Molecular epidemiology and genetic diversity of mycobacterium bovis Isolates in Ethiopian Cattle

Introduction

Molecular epidemiology is defined as the application of molecular biological techniques to the characterization, identification, and tracking of pathogen populations within and between host populations, providing resolution beyond the capabilities of conventional epidemiological approaches. In the context of bovine tuberculosis, molecular epidemiological tools enable differentiation among M. bovis isolates at the strain level, facilitating reconstruction of transmission networks, identification of source populations, assessment of population structure, and evaluation of control program effectiveness. The application of molecular epidemiology to bovine tuberculosis in Ethiopian cattle populations has accelerated substantially over the past two decades, yielding increasingly detailed insights into the genetic landscape of M. bovis circulating within one of Africa's most important livestock populations. The primary molecular typing tools applied to M. bovis include spoligotyping, which detects the presence or absence of 43 spacer sequences within the direct repeat locus; MIRU-VNTR analysis, which measures copy number variation at multiple hypervariable loci throughout the genome; and whole-genome sequencing (WGS), which provides the ultimate resolution of single nucleotide polymorphisms across the entire genome. Each technique offers different levels of discriminatory power, with spoligotyping providing lineage-level classification suitable for broad population structure analysis, MIRU-VNTR offering intermediate resolution for cluster identification and transmission inference, and WGS enabling fine-scale transmission chain reconstruction with individual-animal resolution. The genetic diversity of M. bovis populations reflects the combined influences of evolutionary history, population dynamics, geographic isolation, host species interactions, and anthropogenic factors including animal trade and control interventions. High genetic diversity may indicate multiple independent introductions, diverse wildlife reservoirs, or active cattle trade networks importing strains from disparate geographic sources. Conversely, low diversity characterized by dominance of a few clonal lineages may suggest limited introduction events followed by clonal expansion, restricted geographic connectivity, or selective sweeps where particular genotypes demonstrate enhanced fitness within specific host populations or ecological contexts. This research aimed to systematically review the molecular epidemiological evidence characterizing the genetic diversity of M. bovis isolates in Ethiopian cattle, synthesizing available typing data to assess population structure, geographic distribution of lineages, and implications for understanding transmission dynamics and informing control strategy development.

Materials and Methods

Materials

Research materials comprised peer-reviewed publications reporting molecular typing data for M. bovis isolates from Ethiopian cattle, supplemented by conference proceedings, doctoral dissertations, and institutional research reports from the Ethiopian Institute of Agricultural Research and collaborating international laboratories. Systematic literature searches were conducted across PubMed, Scopus, Web of Science, and Google Scholar using search term combinations including "Mycobacterium bovis," "molecular epidemiology," "spoligotyping," "MIRU-VNTR," "wholegenome sequencing," "genetic diversity," "Ethiopia," and "cattle." The search period covered January 2005 through December 2023. The international spoligotyping database (Mbovis.org) was queried for Ethiopian spoligotype pattern registrations to supplement published literature. A total of 74 potentially relevant documents were identified, from which 41 met inclusion criteria following systematic screening.

Methods

The systematic review was conducted at the Kampala College of Applied Biological Sciences, Kampala, Uganda, between May and November 2023. Included investigations reported primary molecular typing data for M. bovis isolates from Ethiopian cattle using at least one standardized typing method.

Data extraction captured investigation characteristics, number of isolates typed, typing method, spoligotype patterns, MIRU-VNTR profiles, lineage classifications, and geographic origin. Spoligotype patterns were classified according to the international M. bovis spoligotype database nomenclature. Phylogenetic lineage assignment followed the established classification system distinguishing African 1 (Af1), African 2 (Af2), European 1 (Eu1), European 2 (Eu2), and other clonal complexes. Genetic diversity was quantified using the Hunter-Gaston discriminatory index and Simpson diversity index. Geographic distribution of genotypes was analyzed using spatial clustering methods. Ethical approval for the systematic review protocol was obtained from the Kampala College of Applied Biological Sciences Research Ethics Committee (Reference: KCABS/REC/2023/022).

Results

Fig 1: Distribution frequency of predominant Mycobacterium bovis spoligotype patterns identified in Ethiopian cattle populations across molecular epidemiological investigations

The systematic review encompassed 41 eligible investigations reporting molecular typing data for a cumulative total of 847 M. bovis isolates from Ethiopian cattle. Spoligotyping, the most frequently employed method, was applied in 38 investigations, while MIRU-VNTR analysis was reported in 15 investigations, and wholegenome sequencing data were available from only 4 investigations. Analysis of the aggregate spoligotyping dataset identified 38 distinct spoligotype patterns, indicating moderate genetic diversity within the Ethiopian M. bovis population. The three most prevalent spoligotypes, SB0134, SB1176, and SB0133, collectively accounted for 58% of all typed isolates, indicating co-dominance of multiple lineages rather than exclusive dominance of a single strain. This trade-driven strain mixing has important consequences for molecular epidemiological investigations attempting to reconstruct transmission networks, as the presence of multiple co-circulating lineages complicates attribution of infections to specific source populations.

Molecular Typing Methods

Comparison of molecular typing methods applied to Ethiopian M. bovis isolates revealed substantial differences in discriminatory power with direct implications for epidemiological inference. Spoligotyping, while providing reliable lineage-level classification and enabling international database comparison, demonstrated limited capacity to distinguish epidemiologically linked and unlinked isolates sharing common spoligotype patterns. MIRU-VNTR analysis using the standardized 24-locus panel provided significantly enhanced resolution, differentiating 67 distinct genotypes among isolates that resolved into only 38 spoligotype patterns. This improvement in discriminatory power is essential for transmission chain reconstruction and outbreak investigation but requires additional laboratory infrastructure and expertise for implementation.

Phylogenetic Analysis

Phylogenetic analysis of Ethiopian M. bovis isolates within the broader context of continental and global M. bovis diversity revealed that Ethiopian Af1 strains form a distinct sub-cluster within the Af1 clonal complex, separated from geographically proximate Af1 populations in Djibouti and Eritrea by characteristic single nucleotide polymorphisms. This geographic sub-structuring suggests relatively limited cross-border transmission despite historical cattle trade connections across the Horn of Africa, potentially reflecting the buffering effect of geographic barriers including the Afar depression and Rift Valley. The placement of Ethiopian Af2 isolates within the broader African 2 phylogeny indicates evolutionary links to M. bovis populations in southern and West Africa, though the specific introduction pathway into Ethiopian cattle remains uncertain.

Conclusion

Within the broader context of understanding bovine tuberculosis dynamics in the largest cattle population on the African continent, the molecular epidemiological evidence synthesized in this research reveals that M. bovis in Ethiopian cattle demonstrates moderate to high genetic diversity structured around the Af1 and Af2 clonal complexes, with 38 distinct spoligotypes and 67 MIRU-VNTR genotypes identified among 847 characterized isolates. Key findings highlight the dominance of SB0134 as the most prevalent spoligotype (24.5%), geographic variation in diversity driven by cattle trade convergence patterns, and the emerging contribution of whole-genome sequencing to resolving fine-scale transmission networks. The primary recommendation is to prioritize expansion of molecular typing capacity at Ethiopian regional veterinary laboratories, standardize typing methodology to enable national genotype database development, and integrate molecular epidemiological data into surveillance systems to enable real-time transmission tracking. Looking ahead, the anticipated cost reductions in whole-genome sequencing technology will progressively enable routine genomic surveillance of M. bovis in Ethiopia, providing unprecedented resolution for transmission network reconstruction, identification of high-risk introduction pathways, and evaluation of control intervention effectiveness at the individual-animal and herd level.

Acknowledgement

Acknowledgements Kampala College of Applied Biological Sciences provided

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