Emergence of mycoplasmosis in the North American bison

While long established in cattle, M. bovis emerged as a major threat to bison relatively recently, with the first severe outbreaks recognized between 1999 and 2001 in the western United States and Canada (Register et al., Reference Register, Parker, Patyk, Sweeney, Boatwright, Jones, Woodbury, Hunter, Treanor, Kohr, Hamilton, Shury and Nol2021; USDA, 2013). These initial events were marked by severe pneumonia, polyarthritis, and exceptionally high rates of morbidity and mortality, quickly establishing the disease as a primary health concern for the bison industry (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2016; Dyer et al., Reference Dyer, Hansen-lardy, Krogh, Schaan and Schamber2008; Janardhan et al., Reference Janardhan, Hays, Dyer, Oberst and DeBey2010) and leading to its designation as an emerging disease by the USDA in 2013 (USDA, 2013). A defining feature of mycoplasmosis in bison is the role of M. bovis as a primary pathogen, capable of causing severe disease on its own (Janardhan et al., Reference Janardhan, Hays, Dyer, Oberst and DeBey2010; Register et al., Reference Register, Olsen, Sacco, Ridpath, Falkenberg, Briggs, Kanipe and Madison2018). This contrasts with cattle, where it often acts as a secondary invader following an initial viral or bacterial infection associated with BRD (Nicholas et al., Reference Nicholas, Ayling and McAuliffe2008; Oliveira et al., Reference Oliveira, Pelaquim, Flores, Massi, Valdiviezo, Pretto-giordano, Alfieri, Saut and Headley2020). M. bovis isolates from bison also differ from cattle isolates in their interaction with host immune and epithelial cells (Register et al., Reference Register, Olsen, Sacco, Ridpath, Falkenberg, Briggs, Kanipe and Madison2018; Suleman et al., Reference Suleman, Prysliak, Clarke, Burrage, Windeyer and Perez-Casal2016). In cattle, young calves and feedlot animals are most affected (Maunsell and Donovan, Reference Maunsell and Donovan2009), whereas clinical disease in bison calves is rarely reported, although their infection status has not been systematically studied.

Prevalence and geographic distribution

Retrospective studies have fundamentally shifted our understanding of M. bovis epidemiology in bison. Analyses of historical serum samples revealed exposure to the pathogen since at least the 1980s, predating the first recognized clinical outbreaks by over a decade. While the overall seroprevalence was similar in the United States (∼7.1%) and Canada (∼6.7%) in the 35-year study period (1984–2019), there was significant country-specific differences between eras; seroprevalence in the United States was higher pre-2000 and lower post-2000, with the opposite trend observed in Canada (Register et al., Reference Register, Parker, Patyk, Sweeney, Boatwright, Jones, Woodbury, Hunter, Treanor, Kohr, Hamilton, Shury and Nol2021). The presence of antibodies in healthy animals implies that the pathogen circulated cryptically at a subclinical level for years. The emergence of severe clinical disease around the year 2000 therefore signals a likely shift in the host-pathogen-environment dynamic, not a novel introduction. This shift may have been driven by the intensification of bison production, increasing stress and transmission opportunities, or by the evolution and spread of M. bovis strains with heightened virulence or specific adaptation to bison. Evidence also indicates that exposure to M. bovis is widespread among North American bison populations, with approximately 70% of the United States and 90% of Canadian herds showing evidence of exposure (Register et al., Reference Register, Parker, Patyk, Sweeney, Boatwright, Jones, Woodbury, Hunter, Treanor, Kohr, Hamilton, Shury and Nol2021). Despite this widespread exposure, confirmed clinical disease is reported less frequently. For instance, a 2017 survey in western Canada found that only 6% of producers had a laboratory-confirmed M. bovis diagnosis in their herd within a period of six years (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2017a). Geographically, infections have been documented across the bison’s range, including numerous U.S. states and Canadian provinces (Woodbury, Reference Woodbury2012). Within western Canada, Alberta showed the highest prevalence of diagnosed disease in the 2017 survey (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2017a). Before the widespread recognition of clinical disease (prior to 2000), prevalence was significantly higher in the United States than in Canada (9.4% versus 5.0%). In the years following disease emergence (2000–2019),this pattern shifted: prevalence in the United States declined to about 5.0%, while Canadian rates increased to roughly 8.1% (Register et al., Reference Register, Parker, Patyk, Sweeney, Boatwright, Jones, Woodbury, Hunter, Treanor, Kohr, Hamilton, Shury and Nol2021). These contrasting trends point towards possible differences in epidemiological pressures or management practices between the two countries during this period of industry expansion.

A critical epidemiological finding is the presence of M. bovis in apparently healthy animals. Using direct detection methods like culture and polymerase chain reaction (PCR), M. bovis has been detected in the upper respiratory tract (nasal passages and tonsils) of about 3% of clinically normal bison from herds with no history of recent outbreaks. This prevalence is significantly lower than that of other Mycoplasma species considered commensal, such as M. bovirhinis, which was identified in more than 28% of healthy animals sampled in the same studies (Register et al., Reference Register, Jones, Boatwright, Shury, Woodbury, Hamilton, Treanor, Dyer and Nol2021a). Despite this low detection rate, serological surveys indicate that a majority of herds (70–90%) have been exposed to M. bovis (Bras et al., Reference Bras, Suleman, Woodbury, Register, Barkema, Perez-Casal and Windeyer2017b; Register et al., Reference Register, Parker, Patyk, Sweeney, Boatwright, Jones, Woodbury, Hunter, Treanor, Kohr, Hamilton, Shury and Nol2021), underscoring the importance of subclinically infected carriers in disease ecology. The apparent contradiction between high seroprevalence and low direct detection reflects the difficulty of identifying carriers. Carrier animals play a major role in disease perpetuation and introduction into naïve herds, creating a serious biosecurity challenge. The low detection rate may indicate that carriage is often transient, intermittent, or occurs at levels below the sensitivity of current swab-based tests. Alternatively, the primary site of persistence might not always be the nasal cavity or tonsils. Furthermore, serology reflects cumulative lifetime exposure, not necessarily current infection status.

Transmission dynamics and risk factors

Transmission of M. bovis in bison is complex, involving direct contact, asymptomatic carriers, and management-related stressors. Direct animal-to-animal contact, particularly nose-to-nose interactions, is considered the primary route of infection, with inhalation of infectious aerosols generated from respiratory secretions of infected or carrier animals playing a central role (USDA, 2013). Once established, the organism colonizes mucosal surfaces of the respiratory tract but may disseminate systemically, leading to shedding from additional sites (Maunsell et al., Reference Maunsell, Woolums, Francoz, Rosenbusch, Step, Wilson and Janzen2011). Indirect transmission, while less well characterized, is also possible through contaminated feed, water sources, or shared bedding (Justice-Allen et al., Reference Justice-Allen, Trujillo, Corbett, Harding, Goodell and Wilson2010). Although M. bovis is sensitive to drying and sunlight, it can persist for extended periods under cool, moist conditions, potentially surviving in soil or water and contributing to environmental reservoirs (Pfützner, Reference Pfützner1984).

Several specific risk factors shape outbreak dynamics. Stressful events, such as transportation, weaning, and commingling, are major triggers for disease outbreaks, likely by increasing bacterial shedding in carriers while suppressing immunity in other animals. In Western Canada, a case–control study found that larger farms, the presence of feedlot units, frequent visits by rental or third-party trailers, and annual vaccination practices were all associated with heightened disease risk, while herds experiencing fatal respiratory disease the prior year also demonstrated increased vulnerability (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2016). Geographic location further influenced risk, with herds in Alberta exhibiting significantly higher incidence rates (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2017a). In the United States, a comprehensive survey revealed that larger herd size as a consistent risk factor and highlighted shared fence lines with cattle as a major concern, suggesting that proximity to cattle operations can heighten the chances of an outbreak (Martin et al., Reference Martin, Browne and Buttke2025). The introduction of new animals, especially from auction markets, is a primary risk factor for introducing the pathogen into a clean herd. While concurrent infections with other bacterial pathogens like Trueperella pyogenes are often reported and likely exacerbate disease severity (Dyer et al., Reference Dyer, Register, Miskimins and Newell2013), the role of viral coinfections requires further investigation. Finally, although cattle are a known reservoir, direct transmission from cattle to bison has not been confirmed as a primary driver of outbreaks, and genetic data indicate that certain lineages are predominantly associated with bison, consistent with the possibility that some strains may be specifically adapted to bison (Kumar et al., Reference Kumar, Register, Christopher-hennings, Moroni, Gioia, Garcia-fernandez, Nelson, Jelinski, Lysnyansky, Bayles, Alt and Scaria2020).

Clinical signs

Mycoplasmosis in bison is most often expressed as a combination of chronic pneumonia and polyarthritis, a syndrome frequently described as chronic pneumonia and polyarthritis syndrome (CPPS; Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2017a; Dyer et al., Reference Dyer, Hansen-lardy, Krogh, Schaan and Schamber2008; Janardhan et al., Reference Janardhan, Hays, Dyer, Oberst and DeBey2010). However, the disease onset is often insidious, with early signs that can be subtle and easily overlooked, particularly in extensive range conditions. Affected animals typically present with respiratory signs such as coughing, tachypnoea, and dyspnoea, which may progress to laboured breathing. Lameness resulting from painful, swollen joints (polyarthritis) is another frequent manifestation, often causing reluctance to move and abnormal stance (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2017a). Although arthritis can be a primary sign, it often develops subsequent to respiratory infection. As in cattle, the carpal and stifle joints are most commonly affected. Systemic signs include lethargy, depression, and exercise intolerance, a sign frequently reported by producers as an initial indicator (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2017a). In the early stages, animals may remain alert and maintain their appetite, a characteristic possibly explained by the absence of exotoxins produced by M. bovis. As the disease progresses to a chronic state, affected bison show progressive loss in body condition, emaciation, and weakness. Other signs such as intermittent fever and ocular discharge are also reported. Beyond the typical CPPS pattern, additional clinical manifestations described in bison include upper respiratory signs, mastitis, and reproductive disorders such as abortion and dystocia (Janardhan et al., Reference Janardhan, Hays, Dyer, Oberst and DeBey2010; Register et al., Reference Register, Woodbury, Davies, Trujillo, Perez-Casal, Burrage, Clark and Windeyer2013). The disease course is typically chronic and debilitating, with affected animals showing progressive respiratory compromise, lameness, weight loss, and declining body condition over time. Many animals either succumb to the disease or are humanely euthanized due to severe debilitation. During outbreaks, the case fatality risk can be extremely high, ranging up to 25–30% at the herd level and approaching 100% for individuals showing severe clinical signs (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2017a; Martin et al., Reference Martin, Browne and Buttke2025). This contrast between subtle early clinical signs and severe, frequently fatal outcomes complicates timely intervention. By the time overt lameness or respiratory distress is evident, lesions are often advanced and chronic, contributing to poor therapeutic response and high mortality. These features highlight the difficulty of managing established disease and emphasize the importance of preventative, herd-level strategies.

Gross and histological lesions

At necropsy, fibrinous, or fibrinosuppurative pleuritis is a common finding, often appearing as thick mats of yellow fibrin covering the pleural surfaces. The most consistent lesion is severe chronic caseonecrotic bronchopneumonia (Dyer et al., Reference Dyer, Hansen-lardy, Krogh, Schaan and Schamber2008, Reference Dyer, Register, Miskimins and Newell2013; Janardhan et al., Reference Janardhan, Hays, Dyer, Oberst and DeBey2010; USDA, 2013). Affected lungs contain multiple foci of bronchiectasis, ranging from pinpoint cavities to large confluent areas filled with caseous or purulent exudate. Histologically, the classic lesion consists of a necrotic centre, sometimes with mineralization, surrounded by degenerate neutrophils, macrophages, lymphocytes, and plasma cells within affected bronchi or bronchioles (Dyer et al., Reference Dyer, Hansen-lardy, Krogh, Schaan and Schamber2008, Reference Dyer, Register, Miskimins and Newell2013; Janardhan et al., Reference Janardhan, Hays, Dyer, Oberst and DeBey2010). Adjacent alveoli often exhibit necrosis of septal walls, accumulation of protein-rich oedema fluid, deposition of fibrin forming hyaline membranes, and infiltration by inflammatory cells. A characteristic feature of M. bovis infection in bison, as in other species (Radaelli et al., Reference Radaelli, Luini, Loria, Nicholas and Scanziani2008; Rodriguez et al., Reference Rodriguez, Bryson, Ball and Forster1996), is marked bronchiole-associated lymphoid tissue hyperplasia, reflecting the organism’s mitogenic and superantigenic effects on B cells. In chronic cases, firm fibrous adhesions may tether the lungs to the thoracic wall. Polyarthritis is manifested by joint swelling and distension of the capsule with fibrinous or purulent synovial fluid. Histology of the affected joints reveals fibrinosuppurative synovitis with dense infiltration of neutrophils, fibrin, and mononuclear cells within the synovial lining and joint space. Additional lesions include fibrinous inflammation of serosal surfaces such as the pericardium, necrotic lesions in the pharynx, larynx, or liver accompanied by suppurative to granulomatous inflammation, and enlargement of regional lymph nodes (Dyer et al., Reference Dyer, Hansen-lardy, Krogh, Schaan and Schamber2008, Reference Dyer, Register, Miskimins and Newell2013; Janardhan et al., Reference Janardhan, Hays, Dyer, Oberst and DeBey2010; Register et al., Reference Register, Woodbury, Davies, Trujillo, Perez-Casal, Burrage, Clark and Windeyer2013; USDA, 2013).

Sampling methods

Recent studies underscore that sampling method and site of collection strongly influences M. bovis detection in bison. Superficial nasal swabs are simple, minimally invasive, and useful during acute outbreaks when animals shed large numbers of bacteria, but their sensitivity drops quickly once infections transition to subclinical or chronic states. In contrast, deep nasopharyngeal swabs consistently demonstrate superior diagnostic performance, detecting persistent colonization in calves and yearlings that appear clinically normal yet remain infectious for extended periods (Buttke et al., Reference Buttke, Schwartz, Schwalbe, Killion, Sondgeroth, Kaplan and Malmberg2025; Schwartz et al., Reference Schwartz, Schwalbe, Buttke, Bragg, Killion, Sondgeroth and Malmberg2024). Because agreement between superficial and deep swabs is low, paired swabbing provides the most comprehensive approach. This strategy is especially important during outbreaks and when animals are destined for transfer, since the risk of moving undetected carriers into naïve herds is high. At the herd level, pooling samples offer an additional tool to improve surveillance efficiency (Schwartz et al., Reference Schwartz, Schwalbe, Buttke, Bragg, Killion, Sondgeroth and Malmberg2024). Pools of 5, and in some cases 10, reliably identified positives while significantly reducing costs and testing labour, although sensitivity declined as pool size increased, with pools of 15 proving unreliable. Pooling can therefore be considered when screening large numbers of animals in herds with low expected prevalence, but results must be interpreted cautiously, especially when PCR values approach detection thresholds.

Culture-based methods

Isolation of M. bovis by bacterial culture remains the definitive diagnostic approach. It confirms infection and provides isolates for antimicrobial susceptibility testing and molecular characterization. However, the organism is fastidious, requiring specialized, enriched culture media and specific incubation conditions (Parker et al., Reference Parker, Sheehy, Hazelton, Bosward and House2018). Growth is typically slow, often taking several days to weeks before the appearance of small, ‘fried-egg’ colonies visible under the microscope (Quinn et al., Reference Quinn, Markey, Leonard and Hartigan2025). Appropriate diagnostic sample types include lung, liver, joint tissue, and abscess content collected during necropsy, joint fluid aspirated from live animals, bronchoalveolar lavage fluid, milk, or respiratory tract swabs. Success rates are highest when samples are collected from active lesions. Although culture is highly specific, it may miss positive cases if the organism is no longer viable, such as after antimicrobial treatment, autolysis, improper sample handling, or freezing. Overgrowth by faster-growing contaminants is another common cause of false negatives. Moreover, growth of other mycoplasmas or related organisms such as Acholeplasma necessitates confirmatory identification, usually by PCR or biochemical testing (McVey et al., Reference McVey, Kennedy and Chengappa2013). The lengthy turnaround time and technical requirements limit the utility of culture for routine clinical decision-making, though it remains important for research and outbreak investigations.

Serological assays

Enzyme-linked immunosorbent assays (ELISAs) are widely used to detect antibodies against M. bovis, typically IgG in serum, and have become a cornerstone of serological surveillance in cattle. However, their application in bison presents both opportunities and challenges that require careful consideration (Register et al., Reference Register, Sacco and Olsen2013a). Commercial assays have been developed using proteins derived from cattle isolates, but genetic and antigenic divergence between bison and cattle isolates of M. bovis has been documented (Kumar et al., Reference Kumar, Register, Christopher-hennings, Moroni, Gioia, Garcia-fernandez, Nelson, Jelinski, Lysnyansky, Bayles, Alt and Scaria2020; Register et al., Reference Register, Sacco and Olsen2013a), which demonstrated that in-house ELISAs coated with whole-cell antigen from bison isolates consistently produced significantly higher absorbance values from bison compared to assays relying on cattle-derived antigens. This discrepancy was most pronounced in animals with low-to-moderate antibody titres, which are also the most prone to misclassification. Immunoblotting further confirmed broader reactivity against bison-derived antigen preparations, indicating that bison-associated strains express immunoreactive surface proteins that are absent or antigenically divergent in cattle isolates (Register et al., Reference Register, Sacco and Olsen2013a). Collectively, these findings suggest that assays based on cattle-derived antigens may substantially underestimate seroprevalence in bison populations.

Sensitivity is further limited in early infection, as detectable antibody responses usually appear only 2–3 weeks after exposure. In addition, antibody titres may also decline in chronic infections, producing false negatives. Specificity can also be affected by cross-reactivity with antibodies against other mycoplasmas, such as M. bovirhinis, or other unrelated bacterial species (Register et al., Reference Register, Jones, Boatwright, Shury, Woodbury, Hamilton, Treanor, Dyer and Nol2021a). Studies comparing ELISA results with PCR or culture status in healthy bison have shown significant discrepancies with over 50% of bison carrying detectable M. bovis in their upper respiratory tract being seronegative by ELISA, while a smaller percentage (∼5%) of bison negative by PCR/culture were seropositive (Register et al., Reference Register, Jones, Boatwright, Shury, Woodbury, Hamilton, Treanor, Dyer and Nol2021a).

Given these limitations, serology is best interpreted as a herd-level exposure tool rather than a reliable individual-animal diagnostic. At the population level, longitudinal seroprevalence monitoring can identify herd-level introduction events and track exposure dynamics, even when individual test sensitivity is suboptimal. A critical priority for future development is the establishment of validated bison-specific assays. These may be based on antigen panels derived from genetically representative bison isolates or, alternatively, on conserved immunodominant proteins that circumvent Vsp-mediated antigenic variation and have already demonstrated immunogenicity in bison (Kaplan et al., Reference Kaplan, Dassanayake, Briggs, Kanipe, Boggiatto, Crawford, Olsen, Menghwar, Casas and Tatum2024a). Until such assays are available, serological results in individual animals must be interpreted with caution, and negative serology should not be used as the sole criterion for biosecurity clearance decisions.

Despite these limitations, ELISAs offer distinct practical advantages. They are scalable, relatively inexpensive, and well suited for herd-level surveillance in both conservation and production settings where resources may be limited (Kaplan et al., Reference Kaplan, Malmberg, Sondgeroth, Davila, Dassanayake, Sacco, Casas and Buttke2024b). Furthermore, they provide valuable longitudinal data on antibody kinetics, enabling assessment of exposure patterns over time. Consequently, their primary utility lies in epidemiological surveillance rather than clinical diagnosis or case confirmation.

Molecular detection

PCR assays, including quantitative real-time PCR (qPCR), provide a rapid and highly sensitive means of detecting M. bovis DNA in a broad range of specimens (Maunsell et al., Reference Maunsell, Woolums, Francoz, Rosenbusch, Step, Wilson and Janzen2011; Nicholas, Reference Nicholas2011). Suitable materials include fresh or frozen tissues collected at necropsy including lung, joint, lymph node, abscess contents, as well as respiratory or joint swabs, or bronchoalveolar lavage fluid (Nicholas and Ayling, Reference Nicholas and Ayling2003). Compared to culture, PCR generally offers higher sensitivity and markedly shorter turnaround, often yielding results within days. Importantly, unlike serological assays, PCR performance is not constrained by antigenic divergence between cattle and bison isolates.

Target gene selection determines both sensitivity and specificity. Assays targeting species-associated genes, such as uvrC, can provide species-level detection of M. bovis and help distinguish it from commensal or coinfecting mycoplasmas, including M. bovirhinis (Subramaniam et al., Reference Subramaniam, Bergonier, Poumarat, Capaul, Schlatter, Nicolet and Frey1998; Thomas et al., Reference Thomas, Dizier, Linden, Mainil, Frey and Vilei2004). Given the documented genetic divergence of bison M. bovis isolates from cattle strains (Kumar et al., Reference Kumar, Register, Christopher-hennings, Moroni, Gioia, Garcia-fernandez, Nelson, Jelinski, Lysnyansky, Bayles, Alt and Scaria2020), whether primers designed against cattle reference sequences retain equivalent sensitivity across the full diversity of bison-associated strains remains an important validation gap. qPCR can further quantify bacterial load, offering insight into infection intensity and shedding dynamics. PCR also has the practical advantage of detecting DNA from non-viable organisms, which can improve detection when sample quality is compromised by transport delays, autolysis, freezing, or prior antimicrobial therapy. However, this feature also complicates interpretation, because a positive PCR result does not necessarily confirm viable organisms or active infection. PCR is especially useful for confirming cases at necropsy and for detecting potential carriers, although intermittent shedding means that a negative swab does not definitively exclude infection.

Immunohistochemistry

Immunohistochemistry (IHC) is highly specific for M. bovis in cattle and is likely to have similar value in bison (Brown et al., Reference Brown, Laney, Tabatabai, Hassebroek, Lahmers, LeCuyer, Uzal and Carvallo2025; Hermeyer et al., Reference Hermeyer, Peters, Brügmann, Jacobsen and Hewicker-trautwein2012). Although sensitivity may be lower than culture or PCR, particularly in tissues with low bacterial burden, the primary utility of IHC lies in its ability to directly correlate the presence of the pathogen and histopathological lesions. This correlation provides compelling evidence for causation in affected tissues. IHC requires specialized reagents, technical facilities, and interpretation by trained pathologists, limiting its use in routine herd diagnostics. Nonetheless, it remains an important adjunct for confirming the aetiological role of M. bovis in bison lesions.

To summarize, using a combination of tests often gives the clearest picture. For individual cases or post-mortem confirmation, the best approach is to test affected tissues such as lung or joint by PCR or culture, and then pair those results with histopathology and, when possible, IHC. At the herd level, screening for exposure or trying to identify carriers may involve serology, usually ELISA, but results must be interpreted cautiously. Pairing serology with PCR or culture from pooled or repeated samples, such as nasal swabs taken over time, can improve the odds of detecting animals that shed intermittently. Even so, identifying asymptomatic carriers remains extremely difficult, and this gap continues to limit disease control. The lack of diagnostic tests validated specifically for bison, especially serological assays, adds another layer of uncertainty and underscores the urgent need for more focused diagnostic research and development.

Treatment and antimicrobial resistance

Therapeutic management of established infections is generally considered unrewarding. The chronic course of the disease, characterized by caseonecrotic lesions and biofilm formation, severely limits antimicrobial penetration and efficacy (Andrés-Lasheras et al., Reference Andrés-Lasheras, Zaheer, Jelinski and McAllister2024; McAuliffe et al., Reference McAuliffe, Ellis, Miles, Ayling and Nicholas2006). Mycoplasmas lack a cell wall, rendering all β-lactam antibiotics (penicillins, cephalosporins) ineffective, and sulphonamides are also inactive because the organisms do not synthesize folic acid (Maunsell et al., Reference Maunsell, Woolums, Francoz, Rosenbusch, Step, Wilson and Janzen2011). Drugs generally considered active against mycoplasmas are those interfering with protein synthesis (tetracyclines, macrolides lincosamides, and phenicols) or DNA replication (fluoroquinolones) (Cai et al., Reference Cai, McDowall, Parker, Kaufman and Caswell2019).

The antimicrobial resistance (AMR) landscape in bison is alarming and distinguishes bison mycoplasmosis from the disease in cattle in practically important ways. A study on in vitro susceptibilities of isolates collected from bison herds in Canada showed spectinomycin, tiamulin, and tulathromycin to be the most effective, while with 100% of isolates were resistant to tetracyclines and to the macrolide tilmicosin, and 97.5% resistant to fluoroquinolones (Suleman et al., Reference Suleman, Prysliak, Windeyer and Perez-Casal2015). Critically, two lung isolates from the group were also found to be resistant to both spectinomycin and tulathromycin, raising concerns of emerging pan-resistant strains. These results emphasize the necessity of testing antimicrobial susceptibility of M. bovis bison isolates.

The molecular mechanisms underlying AMR in M. bovis are well-characterized in cattle isolates. Tetracycline resistance is predominantly conferred by ribosomal protection genes (tet(M) and tet(O)) and, less commonly, by efflux pumps. Macrolide resistance arises primarily through mutations in the 23S rRNA gene (particularly at positions A2058G and A2059G) and through acquisition of erm genes encoding ribosomal methylases. Fluoroquinolone resistance results from point mutations in DNA gyrase subunits (gyrA and gyrB) and topoisomerase IV subunits (parC and parE), which reduce drug binding affinity at the target enzymes (Cai et al., Reference Cai, McDowall, Parker, Kaufman and Caswell2019). Whether the same molecular mechanisms operate in bison isolates and whether additional, bison-specific resistance determinants exist, is largely unknown.

The public health and food safety implications of AMR in bison M. bovis deserve particular attention. No antimicrobial drugs are specifically licensed for M. bovis in bison in either the United States or Canada, so all use is extra-label under veterinary oversight. Rational drug selection is further complicated because AMR surveillance in bison M. bovis is not systematic. The published bison AMR data was derived almost entirely from a single Canadian study conducted over a decade ago (Suleman et al., Reference Suleman, Prysliak, Windeyer and Perez-Casal2015). This creates a critical evidence gap: current resistance trends, the geographic distribution of resistant strains, and the potential for ongoing resistance evolution or inter-species transfer of resistance determinants between bison and cattle M. bovis populations are entirely uncharacterized. Given the global rise of AMR as a One Health concern and the close proximity of bison and cattle in many production systems, the absence of a structured AMR surveillance programme for bison M. bovis is a significant deficiency.

Furthermore, unsuccessful or prolonged antimicrobial therapies may select for resistant strains and favour their persistence within herds. This risk is compounded by the chronic, relapsing nature of bison mycoplasmosis, which often prompts repeated or prolonged treatment courses. Collectively, these findings underscore the difficulty of treating bison mycoplasmosis, highlight the urgent need for antimicrobial susceptibility testing of outbreak isolates to guide rational, albeit still off-label antimicrobial choices, and reinforce the imperative for non-antibiotic control strategies.

Vaccines

At present, no commercially available vaccines have been demonstrated to effectively prevent or control mycoplasmosis in bison. This absence of effective immunoprophylaxis remains one of the greatest impediments to managing the disease. A variety of vaccine strategies have been investigated in cattle, with mixed outcomes, but none have shown consistent efficacy under field conditions, and none are currently recommended for use in bison (Maunsell et al., Reference Maunsell, Donovan, Risco and Brown2009; Nicholas et al., Reference Nicholas, Ayling and Stipkovits2002). A major factor underlying this poor performance is the pathogen’s extraordinary ability to undergo antigenic variation, particularly through its variable surface lipoproteins (Vsps). Because Vsps are highly immunogenic, the organism’s ability to alter their expression allows it to evade vaccine-induced immune responses directed against previously encountered epitopes (Beier et al., Reference Beier, Hotzel, Lysnyansky, Grajetzki, Heller, Rabeling, Yogev and Sachse1998; Sachse et al., Reference Sachse, Helbig, Lysnyansky, Grajetzki, Müller, Jacobs and Yogev2000). Autogenous vaccines, produced from M. bovis strains isolated from affected herds, are sometimes used in attempts to control persistent herd problems. The rationale is to provide protection against the specific strains circulating locally. Despite their widespread use in cattle and occasional use in bison operations, no published data confirm their efficacy in bison, and anecdotal reports suggest inconsistent outcomes (Kaplan et al., Reference Kaplan, Dassanayake, Briggs, Kanipe, Boggiatto, Crawford, Olsen, Menghwar, Casas and Tatum2024a). More recently, attention has turned to subunit vaccines, which incorporate purified antigens rather than whole organisms. These strategies aim to overcome the problem of antigenic variation by targeting conserved proteins critical to M. bovis survival or virulence. A promising experimental study in bison evaluated a recombinant protein subunit vaccine containing elongation factor Tu (EFTu) and heat shock protein 70 (Hsp70) (Kaplan et al., Reference Kaplan, Dassanayake, Briggs, Kanipe, Boggiatto, Crawford, Olsen, Menghwar, Casas and Tatum2024a). Vaccination with recombinant EFTu and Hsp70 induced strong antigen-specific antibody and T-cell responses in bison and improved control of infection following the coinfection of BHV-1 and M. bovis challenge. Vaccinated animals demonstrated reduced clinical severity, less extensive lung lesions, lower pulmonary bacterial loads, and reduced dissemination to joints compared with unvaccinated controls following experimental challenge. These results demonstrate that the EFTu/Hsp70 subunit vaccine enhanced host control of infection and reduced disease severity in infected bison, providing strong support for continued development of protein subunit vaccines specifically tailored to bison.

Prevention and control strategies

In the absence of effective treatment or validated vaccines, mycoplasmosis prevention relies primarily on biosecurity and sound herd management (Martin et al., Reference Martin, Browne and Buttke2025). Maintaining closed herds is the most effective strategy, while any necessary introductions should come from herds with a documented negative history for mycoplasmosis (USDA, 2013). New arrivals should undergo strict quarantine (30–60 days) with diagnostic testing considered, despite the limitations of current assays. Minimizing contact between bison and potentially infected cattle herds is also advisable. Within affected herds, efforts should focus on reducing transmission and mitigating stress. Lower stocking densities, careful handling, adequate nutrition, and environmental management all help reduce susceptibility and shedding. Routine cleaning of shared equipment, feed bunks, and waterers, along with prompt isolation or removal of clinically affected animals, can further limit spread. Where feasible, separating age groups may also be beneficial (Bras et al., Reference Bras, Barkema, Woodbury, Ribble, Perez-Casal and Windeyer2016; Martin et al., Reference Martin, Browne and Buttke2025). Regular health monitoring and targeted diagnostics, such as necropsies of unexplained deaths or testing chronically affected animals, are essential for early detection. USDA APHIS is actively investigating risk factors for M. bovis in U.S. bison herds to develop more tailored biosecurity guidelines. Nonetheless, significant gaps remain, particularly in the ability to identify asymptomatic carriers. Until effective vaccines or treatments are available, prevention through management and biosecurity will remain the cornerstone of control.