Abstract Background Zoonotic tuberculosis (zTB) is the transmission of Mycobacterium tuberculosis complex (MTBC) subspecies from animals to humans. zTB is generally quantified by determining the proportion of human isolates that are Mycobacterium bovis . Although India has the world’s largest number of human TB cases and the largest cattle population, where bovine TB is endemic, the burden of zTB is unknown. Methods To obtain estimates of zTB in India, a PCR-based approach was applied to sub-speciate positive MGIT® cultures from 940 patients (548 pulmonary, 392 extrapulmonary disease) at a large referral hospital in India. Twenty-five isolates of interest were subject to whole genome sequencing (WGS) and compared with 715 publicly available MTBC sequences from South Asia. Findings A conclusive identification was obtained for 939 samples; wildtype M. bovis was not identified (95% CI: 0 – 0.4%). There were 912 M. tuberculosis sensu stricto (97.0%, 95% CI: 95.7 – 98.0), 7 M. orygis (95% CI: 0.3 – 1.5%); 5 M. bovis BCG, and 15 non-tuberculous mycobacteria. WGS analysis of 715 MTBC sequences again identified no M. bovis (95% CI: 0 – 0.4%). Human and cattle MTBC isolates were interspersed within the M. orgyis and M. tuberculosis sensu stricto lineages. Interpretation M. bovis prevalence in humans is an inadequate proxy of zTB in India. The recovery of M. orygis from humans, together with the finding of M. tuberculosis in cattle, underscores the need for One Health investigations to assess the burden of zTB in countries with endemic bovine TB. Funding Bill & Melinda Gates Foundation, Canadian Institutes for Health Research
We thank Sven David Charles Parsons for highlighting the many unknowns concerning zoonotic tuberculosis and Mycobacterium orygis. We concur with the assessment that the evidence from the literature, including our Article,1Duffy SC Srinivasan S Schilling MA et al.Reconsidering Mycobacterium bovis as a proxy for zoonotic tuberculosis: a molecular epidemiological surveillance study.Lancet Microbe. 2020; 1: e66-e73Summary Full Text Full Text PDF PubMed Scopus (25) Google Scholar precludes a definitive understanding of whether M orygis is a marker of zoonosis, zooanthoponosis, or both. Whether cattle or humans (or other species, such as oryxes) serve as reservoir hosts for M orygis is unknown, highlighting the need for genomic epidemiological investigations to refine our understanding of the transmission dynamics of this pathogen. This is not only the case for M orygis, definitive knowledge of reservoir host and host-range remain unknown for most of the animal-associated lineages within the Mycobacterium tuberculosis complex, including those that have been reported to infect and cause tuberculosis in humans.2Rodríguez S Bezos J Romero B et al.Mycobacterium caprae infection in livestock and wildlife, Spain.Emerg Infect Dis. 2011; 17: 532-535Crossref PubMed Scopus (82) Google Scholar, 3Jurczynski K Lyashchenko KP Gomis D Moser I Greenwald R Moisson P Pinniped tuberculosis in Malayan tapirs (Tapirus indicus) and its transmission to other terrestrial mammals.J Zoo Wildl Med. 2011; 42: 222-227Crossref PubMed Scopus (22) Google Scholar Parsons notes there is no published evidence of direct animal to human transmission of M orygis. We add that there are no reports of human to human transmission of M orygis either. Accumulating evidence suggests that M orygis, like many other members of the M tuberculosis complex, is a multispecies pathogen.4Rahim Z Thapa J Fukushima Y et al.Tuberculosis Caused by Mycobacterium orygis in dairy cattle and captured monkeys in Bangladesh: a new scenario of tuberculosis in south Asia.Transbound Emerg Dis. 2017; 64: 1965-1969Crossref PubMed Scopus (26) Google Scholar Parsons cites that most M orygis cases are reported from humans and a variety of animals in captivity; but it remains unclear whether this reflects a sampling bias because free-living animals are also reported to be infected.5Thapa J Paudel S Sadaula A et al.Mycobacterium orygis–associated tuberculosis in free-ranging rhinoceros, Nepal, 2015.Emerg Infect Dis. 2016; 22: 570-572Crossref PubMed Scopus (19) Google Scholar Therefore, the host range of M orygis is not fully defined. This highlights a need for comprehensive genomic epidemiology investigations to establish the host range of M orygis and other animal-lineage members of the M tuberculosis complex and determine their potential for zoonotic and or zooanthroponotic infection in regions where they are endemic. As reflected by the title, the central message of our Article was to broaden the operational definition of zoonotic tuberculosis to include any animal lineage M tuberculosis complex subspecies capable of causing human disease.1Duffy SC Srinivasan S Schilling MA et al.Reconsidering Mycobacterium bovis as a proxy for zoonotic tuberculosis: a molecular epidemiological surveillance study.Lancet Microbe. 2020; 1: e66-e73Summary Full Text Full Text PDF PubMed Scopus (25) Google Scholar This recommendation was based on the observation of M orygis circulating in both humans and cattle, and the inability to recover M bovis from humans in a region where bovine tuberculosis is endemic with considerable opportunities for zoonotic transmission. This broader definition should sensitise medical and veterinary practitioners and laboratories to the M tuberculosis species that might be associated with zoonotic or zooanthroponotic infections and enable future definitive studies of the important questions raised by Parsons and in our Article. MAB reports grants from the Canadian Institutes for Health Research, during the conduct of this study. VK reports grants from the Bill & Melinda Gates Foundation, during the conduct of this study. All other authors declare no competing interests. Reconsidering Mycobacterium bovis as a proxy for zoonotic tuberculosis: a molecular epidemiological surveillance studyM bovis prevalence in humans is an inadequate proxy of zoonotic tuberculosis. The recovery of M orygis from humans highlights the need to use a broadened definition, including MTBC subspecies such as M orygis, to investigate zoonotic tuberculosis. The identification of M tuberculosis in cattle also reinforces the need for One Health investigations in countries with endemic bovine tuberculosis. Full-Text PDF Open AccessMycobacterium orygis: a zoonosis, zooanthroponosis, or both?Shannon Duffy and colleagues propose that the operational definition of zoonotic tuberculosis should be broadened to include disease caused by Mycobacterium orygis and that human M orygis infection should be regarded as evidence of zoonotic transmission.1 However, although human infection has often been assumed to be of animal origin there is no direct evidence of this.2 Full-Text PDF Open Access
BackgroundZoonotic tuberculosis is defined as human infection with Mycobacterium bovis. Although globally, India has the largest number of human tuberculosis cases and the largest cattle population, in which bovine tuberculosis is endemic, the burden of zoonotic tuberculosis is unknown. The aim of this study was to obtain estimates of the human prevalence of animal-associated members of the Mycobacterium tuberculosis complex (MTBC) at a large referral hospital in India.MethodsWe did a molecular epidemiological surveillance study of 940 positive mycobacteria growth indicator tube (MGIT) cultures, collected from patients visiting the outpatient department at Christian Medical College (Vellore, India) with suspected tuberculosis between Oct 1, 2018, and March 31, 2019. A PCR-based approach was applied to subspeciate cultures. Isolates identified as MTBC other than M tuberculosis or as inconclusive on PCR were subject to whole-genome sequencing (WGS), and phylogenetically compared with publicly available MTBC sequences from south Asia. Sequences from WGS were deposited in the National Center for Biotechnology Information Sequence Read Archive, accession number SRP226525 (BioProject database number PRJNA575883).FindingsThe 940 MGIT cultures were from 548 pulmonary and 392 extrapulmonary samples. A conclusive identification was obtained for all 940 isolates; wild-type M bovis was not identified. The isolates consisted of M tuberculosis (913 [97·1%] isolates), Mycobacterium orygis (seven [0·7%]), M bovis BCG (five [0·5%]), and non-tuberculous mycobacteria (15 [1·6%]). Subspecies were assigned for 25 isolates by WGS, which were analysed against 715 MTBC sequences from south Asia. Among the 715 genomes, no M bovis was identified. Four isolates of cattle origin were dispersed among human sequences within M tuberculosis lineage 1, and the seven M orygis isolates from human MGIT cultures were dispersed among sequences from cattle.InterpretationM bovis prevalence in humans is an inadequate proxy of zoonotic tuberculosis. The recovery of M orygis from humans highlights the need to use a broadened definition, including MTBC subspecies such as M orygis, to investigate zoonotic tuberculosis. The identification of M tuberculosis in cattle also reinforces the need for One Health investigations in countries with endemic bovine tuberculosis.FundingBill & Melinda Gates Foundation, Canadian Institutes for Health Research.
Abstract The Mycobacterium tuberculosis complex (MTBC) includes M. tuberculosis (M. tb), the cause of human tuberculosis, and the animal pathogens, M. bovis and M. orygis, causes of tuberculosis in a broad range of mammalian hosts, including humans. Since the late 1800s, M. tb and M. bovis have shown distinct differences in clinical presentation and virulence following experimental infection. However, the pathogenicity of M. orygis has yet to be subject to experimental investigation. Despite possessing the smallest genome of the three, M. orygis is hypervirulent in C57BL/6 mice, comparable to M. bovis, but markedly different from M. tb. Proteomic comparisons, gene disruptions and infection studies reinforced the importance of the canonical virulence factor, ESAT-6, and identified MPT70 as a non-canonical virulence factor. The demonstration of differential virulence among these MTBC lineages necessitates a reconsideration of M. tb as the default organism for pathogenesis studies and has fundamental and translational implications for tuberculosis research.
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