITT. They require only a single blood sample, can be repeated without desensitization, and have higher sensitivity (often exceeding 90 percent) particularly in early infection. However, they are more expensive and require laboratory infrastructure. The combination of ITT and IGRA in parallel or serial testing regimens is now recommended by many veterinary authorities to maximize diagnostic accuracy. Recent studies have also described the use of MycoPac dual lateral flow assays for rapid field detection of antibodies, though these are less established than cellular assays [2].

Molecular Detection of Mycobacterium bovis

Molecular diagnostics have become indispensable for confirmatory identification of M. bovis from tissue samples, milk, and nasal swabs. Conventional PCR targeting the IS6110 insertion element is widely used, but this element may be present in low copy numbers in some M. bovis strains, necessitating the inclusion of additional targets such as IS1081 or RvD1 deletion-specific regions. Real-time quantitative PCR (qPCR) using TaqMan probes provides rapid quantification and has been validated for detection of M. bovis from lung tissue and lymph nodes [5].

A further advancement is the PCR-BIO/DIG-ELISA assay, which couples biotin-digoxigenin labeled PCR products with an ELISA-based detection step to enhance analytical sensitivity [6]. This approach is particularly useful when bacterial loads are low or when samples are degraded. MIRU-VNTR (mycobacterial interspersed repetitive unit-variable number tandem repeat) typing has emerged as the gold standard for genotyping M. bovis isolates. A 12-locus MIRU-VNTR scheme can discriminate between strains and trace transmission pathways within and between herds, as well as between livestock and wildlife [7]. Similar MIRU-VNTR approaches have been applied to M. tuberculosis isolates from human populations, but cross-species comparisons remain methodologically distinct [8].

The detection of nontuberculous mycobacteria in milk and nasal swabs from cattle that test positive by ITT or IGRA highlights the challenge of false-positive immunological results. Molecular assays that differentiate MTBC members from NTM are essential to avoid unnecessary culling [2]. TaqMan-based qPCR assays that specifically target M. bovis (e.g., targeting the pncA or oxytR genes) provide definitive species-level identification.

Wildlife Reservoirs and Transmission Dynamics

Wildlife species serve as maintenance hosts for M. bovis and play a critical role in the epidemiology of bTB in many countries. The European badger (Meles meles) is the principal wildlife reservoir in the United Kingdom and Ireland. Badgers shed M. bovis in urine, feces, sputum, and wound exudates, contaminating pasture and farm infrastructure. Recent fecal microbiome studies have shown that the gut microbial community of badgers varies with social group, age, and bTB infection status, suggesting that microbiome perturbations may influence susceptibility or shedding patterns [9].

Deer species, including white-tailed deer (Odocoileus virginianus) in North America and red deer (Cervus elaphus) in Europe, are also important reservoirs. Deer may develop pulmonary and non-pulmonary lesions, and their gregarious behavior facilitates direct contact transmission. In New Zealand, the brushtail possum (Trichosurus vulpecula) acts as a maintenance host. A cross-sectional study in wildlife-rich areas of South America found that human-animal contact with wild and domestic animals is frequent, providing pathways for spillover [10]. The interface between cattle and wildlife is of particular concern in areas where livestock graze on shared pastures or where supplemental feeding attracts wildlife.

Feline tuberculosis caused by M. bovis in domestic cats has been documented, including household outbreaks suggesting possible zoonotic transmission from cats to humans [11]. This underscores the multi-host nature of M. bovis and the need for a One Health surveillance approach. In Algeria, a five-year epidemiological survey of tuberculosis-like lesions at slaughterhouses revealed a substantial economic impact and highlighted the need for integrated surveillance across livestock and wildlife sectors [12]. Slaughterhouse-based post-mortem inspection in Nigeria has similarly emphasized the role of carcass-processing practices in meat quality and potential human exposure [13].

Comparative Diagnostics and Algorithmic Decision-Making

Integration of immunological and molecular tests requires a structured decision algorithm to optimize herd-level management. The following Mermaid diagram illustrates a typical diagnostic workflow for bTB in cattle:

flowchart TD
    A[Herd Screening with ITT], > B{Comparative ITT Result}
    B, >|Positive| C[Confirmatory IGRA]
    B, >|Negative| D[Continue Routine Surveillance]
    C, > E{IGRA Positive?}
    E, >|Yes| F[Slaughter and Tissue Sampling]
    E, >|No| G[Repeat ITT in 60 days]
    F, > H[Gross Pathology Examination]
    H, > I{Lesions Present?}
    I, >|Yes| J[PCR/qPCR for M. bovis]
    I, >|No| K[NTP or NTM?]
    J, > L[Species Confirmation + MIRU-VNTR]
    K, > M[Incubate for Culture]

Key performance parameters for each diagnostic method are summarized in Table 1.

Table 1. Comparative Performance of Diagnostic Platforms for Bovine Tuberculosis

N/A: Not applicable for sensitivity/specificity as this is a typing method, not a diagnostic screening test.

The advent of high-throughput sequencing and computational models (as described in the article Biological Foundation Models for Veterinary Virology: Predicting Host Tropism and Pathogenicity) is beginning to influence bTB research, particularly for predictive modeling of cross-species transmission. Similarly, antimicrobial resistance patterns in livestock-associated bacteria, as discussed in Antimicrobial Resistance in Livestock-Associated Staphylococcus aureus: Genomic Epidemiology and One Health Implications, provide a parallel framework for understanding genomic surveillance of M. bovis.

Genotyping and Strain Diversity

MIRU-VNTR analysis has revealed substantial genetic diversity among M. bovis isolates. A 12-locus panel applied to dairy products and cattle tissues in northwest Iran identified multiple genotypes, indicating multiple introductions and ongoing circulation within the region [7]. Similar diversity has been observed in M. tuberculosis isolates from human populations using the same typing scheme, but host-associated clustering patterns differ [8]. The discriminatory power of MIRU-VNTR is superior to that of IS6110 restriction fragment length polymorphism (RFLP) for M. bovis due to the typically low copy number of IS6110. The inclusion of additional loci, such as those targeting QUB and MtUB marker regions, can further refine typing resolution.

In M. bovis strains, comparative genomics has identified variable presence of the RD1 region, which includes esat-6 and cfp-10. These genes are critical for virulence and for the host immune response detected by IGRA. Strain-dependent differences in ESAT-6 and CFP-10 expression influence inflammasome activation in bovine macrophages, which may affect both pathogenesis and diagnostic assay performance [4]. The molecular detection of Mycobacterium orygis (a member of the MTBC that infects cattle and wildlife) using conventional and TaqMan qRT-PCR assays further expands the scope of diagnostic targets beyond M. bovis sensu stricto [5]. This species has been reported in South Asia and the Middle East and may be underdiagnosed where only M. bovis-specific tests are used.

Challenges in Slaughterhouse Surveillance

Abattoir surveillance is a cornerstone of bTB control programs, providing data on lesion prevalence and enabling sample collection for culture and molecular typing. A systematic review of slaughterhouse data from northern Algeria spanning 2019 to 2024 documented tuberculosis-like lesions in a considerable proportion of cattle, sheep, and goats, with significant economic losses [12]. In Nigerian municipal slaughterhouses, poor carcass-processing practices were identified as a risk factor for contamination and potential human exposure to M. bovis [13]. Lesions in cattle typically appear as caseous, calcified granulomas within the respiratory tract and associated lymph nodes. Detection of such lesions at post-mortem inspection, followed by confirmatory PCR, remains the definitive diagnostic pathway.

One persistent challenge is the inability of gross pathology to differentiate bTB lesions from those caused by NTM or other granulomatous diseases. Molecular tools are therefore essential. The use of a dual approach combining culture on selective media (e.g., Stonebrink or Lowenstein-Jensen) and PCR has been advocated [2, 6]. However, culture requires 4 to 8 weeks, limiting its utility for rapid decision-making. Molecular detection directly from tissue homogenates allows same-day results.

Role of Wildlife in Persistence and Spillover

Evidence from longitudinal studies demonstrates that wildlife reservoirs can maintain M. bovis infection even in the absence of cattle, complicating eradication efforts. Badgers in the UK have been culled in containment zones, but population reduction is controversial due to ethical concerns and potential perturbation effects that may increase ranging behavior. The fecal microbiome study by Meadows et al. [9] suggests that bTB infection in badgers alters gut microbial composition; this could influence host immune status or pathogen shedding. Further research is needed to determine whether microbiome modulation could be leveraged for disease control.

Deer populations in North America have been managed through reduced supplemental feeding and targeted culling. In Michigan, white-tailed deer are the primary reservoir, and M. bovis strains isolated from deer share genotypes with those from cattle, confirming bidirectional transmission [7]. The MIRU-VNTR profiles of deer isolates often match those from nearby cattle herds, emphasizing the importance of geographical clustering. In other regions, wild boar and feral pigs serve as reservoirs, with transmission to cattle occurring through direct contact or environmental contamination.

A cross-sectional study across Bolivia, Chile, and Guatemala assessed human-animal contact patterns and found that individuals in wildlife-rich areas frequently interact with both wild and domestic animals, increasing the risk of zoonotic pathogen exposure including M. bovis [10]. Such data inform public health risk assessments and highlight the need for integrated surveillance that spans livestock, wildlife, and human populations under a One Health framework.

Future Directions

The diagnostic landscape for bTB is evolving with the incorporation of point-of-care molecular platforms, digital PCR, and next-generation sequencing. These technologies promise faster turnaround times and higher discriminatory power. The development of portable, battery-operated qPCR instruments could enable field-based confirmation of suspect cases without the need for centralized laboratory transport. Computational modeling of transmission dynamics, as exemplified by African Swine Fever: Computational Models for Early Detection and Spread Prediction in Wild Boar Populations, could be adapted to bTB to predict spillover risk from wildlife to livestock.

In parallel, vaccine development remains an active area of research. While M. bovis BCG offers variable protection, novel vaccines based on attenuated M. bovis strains or subunit formulations may improve control. Diagnostic tests that can distinguish vaccinated from infected animals (DIVA) will be necessary if vaccination is adopted. The strain-dependent effects of ESAT-6 and CFP-10 on bovine innate immunity [4] inform the design of such DIVA-compatible antigens.

References

[1] Wang Q, Wang XN, Liu XQ, et al. Prevalence and risk factors of bovine tuberculosis in dairy cattle, 2020-2025: A systematic review and meta-analysis of available literature. Prev Vet Med. 2026.

[2] Pal S, Samanta S, Haque MZ, et al. Detection of nontuberculous mycobacteria in milk and nasal swab of cattle tested positive by tuberculin skin test, Interferon-γ release assay and MycoPac dual lateral flow assay. Antonie Van Leeuwenhoek. 2026.

[3] Rikhotso BO, Pretorius O, Thompson PN, et al. Benefits of a tuberculin skin testing refresher training programme for veterinary field technicians in the Bushbuckridge Municipality, South Africa. J S Afr Vet Assoc. 2026.

[4] Blanco FC, Vazquez CL, Bigi MM, et al. Mycobacterium bovis Strain-Dependent Effects of ESAT-6 and CFP-10 on Inflammasome Activation in Bovine Macrophages. Int J Mol Sci. 2026.

[5] Prajapati SK, Narang D, Chandra M, et al. Molecular detection of Mycobacterium orygis in lung tissues and lymph nodes using conventional and TaqMan qRT-PCR Assay. Vet J. 2026.

[6] Fabiano SN, Cislaghi AP, Canal AM, et al. Strategy for Improving the Diagnostic Efficacy of Bovine Tuberculosis Using a Novel PCR(BIO/DIG)-ELISA Assay. Vet Res Commun. 2026.

[7] Aminoleslami L, Hanifian S, Shayegh J, et al. Genetic diversity of Mycobacterium bovis in dairy products and cattle tissues from northwest Iran using a 12-locus MIRU-VNTR approach. Sci Rep. 2026.

[8] Yazdanmanesh M, Tadayon K, Kazemian H, et al. Molecular Characterization of Mycobacterium tuberculosis Isolates From West of Iran Using Mycobacterial Interspersed Repetitive Unit-Variable Number Tandem Repeats: A Cross-Sectional Study. Health Sci Rep. 2026.

[9] Meadows NML, Delahay RJ, McDonald RA, et al. Fecal Microbiome Varies With Social Group, Age and Bovine Tuberculosis Infection in the European Badger (Meles meles). Mol Ecol. 2026.

[10] Kuhn C, Radon K, Pérez Morales FM, et al. Human-animal contact and zoonotic exposure from wild and domestic animals: A cross-sectional study in wildlife-rich areas of Bolivia, Chile, and Guatemala. One Health. 2026.

[11] Peters H, Kaspers L, Brangsch H, et al. Outbreak of feline tuberculosis caused by Mycobacterium bovis in a German household, a possible domestic zoonosis. J Vet Diagn Invest. 2026.

[12] Balla EH, Besseboua O, Dergal NB, et al. Epidemiological Survey and Economic Impact of Ruminant Tuberculosis-like Lesions at Slaughterhouses in Two Areas of Northern Algeria (2019-2024): A One Health Assessment. Pathogens. 2026.

[13] Njoga EO, Kperegbeyi JI, Onwumere-Idolor OS, et al. Animal Welfare, Carcass-Processing Practices and Post-Mortem Lesions in Nigerian Municipal Slaughterhouses: Implications for Meat Quality and Public Health Security. Vet Sci. 2026.

[14] Chen M, Yan X, Zhao J, et al. Detection of Bovine Brucellosis Antibodies in Serum and Milk Using Quantum Dot Microspheres Immunochromatographic Assay. Microorganisms. 2026.

[15] Jafari-Rouhi AH, Tizfahm Tikmehdash H, Mosavari N, et al. Molecular identification of Burkholderia cepacia complex from people with cystic fibrosis in northwestern Iran: a research note. BMC Res Notes. 2026.