Abstract
Mycobacterium bovis causes tuberculosis (TB) in cattle, which in turn can transmit the pathogen to humans. Tuberculosis in dairy cattle is of particular concern where the consumption of raw milk and dairy products is customary. Baja California (BCA), Mexico, presents high prevalence of TB in both cattle and humans, making it important to investigate the molecular epidemiology of the disease in the region. A long-term study was undertaken to fully characterize the diversity of M. bovis genotypes circulating in dairy cattle, cheese and humans in BCA by whole-genome sequencing (WGS). During a 2-year period, 412 granulomatous tissue samples were collected from local abattoirs and 314 cheese samples were purchased from local stores and vendors in BCA and sent to the laboratory for mycobacterial culture, histology, direct PCR and WGS. For tissue samples M. bovis was recovered from 86.8%, direct PCR detected 90% and histology confirmed 85.9% as mycobacteriosis-compatible. For cheese, M. bovis was recovered from 2.5% and direct PCR detected 6% of the samples. There was good agreement between diagnostic tests. Subsequently, a total of 345 whole-genome SNP sequences were obtained. Phylogenetic analysis grouped these isolates into 10 major clades. SNP analysis revealed putative transmission clusters where the pairwise SNP distance between isolates from different dairies was ≤3 SNP. Also, human and/or cheese isolates were within 8.45 (range 0–17) and 5.8 SNP (range 0–15), respectively, from cattle isolates. Finally, a comparison between the genotypes obtained in this study and those reported previously suggests that the genetic diversity of M. bovis in BCA is well-characterized, and can be used to determine if BCA is the likely source of M. bovis in humans and cattle in routine epidemiologic investigations and future studies. In conclusion, WGS provided evidence of ongoing local transmission of M. bovis among the dairies in this high-TB burden region of BCA, as well as show close relationships between isolates recovered from humans, cheese, and cattle. This confirms the need for a coordinated One Health approach in addressing the elimination of TB in animals and humans. Overall, the study contributes to the knowledge of the molecular epidemiology of M. bovis in BCA, providing insight into the pathogen's dynamics in a high prevalence setting.
Highlights
Bovine tuberculosis, most commonly caused by Mycobacterium bovis, is characterized by the formation of granulomas in the lymph nodes and lungs of infected individuals, though other organs may be affected [1, 2]
The overall detection rate for M. bovis from tissue samples was 86.9%, with lymph nodes achieving the highest rate at 91.7% (Table 1)
726 WGS sequences, including cattle and cheese have been collected from this region, and based on the significant overlap, we suggest that the genetic diversity of M. bovis in Baja California (BCA) is wellrepresented
Summary
Bovine tuberculosis (bTB), most commonly caused by Mycobacterium bovis, is characterized by the formation of granulomas in the lymph nodes and lungs of infected individuals, though other organs may be affected [1, 2]. It is an OIE (World Organization for Animal Health) reportable disease that infects a broad variety of mammals including humans. Infection in cattle can occur through direct contact by the inhalation of infected aerosols from sick animals and through oral ingestion of contaminated milk, fodder and pastures [3]. Significant success has been achieved, but wildlife reservoirs have challenged total eradication [5, 6]; in least-developed or developing countries, the lack of economic compensation for culled animals due to test and slaughter strategies, or the absence of such strategies, complicates control [7, 8]
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