Tuberculosis (TB) is a pandemic infectious disease and poses an important threat to global public health (1). Indeed, millions of people worldwide lose their lives to TB every year (2). Mycobacterium tuberculosis complex (MTBC) is responsible for TB in both humans and animals (3), and MTBC consists of well-defined members of Mycobacterium species such as M. tuberculosis, Mycobacterium bovis, Mycobacterium caprae, Mycobacterium pinnipedii, Mycobacterium africanum, Mycobacterium mungi, Mycobacterium canettii, and Mycobacterium microti (4). All species in this group have identical nucleotides (99.9%) at their genome level and have homogeneity in their 16S rRNA sequences (5). However, they differ from each other by their phenotypic characteristics, host spectrum, pathogenicity, and disease epidemiology (6).

Among the MTBC, TB due to the oryx bacillus has been recorded in oryx and other animals. This name conveys the fact that this pathogen was primarily isolated from antelopes, a member of the Bovidae family (7, 8). Oryx bacillus was considered a phenotypically divergent subtype of M. bovis due to the sharing of a unique spoligotype pattern with this species (9). For instance, spoligotyping patterns and IS6110 copy numbers of oryx bacilli, namely, Kremer 24 and Kremer 69 isolates, were matched with M. bovis. On the other hand, these two isolates had five specific deletions (RDoryx_wag22, RDoryx_1, RD5oryx, RD12oryx, and RDoryx_4), and these deletions were absent in other members of MTBC. Therefore, oryx bacilli were considered to be genetically different from M. bovis strains and deserve an exclusive phylogenetic position within the MTBC (6). In 2012, 22 oryx bacilli isolated from different hosts were analyzed by restriction fragment length polymorphism (RFLP) targeting IS6110 gene sequences, region of difference (RD), and single-nucleotide polymorphism (SNP) and renamed this zoonotic pathogen as M. orygis, which is now considered as a new subspecies under MTBC with a distinct lineage (4).

Though there are currently no reliable estimates based on thorough surveys to evaluate the impact that zTB poses to the health of humans and animals (10), the impact of zTB is wide-ranging and multifaceted, affecting both humans and animals. TB in animals represents a threat to animal health, leading to chronic disease, economic loss due to decreased productivity, trade restrictions, challenges in disease surveillance, and costs involved in the implementation of control programs (11). It would be very expensive and difficult to eradicate zTB from wild animals, and these animals may act as a source of infection to the entire ecosystem (12). Deforestation and rapid urbanization increase contact between wild animals, livestock, and humans, and as a result of repercussions, many diseases, including zTB, may increase in the future. Nonetheless, it is evident that the existence of zTB in livestock and wildlife populations affects human health and food safety (10). M. orygis can cause TB in animals but also affects humans (13). The true burden of M. orygis and its zoonotic potential remains unknown worldwide. However, based on scientific literature, it seems that the number of M. orygis cases has increased in recent years. Advances in genome sequencing technologies and bioinformatics tools have made it possible to identify members of the MTBC up to the subspecies level, which was not feasible in the past by traditional laboratory techniques (14). This could be a reason for the increased reporting of M. orygis cases nowadays. Therefore, increased awareness among physicians, veterinarians, laboratories, and public health personnel under ‘One-Health’ approach is imperative to understand the epidemiology, transmission cycle, host spectrum, reservoirs, and zoonotic potential of M. orygis. This review aims to highlight the challenges and threats of this neglected M. orygis for humans, animals, and the environment.

Mycobacterium orygis is a very slow-growing, acid-fast bacterium with an incubation period of 5–8 months. Its colony morphology is smooth to greasy, moist, non-chromogenic, and domed-shaped on solid media (15, 16). Information on the specific phenotypic and biochemical characteristics of M. orygis is scarce and requires further study. The whole-genome sequencing of M. orygis (strain NIRTAH144) reveals that it has a ~4.29 Mb genome with 65.59% GC content (17). Complete genome sequencing of another M. orygis (strain 51,145) yielded 4,352,172 bp and a 65.6% GC content (18). Recently, human (n = 322)- and animal (n = 529)-adapted MTBC were analyzed by maximum likelihood topology and thus placed M. orygis under animal-adopted MTBC clade A3 (19). This phylogenetic study suggested that M. orygis shares an ancestor that is common to M. bovis and M. caprae (19). Another study proposed that it should be positioned below the M. africanum but above the M. bovis and M. caprae in phylogeny (9). However, its exact phylogenetic position remains unsettled. Since M. orygis and the other members of MTBC are closely linked at the genomic level, identifying and differentiating them from one another using traditional approaches becomes a challenging task, which led to a paucity of data regarding the clinical characteristics of M. orygis.

Genotyping studies reveal that there are genomic insertions and deletions in the members of MTBC; these large sequence polymorphisms are known as RD and are used for the differentiation of the MTBC (20). Comparative genomic studies showed that there is a genomic insertion of 2–12.7 Kb in the M. tuberculosis H37Rv strain, and this region is absent in other species of MTBC. Thus, the deletion of this RD creates diversity among other members of MTBC (21). Recently, RD motif analysis by PCR-based techniques such as single tube multiplex qPCR has been explored to discriminate the members of MTBC (9, 22), and the findings are shown in Table 1. According to the RD analysis, M. orygis has RD1 and RD4 and lacks RD7, RD8, RD9, RD10, and RD12 (4, 17). Single-nucleotide polymorphisms in the gyrB, pncA, oxyR, leuS, mmpS6, PPE55, Rv0444c, and hsp genes of MTBC are also useful for discriminating between MTBC species (23–25). M. orygis shows specific nucleotide transitions such as T to G at the 38th codon of Rv2042C, G to C at the 698th codon of Rv0444c, C to G at the 551st codon of mmpL6, the C to T position in PPE55, and other SNPs (4, 9, 24, 26). Of these, a novel GGC mutation in the Rv2042³⁸ gene is a specific SNP marker for M. orygis (Table 2). However, other researchers claim that the SNP at the 1,113 (G to A) position of the gyrB gene is a more useful marker to discriminate M. orygis from other MTBC species (27).

In 1976, M. orygis was first reported in two East African oryx and later in 1987 from an oryx kept in a zoo in the Netherlands. Thereafter, M. orygis has been identified as a potential pathogen for members of the Bovidae family (28). Nowadays, it has been reported in a variety of animals, such as African buffalo, domestic cattle, gazelles, waterbucks, deer, rhinoceros, and blue bulls (7, 13, 28). Until now, a vast number of M. orygis cases have been reported from human patients suffering from TB (19), and it has been reported from rhesus monkeys (26, 29). It is necessary to differentiate the maintenance host from the spillover host to identify M. orygis host tropism. Some of the studies suggest that cattle are the maintenance host for M. orygis, and other animals are considered to be spillover hosts, which are infected with M. orygis due to contact with infected cattle. Some researchers also speculated that M. orygis and M. bovis were adapted to Bos indicus and Bos taurus, respectively (19). Nevertheless, further study is required to identify the precise host range of M. orygis (28).

The exact route of transmission of M. orygis is not ruled out yet. Recent studies suggest that aerosol transmission is the most common route in disease spread (26). Other possible routes are through the ingestion of M. orygis-contaminated water and food, as well as vertical and cutaneous transmission (Figure 1). Close contact with infected animals and consumption of raw milk and undercooked meat of infected animals could also act as a source of disease transmission to humans (15, 30–32). There is a possibility that M. orygis may spread from human to human, and it needs more epidemiological studies to understand the transmission route of this pathogen (29).

Specific clinical symptoms and incubation periods of M. orygis are poorly understood and not well documented yet. However, in general, infected humans typically show clinical symptoms such as fever, dry cough, body pain, loss of appetite, dysuria, loss of body weight, nausea, and lymph node enlargement (33). Infected animals may exhibit non-specific clinical signs such as nasal discharge, dullness and depression, sneezing, anorexia, weakness, cough, diarrhea, gradual weight loss, pneumonia, enlarged lymph nodes, and dyspnea (13, 26, 31). Sometimes, in the environment, performing a postmortem examination is not possible considering the risk of contamination of the environment and spreading of organisms to other susceptible hosts (34). Thus, this limits the availability of comprehensive descriptive tuberculous lesions caused by M. orygis (15). Based on the limited information on the gross pathology, in most of the cases, granulomatous lesions were observed in the lungs, lymph nodes, liver, and gastrointestinal tract of animals and are responsible for both pulmonary and extra-pulmonary TB (13, 26, 30, 35, 36). Pathological findings in a wild African buffalo showed a focal well-encapsulated pulmonary granuloma in the lower half of the dorso-caudal portion of the right cranial lung lobe. Below the capsule, an area of apparent calcification was also observed and a cut section of the granuloma had a central caseous necrosis with a partial liquefaction. In a deer, both extra-pulmonary and pulmonary tuberculous nodules of varying sizes (1–2 cm) were noticed; these nodules were encapsulated and liquefied in nature (15). In another study, calcified, pale-yellow lesions were recorded in the lungs and lymph nodes of blackbucks and spotted deer (37). Neural granulomatosis was also reported in a recent study (38). Multifocal, single-to-coalescing calcified caseous nodules were seen in the lungs and lymph nodes of the buffalo, cattle, and deer and in the liver of cattle infected with M. orygis (39).

The treatment regimen for TB is outside the scope of this review. However, various treatment options are available, depending on the drug sensitivity and resistance of the TB infection, and these guidelines are regularly updated by the WHO. In general, a combination of isoniazid, rifampin, pyrazinamide, and ethambutol for drug-sensitive TB is considered the first-line treatment for TB (40), and the duration varies between 4 and 6 months (41). For drug-resistant TB cases, repurposed drugs such as moxifloxacin, levofloxacin, beta-lactams, linezolid, and clofazimine, as well as newer drugs such as delamanid, bedaquiline, and pretomanid, are commonly used (40). The treatment duration for drug-resistant TB ranges from 6 to 18 months (42). According to Centre for Disease Control (CDC) guidelines, latent TB can be treated with (i) isoniazid and rifapentine, once weekly for 3 months; (ii) isoniazid and rifampin, daily for 3 months; (iii) rifampin, daily for 4 months; or (iv) isoniazid, daily for 6–9 months or twice weekly for 52–76 weeks (43). Treatment for TB caused by M. orygis in humans is similar to that provided for TB caused by M. tuberculosis, and it generally lasts from 9 months to 2 years (33). For more detailed information on TB treatment, readers can refer to the WHO and CDC guidelines. In animals, TB infection can be treated with the same drugs used in humans; however, no systematic studies have been conducted to evaluate the efficacy of these drugs in eliminating TB agents (44).

Due to a lack of systematic surveillance, the global prevalence of TB caused by M. orygis is unknown. Various molecular and epidemiological studies on MTBC strains indicated that M. orygis diverges phylogenetically from other MTBC members (4, 45). It has been postulated that with the migration of humans and the subsequent global spread of MTBC, members could have resulted in the introduction of distinct MTBC subspecies into various geographic regions at different rates (26). Hence, M. orygis may have dispersed to South Asia before the arrival of M. bovis and evidence in past studies suggested that M. orygis might be endemic in South Asian countries (4, 26, 45). Later, due to globalization, urbanization, and international trade of animals and their products, the disease has spread to various regions of the world. Based on the limited information (Table 3), infections have been reported in both wild and domestic animals in South Asia, Africa, and the Arabian Peninsula (46) (Figure 2). Between 1987 and 1988, M. bovis strains isolated from waterbuck, oryx, and antelopes in a single zoo in the Netherlands were characterized by IS6110-associated RFLP. These isolates had a highly unusual copy number of IS6110 bands (n = 20), and later on, some of these isolates were re-identified as M. orygis (4, 7). Saudi Arabia had an outbreak in Arabian oryx in 1994, and possibly, this outbreak was caused by M. orygis (27, 47). In the United Arab Emirates, mycobacteria were isolated from dromedary camels suffering from TB and gene deletion analysis showed that these isolates lacked RD7, RD8, and RD9. Furthermore, SNP studies of the mmpL6 gene of the isolates revealed the presence of a G residue in the 551 codon position, and these properties were matched with members of the antelope clade of M. bovis (35, 48). The authors suspected that the ingestion of bacilli excreted from the gazelle could be a possible route for transmission to camels. Recently, in Southern Africa, a free-ranging African buffalo (Syncerus caffer) was found to be positive for TB by three consecutive comparative intradermal tuberculin tests. Subsequently, M. orygis was isolated from a lung sample and confirmed by comprehensive molecular assays (27). In 2024, the United States reported the presence of M. orygis in 26 Macaca fascicularis (Cynomolgus macaques) that were imported from Southeast Asia for scientific studies (49).

In South Asia, most of the cases were reported from India, Nepal, Pakistan, and Bangladesh (Table 3), and it has been proposed that M. orygis is endemic in these countries (29, 34) (Figure 2). A study conducted in Bangladesh between 2009 and 2010 identified a total of 21 TB-positive cattle by postmortem examination, and two monkeys died due to pneumonia, of which M. orygis has been isolated from 18 cattle and two rhesus monkeys. Interestingly, 15 cattle and two monkey isolates had similar profiles based on multiple-locus variable-number tandem repeat analysis (MLVA). On the other hand, the remaining three cattle isolates had different MLVA patterns, and this shows the existence of strain diversity among M. orygis and its establishment over a period of time in the study area (26). M. orygis was isolated from a deer and a blue bull from a zoo in Nepal. Intriguingly, spoligotyping shows that spacer 3 was absent, although it is commonly reported in this species. This may be due to two point mutations in this spacer (15). M. orygis from bull and deer had similar patterns based on mycobacterial interspersed repetitive unit-variable number of tandem repeats (MIRU-VNTR) analysis of 22 loci. Thus, the same strain is circulating in captivity and acts as a source of infection to other susceptible animals through aerosol, contaminated food, and water (15).

In 2015, M. orygis was isolated and characterized from a free-ranging rhinoceros in Nepal. Dendrogram construction based on MIRU-VNTR results revealed that the rhinoceros isolate falls into a unique position and had a single difference in the MIRU 424 locus (13). Another study screened 3,581 cattle and buffaloes in slaughterhouses in Pakistan for bovine TB and 34 animals had TB-like lesions, of which 10 animals were found to be positive for M. orygis (34). In New Zealand, M. orygis infection was documented in cows and possibly transmitted from an animal attendant (29). Moreover, recently in India, M. orygis was isolated from cattle TB, which was found to be positive by comparative intra-dermal test (17). In the western hemisphere, M. orygis was recovered from a one-horned rhinoceros suffering from neural granulomatosis infection (38). Recently, in India, isolation of M. orygis was reported from two blackbucks, eight spotted deer, eight cattle, nine buffaloes, and one bison, and the species was confirmed by PCR amplification and whole-genome sequencing (37, 50). In another study, 567 samples were collected from 500 animals, including bulls, bull calves, and bullocks. Of these, 16 animals tested positive for MTBC cultures, with 13 of them identified as M. orygis (51). Considering all these reports, it is speculated that M. orygis infections in wild and domestic mammals have been underreported and received less attention. This may be due to a lack of an in-depth epidemiological surveillance mechanism and control program for both domestic and wild animals at the national level in many countries. At the field level, most of the animal TB cases are not identified up to the species level due to a lack of standard diagnostic assays to distinguish each member of MTBC.

Zoonotic tuberculosis is an extremely overlooked concept, and approximately 1.4% of cases of human TB were reported as zoonotic (52). The exact incidence rates of zTB remain uncertain because of the dearth of surveillance data in most countries. However, the data that are now available show that zTB has an impact on particular populations and environments (53). Previously, it was estimated that zTB was mainly caused by M. bovis, rarely by M. caprae, but in recent times M. orygis has also been included in the list (19, 54). In India, a total of 940 mycobacterial cultures from human TB patients were analyzed to estimate the zoonotic burden of animal-adopted MTBC, of which 0.7% (n = 7) isolates were M. orygis (Table 3). Interestingly, M. bovis was not identified in any of these cases (45). Between 2005 and 2010, a total of 1,763 patients from Australia were diagnosed with MTBC infection; of these, eight patients were infected with M. orygis, whereas M. bovis was identified in two cases (55). In New York, from 2005 to 2016, eight cases of M. orygis have been reported based on the whole-genome sequence analysis of 6,322 MTBC species (30). M. orygis was isolated from an immunocompetent patient suffering from lymphadenitis in New York (30) (Table 3). One case was reported in New Zealand, wherein a dairy handler was found to be infected with M. orygis (29). In the United Kingdom, out of 3,128 clinical MTBC isolates analyzed by SNP-IT tools, 24 were identified as M. orygis (14). Of these, two pairs of M. orygis were identical and thus speculate on the possibility of human-to-human transmission of this pathogen or common exposure to the same infected animal (14). Furthermore, M. orygis was isolated from five patients in Norway, and the author concluded that all five patients were from South Asian countries and imported M. orygis to Norway (56). In India, a study was conducted, and out of 1,105 patients, eight were infected with TB caused by M. orygis. Possible risk factors such as close contact with infected animals or consumption of unpasteurized animal products for acquiring zTB were not ruled out (33). We can speculate that the same species of M. orygis isolated from humans that was earlier identified in animals has raised concerns about the wide host spectrum of this pathogen. Recently, a study conducted in Canada found that out of 42 patients infected with animal lineages, 21 were infected with M. orygis (25). All these cases prompt the international health authorities to reconsider M. bovis as the sole cause of zTB. In the human health sector, the major focus is to detect and treat each TB case to achieve the goal of “ending TB, “rather than identifying each member of the MTBC. Data on zTB are only available from 61 countries (57), suggesting that the true extent of zTB in many developing and underdeveloped countries with large livestock populations and high TB burdens is likely underreported.

Despite being an ancient and curable disease, TB remains one of the world’s leading causes of mortality in humans. A target set by the WHO in 2018 is to reduce TB death and incidence rates by 95% and 90%, respectively, by 2035. Nevertheless, it is not possible to reach this milestone without adequate surveillance, monitoring, and control programs for zTB, including domestic and wild animals. In this study, we are highlighting some of the major challenges and areas to focus on for the control of zTB caused by M. orygis and M. bovis.

Mycobacterium orygis is a member of the MTBC, and its zoonotic potential is often underestimated. The One Health approach calls for enhanced vigilance and collaboration between medical, veterinary, and environmental disciplines to better understand the epidemiology, transmission, host range, reservoirs, and zoonotic potential of this pathogen. More research is needed to determine the global prevalence and burden of M. orygis. Developing rapid and accurate diagnostic tools and effective treatment protocols will be critical in mitigating the public health threat posed by M. orygis. There is a need for systematic surveillance of M. orygis in humans, animals, and the environment for early detection and control. Strengthening bio-safety measures in laboratories and educating health professionals and the public about the risks associated with zTB can help in managing this pathogen.