Bovine respiratory disease (BRD), also known as pneumonia, is a multifactorial condition that affects cattle across all age groups [1,2,3,4,5,6,7,8,9,10]. It is the leading cause of mortality in pre-weaned dairy calves [11,12]. The disease arises from a complex interplay of viral and bacterial pathogens, host-related factors, environmental stressors, and management practises [13,14,15]. The term BRD is commonly used to encompass both upper (URT) and lower respiratory tract (LRT) illnesses in cattle, which frequently involve mixed infections of viral and bacterial origin [16]. The crises associated with BRD are multifaceted. Clinically, it compromises animal welfare, hinders growth rates, and can result in chronic pulmonary damage. From an economic standpoint, BRD is one of the most costly diseases in cattle production systems, contributing to losses due to treatment expenses, reduced weight gain, increased culling rates, and mortality. Estimates suggest that BRD costs the U.S. beef industry alone over USD 1 billion annually in direct and indirect losses [17,18,19]. Similar burdens are reported globally, affecting both intensive and extensive farming operations. Despite decades of research, BRD remains a persistent challenge due to its complex etiology and the variability in diagnostic criteria, prevention strategies, and treatment responses. A deeper understanding of the disease dynamics, including pathogen–host–environment interactions, is critical for developing effective interventions. Moreover, antimicrobial resistance associated with BRD pathogens adds urgency to adopting evidence-based management and therapeutic strategies [20]. The objective of this comprehensive review is to provide an overview of the existing literature on bovine respiratory BRD, with a particular emphasis on clinical diagnostics as well as evaluating the complex interplay of infectious agents, host factors, stressors, and management practises that contribute to its onset and progression.

A comprehensive literature review was conducted to identify relevant studies addressing bovine respiratory disease (BRD) in cattle. Multiple electronic databases were searched, including Web of Science, PubMed, Science Citation Index Expanded (SCI-Expanded), Scopus, and CAB Abstracts (CABI). Additional searches were performed via Google Scholar to capture grey literature and articles not indexed in the primary databases. A wide range of keywords and search phrases were employed using Boolean operators (AND, OR) to ensure both breadth and specificity. The search focused on six thematic areas: disease-related terms (e.g., “bovine respiratory disease”, “BRD complex”, “respiratory tract infections in cattle”, “cattle pneumonia”, “BRD pathogens”, “respiratory co-infections”, “viral and bacterial synergy”); diagnosis and clinical signs (e.g., “clinical respiratory scoring”, “respiratory scoring systems”, “rectal temperature in BRD”, “visual assessment of cattle”, “clinical signs of BRD”); animal behaviour (e.g., “behavioural changes in sick cattle”, “depression scores in BRD”, “feeding behaviour and BRD”, “locomotion and respiratory disease”); epidemiology and prevalence (e.g., “BRD prevalence”, “incidence of respiratory disease in feedlot cattle”, “risk factors for BRD”, “BRD outbreaks”, “seasonal variation in BRD”); economic aspects (e.g., “economic impact of BRD”, “cost of treatment for BRD”, “losses due to BRD”, “feedlot economics and respiratory disease”, “economic burden of BRD in beef/dairy industry”); and treatment and prevention (e.g., “antimicrobial treatment for BRD”, “vaccination against BRD”, “antimicrobial resistance in BRD pathogens”, “BRD metaphylaxis”, “preventive strategies for respiratory infections”). Search filters were applied to prioritize literature published within the last 10 to 15 years; however, high-impact foundational studies published earlier were also included when deemed scientifically relevant to contextualize current findings. Following the application of filters, the search identified a total of 181 studies. Articles were included if they: focused on BRD in cattle (beef or dairy); reported primary data or were systematic/narrative reviews relevant to the six thematic areas; were published in English. The extracted data included: study design (e.g., observational, experimental, clinical trial, review); population (animal) characteristics (e.g., cattle breed, age, production system); definitions and diagnostic criteria for BRD; identified pathogens (viral and/or bacterial); interventions or treatment strategies, including vaccinations; outcome measures (e.g., morbidity, mortality, economic impact). Studies were assessed using the following criteria: risk of bias, consistency of results, directness of evidence, precision, and publication bias. Based on these criteria, the quality of evidence was categorized as high, moderate, low, or very low. In cases where conflicting results were identified among studies, findings were analyzed in the context of their methodology, diagnostic criteria and study population.

The health of livestock and productivity levels are intrinsically linked. Animal diseases pose a threat to animal welfare, the environment, public health and the economy [17]. Animal diseases pose significant threats to animal welfare, the environment, public health, and the economy [17]. The costs associated with animal disease outbreaks can be categorized as direct or indirect [17]. Direct costs include visible losses such as animal deaths, reduced production yields, and slower growth rates, as well as less obvious impacts like decreased fertility leading to fewer offspring or restricted market access due to disease presence [18]. Indirect costs are those that occur in other markets (international trade, tourism) after the declaration of disease absence [17]. Often disease outbreaks can lead to reduced productivity and performance throughout the animal’s lifetime [18].

In Australia, a study evaluated the economic impact of BRD in Australian feedlot cattle reported that animals exhibiting subclinical and clinical BRD had carcass weights reduced by 16 kg and 24.1 kg, respectively, and generated returns that were AUD 67.10 (EUR 37.40) and AUD 213.90 (EUR 119.23) lower compared to healthy animals [19]. Additionally, mortality among early fetal and neonatal calves significantly contributes to elevated production costs [20].

An US national survey [21] reported that the median annual costs for medicine and labour to treat pre-weaned calves for BRD were USD 11.00 (EUR 9.42) and USD 15.00 (EUR 12.84)/affected calf, respectively. Adjusted mean annual BRD vaccine cost for pre-weaned calves (USD 7.67 (EUR 6.57)/animal) was significantly greater than that for cows (USD 3.18 (EUR 2.72)/animal) and heifers (USD 4.48 (EUR 3.84)/animal) while vaccination costs varied by age and ranged from USD 2.25 (EUR 1.93) to USD 6.25 (EUR 5.35)/head. In the US, using longitudinal treatment data from a large cohort of 11,470 pre-weaned dairy calves in California it was estimated that the average short-term economic cost of BRD was USD 42.15 (EUR 36.08) per affected calf [22]. This estimate included expenses such as treatment costs, including the use of anti-inflammatory medications [22]. Another study, across the US [23] reported that of 104,100 dairy replacement heifers from across the US, 36.6% had one or more cases diagnosed within the first 120 days of age with the highest risk of new cases occurring prior to weaning. Comparison of the raising cost for heifers with BRD and those without a recorded history of BRD resulted in an estimated cost per incident occurring in the first 120 days of age of USD 252 (EUR 215.66) or USD 282 (EUR 241.33). Efforts to treat these infections continue to rely largely on the use antimicrobial drugs, which through overuse, specifically in the agricultural sector, can lead to antimicrobial resistance (AMR) [24]. A further Australian study estimated that cattle that received three or more antibiotic treatments for BRD generated, on average, (covering treatment, reduced growth, and raising costs) AUD 384.97 (EUR 214.55) less in returns compared to untreated animals [19].

A study in the Netherlands [25] quantified the economic impact of BRD on veal calves, by measuring losses in hot carcass weight (HCW) and carcass quality. Calves experiencing one BRD episode lost on average 8 kg of HCW, resulting in financial losses ranging from EUR 36.8 to EUR 64.8 depending on the breed (Belgian Holstein Friesian (BHF), Red Holstein Friesian (RHF), crossbreds, or Belgian Blue (BB)). In veal calves with three BRD episodes, particularly BB and BHF, losses were much higher. BRD also increased the odds of inferior carcass quality and undesirable meat colour, further compounding financial losses. In France, a modelling study [26] suggested that eliminating BRD in beef calves could boost national beef-sector productivity by ~5.1%, equivalent to EUR 95.5 million annually in revenue and would have a much lower effect on the dairy systems (dairy calves, dairy young bulls or veal calves). In summary, BRD imposes substantial direct economic costs (treatment, mortality, weight losses) and indirect costs (impaired fertility, degraded carcass quality, trade limitations). Addressing both treatment costs and productivity losses is essential to mitigate the disease’s multifaceted effects, thereby improving animal health outcomes, optimizing production efficiency, and enhancing the overall sustainability of livestock industries worldwide. Furthermore, there is a critical need for more comprehensive economic modelling that integrates short-term treatment costs with long-term productivity losses across different production systems and regions. Such models would improve the accuracy of economic impact assessments and inform targeted resource allocation to enhance disease management and industry sustainability globally.

BRD is present among cattle populations worldwide. The prevalence of BRD varies widely across regions, seasons, and production systems. This variation can be attributed to a complex interplay of environmental, management, animal-specific, and diagnostic factors. Cold, damp weather and high stocking densities increase stress and facilitate the spread of respiratory pathogens. Poor ventilation and management practises, including inadequate colostrum intake or vaccination, also contribute to higher disease rates. Additionally, differences in diagnostic methods, such as clinical scoring versus thoracic ultrasonography, significantly affect reported prevalence. Production systems (e.g., intensive indoor vs. pasture-based) further influence exposure risk and disease outcomes. In Ireland, BRD was the largest cause of mortality in calves aged between one and six months of age in 2023 [27]. The estimated prevalence of BRD in Irish spring calving dairy herds was reported to be 4% [28], which is lower or like those found in other dairy production systems. A large-scale study of BRD on dairy farms in California found a BRD prevalence of 8.2% based on calves scoring positive by observed clinical signs [22]. A separate study, which did not use thoracic auscultation or TUS, but included a larger sample of dairies, estimated a 6.9% prevalence of BRD in 4636 calves across 100 dairies in California [29]. A BRD prevalence of 14% was reported in calves in Wisconsin during the winter months, based on assessments using the Wisconsin respiratory scoring system [30]. In a separate study, herd-level BRD prevalence on dairy farms in Quebec, identified by lung consolidation of ≥3 cm on thoracic ultrasonography (TUS), was recorded at 8% in the summer and rose to 15% in the winter [3]. In France, the farm level incidence of acute BRD was 9.8%, with the cumulative incidence at animal level at 2.1% [31].

A Belgian study reported that 20.2% of calves exhibited a CRS of 5 or higher, while 16.1% of calves had a TUS of 6 cm or greater [32]. In Swiss veal calves, the prevalence of key BRD viruses were evaluated from nasal swabs with findings showing a prevalence of 2.1%, 3.3%, 53.5%, 4.1% and 0% for bovine respiratory syncytial virus (BRSV), bovine parainfluenza virus type 3 (BPI3V), bovine coronavirus (BCoV), influenza D virus (IDV), and influenza C virus (ICV), respectively [8]. The wide variation in BRD prevalence is mainly driven by differences in several factors including: Climate: colder, wetter conditions promote pathogen survival and cattle stress. Herd density and management: overcrowding, poor ventilation, and inadequate preventive measures increase risk. Diagnostic criteria: variation in how BRD is detected (clinical signs vs. ultrasound) causes differences in reported prevalence. Production systems: indoor, intensive systems generally show higher prevalence than pasture-based systems. These factors highlight the need for regionally tailored prevention and control strategies that consider local climate, management practises, and diagnostic methods. In summary, diagnostic approaches, including clinical scoring systems and TUS, strongly influence reported prevalence. These findings highlight the need for standardized diagnostic criteria and support the importance of targeted prevention strategies and surveillance tailored to regional risk factors. Additionally, the marked seasonal and management-related variations in BRD prevalence suggest that prevention and treatment plans must be tailored to local risk factors rather than applying a one-size-fits-all approach.

Although not the only mechanism of BRD infection, it is believed that initial viral infections compromise the respiratory tract epithelium making it more susceptible to bacterial colonization and subsequently leading to secondary bacterial infections [33,34]. The often polymicrobial nature of BRD can impact disease severity, with certain combinations of pathogens resulting in enhanced disease [15]. Although BRD affects cattle of all ages, it is often more severe in younger calves [35]. In 2023, one to six months old calves (n = 285) accounted for 41% of all cases submitted to the Irish national veterinary diagnostic labs [27]. For example, out of the 285 calves that were submitted for post-mortem BRD diagnostics, the distribution of identified agents was as follows: bacterial infections (73.3%), parasitic infections (12.6%), viral infections (10.2%), and cases with no identifiable agent represented 3.9% of the total submissions. There are a range of viral and bacterial agents that are often implicated in BRD cases, many of which are commensals within the bovine respiratory tract [36]. Due to the multifactorial nature of disease onset and the interaction between the various viral and bacterial pathogens, BRD is difficult to control and prevent [37,38]. Although single-pathogen infections can initiate outbreaks of BRD, co-infections—most commonly involving a primary viral infection that disrupts the URT microbiota and facilitates secondary bacterial colonization of the LRT—are the more prevalent pathogenic mechanism. The following sections of this review will detail the main viral and bacterial pathogens commonly associated with BRD, highlighting their roles in disease progression and interactions within the respiratory tract.

A thorough physical examination remains a cornerstone in the clinical diagnosis of BRD. Key examination skills include auscultation of the thorax to detect abnormal lung sounds such as crackles, wheezes, or decreased breath sounds, which may indicate pneumonia or airway obstruction. Measurement of rectal temperature is critical for identifying febrile responses; however, fever patterns can vary depending on the causative pathogen and stage of infection. Assessment of hydration status through evaluation of mucous membrane moisture, skin tenting, and eye recession provides insight into systemic illness and disease severity. Additionally, observation of rumen motility can offer indirect information on the animal’s overall health and response to disease. Respiratory rate and effort, nasal and ocular discharge, coughing, and general demeanour should also be systematically evaluated. Integrating these physical examination findings with clinical respiratory scoring enhances diagnostic accuracy and supports timely intervention. Nonetheless, the accurate and timely diagnosis of BRD remains a considerable challenge within both clinical (veterinary clinics, laboratories) and field (on-farm) settings. Affected animals can display a wide range of symptoms including fever, fatigue, nasal and ocular discharge, coughing and loss of appetite [35]. The severity and duration of these clinical signs can vary depending on the causative pathogen [3]. With disease diagnosis commonly based upon the observation of clinical signs in sick animals, subclinical infections can often go undetected [3]. Accurately diagnosing BRD, especially in distinguishing between viral and bacterial aetiologies and determining whether the infection involves the upper or lower respiratory tract, continues to be a significant challenge [125]. In addition, many of the clinical symptoms are non-specific to BRD meaning a diverse range of pathogens can cause them, thereby making it difficult to pinpoint the exact cause of illness. To address these diagnostic challenges, the subsequent sections will review the clinical diagnostic tools available for BRD, highlighting their applications in identifying the nature of the infection and localizing respiratory tract involvement. An integrated decision-making flowchart for BRD diagnosis is presented in Figure 1. The flowchart shows the sequential application from routine herd health monitoring, clinical scoring, thoracic ultrasound (TUS), and computer-aided lung auscultation (CALA) to guide treatment decisions, pathogen diagnostics, vaccination, protocols and biosecurity measures.

Variations in vaccine efficacy and optimal vaccination timing between beef and dairy calves are influenced by distinct physiological, immunological, and management factors inherent to each production system. Beef calves are commonly exposed to acute stressors, such as weaning, transportation, and commingling, collectively known as “shipping fever,” which induce transient immunosuppression [4,6,34,42,45]. This immunosuppression heightens susceptibility to bovine respiratory pathogens and can impair the development of protective immunity if vaccination is not administered sufficiently prior to these stress events. Therefore, timely vaccination and appropriate booster administration before feedlot entry is critical for achieving effective immunoprotection in beef calves. In contrast, dairy calves typically develop enzootic pneumonia during the pre-weaning to early post-weaning period (2–6 months of age), often within group housing systems characterized by continuous exposure to respiratory pathogens [3,7,11,12,28,142]. At this stage, maternally derived antibodies (MDA) are generally present at high titers and may interfere with vaccine antigen recognition, attenuating the calves’ active immune responses, particularly to inactivated vaccines. Furthermore, the temporal pattern of BRD incidence in dairy calves is less associated with discrete stressors compared to beef calves, complicating the identification of an optimal vaccination window. Recognizing these mechanistic distinctions is essential for the development of tailored vaccination protocols that maximize protective efficacy against BRD within specific cattle production systems.

Antimicrobial metaphylaxis has historically been employed as a preventative strategy to reduce the incidence of BRD development in high-risk cattle populations. However, increasing regulatory restrictions on antimicrobial use, coupled with concerns regarding the contribution of metaphylaxis to AMR, have prompted a re-evaluation of this approach. In the context of BRD, the reliance on antibiotics is particularly critical, as they remain a primary treatment option for secondary bacterial infections, especially in severe disease cases. Consequently, emphasis has shifted toward the implementation of comprehensive disease prevention strategies that prioritize good husbandry and management practises, alongside effective vaccination programmes. These integrated approaches aim to reduce disease incidence, improve animal welfare, and mitigate the risk of AMR development. Although vaccines are currently used to prevent BRD outbreaks, the disease continues to be a significant challenge in dairy and beef production systems globally [6,28,29,145,146].

There are many types of vaccines used against both viral and bacterial BRD pathogens in both beef and dairy cattle. Viral vaccines either contain modified-live virus (MLV) or inactivated virus [147,148,149]. Multivalent viral vaccines, targeting more than one virus, are commercially available in Ireland. Generally, MLV vaccines have been shown to induce strong humoral and cell-mediated immunity with fewer doses required to provide protection [150,151,152]. Killed or inactivated viral vaccines induce robust humoral responses but weaker cell-mediated immunity and require at least two doses to provide adequate protection [153]. Additionally, bacterin and/or leukotoxoid vaccines are available against bacterial BRD pathogens such as M. haemolytica, P. multocida and H. somni [154].

The effectiveness of BRD vaccines for disease prevention has been the subject of many studies. The duration of immunity provided by a quadrivalent vaccine targeting BoHV-1, PI3, BVDV, and BRSV was studied, with results indicating reduced viral shedding in vaccinated animals compared to controls [155]. Beef steers that received a modified live vaccine (MLV) against BVDV demonstrated improved feed intake and feeding behaviours after viral challenge compared to those given either a killed vaccine or no vaccine [156]. Additionally, vaccination with a quadrivalent vaccine (inactivated BoHV-1, MLV BRSV, inactivated BVDV, and MLV PI3) followed by a booster 21–27 days later significantly decreased morbidity and mortality rates from BRD in vaccinated animals compared to unvaccinated controls [157]. A recent study reported a BRSV outbreak on a single farm where vaccinated animals showed significantly reduced morbidity (20.4%) and disease severity (score of 1.70) compared to unvaccinated controls, which had morbidity of 53.7% and a severity score of 2.11 [158]. Overall, vaccinated animals had a significantly lower number of cases (0.36 vs. 0.64 cases/calf), lower morbidity (26.78% vs. 41.24%), and lower antimicrobial treatments (33.3% vs. 57.4%). In addition, vaccinated animals exhibited a numerically higher average daily weight gain (ADWG) and a significantly greater carcass weight compared to controls, with increases of 35.78 g/day and 6.58 kg, respectively. However, vaccination against BRD is not always successful and certain factors can influence the efficacy of vaccines. Calves which had pre-existing maternally derived antibodies (MDAs) were reported to respond poorly to foot and mouth disease vaccination [159]. The administration of an intramuscular MLV vaccine against BVDV 1 and 2, BRSV, BoHV-1 and PI3 to dairy heifer calves found no difference in the risk of developing BRD or the risk of mortality between vaccinated and unvaccinated calves, with MDA interference suggested as a possible reason for these findings [160]. In a study comparing the efficacy of a bivalent modified-live (ML) vaccine against BRSV and PI3 virus in Czech-pied Holstein calves with and without MDAs, vaccinated calves showed significantly lower clinical scores and reduced viral nasal shedding compared to unvaccinated controls, regardless of the presence of MDAs [161]. Furthermore, it was demonstrated that a live intranasal vaccine triggered an anamnestic IgG response after a BRSV challenge in calves regardless of the presence of MDAs [162]. This indicates that early intranasal vaccination may be an effective approach to protect calves during the critical period when MDA levels are declining but active immunity has not yet been fully established [162].

BRD pathogenesis often differs between beef and dairy cattle, which can impact vaccine effectiveness. Dairy calves often develop enzootic pneumonia between 2 and 6 months of age, while beef calves typically experience ‘shipping fever’ after weaning, often due to stress [2]. These differences affect the timing of peak BRD incidence, vaccination schedules, and the influence of maternal antibodies. It was suggested that these factors likely played a role in the significantly lower morbidity and mortality seen in vaccinated beef calves, a benefit that was not observed in vaccinated dairy calves compared to unvaccinated controls during natural exposure trials [147]. Additionally, the timing of vaccination is a crucial element influencing vaccine efficacy. Vaccinating feedlot cattle 15 days before weaning, followed by a booster 15 days before entering the feedlot, led to higher serum antibody titers against BRD viruses (BVDV, BoHV-1, BRSV, and PI3) at feedlot entry and a decreased incidence of BRD during the feedlot period compared to both control and delayed vaccination groups [163]. Recently, a study investigating the timing of administering a polyvalent (BoHV-1, PI3, BVDV-1, BVDV-2 and BRSV) vaccine found that animals that received a vaccination upon feedlot arrival had significantly lower antimicrobial use, greater average daily gain, and reached slaughter age earlier than animals that were vaccinated later [164]. While challenge studies have proven effective in evaluating vaccine efficacy, they are typically carried out under highly controlled conditions, which differ significantly from the more variable environments found on farms [165]. Additionally, the timing of viral challenge may not always concur with the timing of viral/bacterial outbreaks in a natural setting [149]. Therefore, further research on the efficacy of vaccines during natural field infections is warranted. Although evidence shows that many vaccines are effective in reducing morbidity and mortality in BRD cases, disease burden remains high in cattle populations and so further preventative measures are required to lower the incidence of BRD.

Recent studies have identified emerging pathogens, such as Influenza D virus, as contributing agents in the complex etiology of BRD. Currently, these pathogens are not incorporated into existing vaccination programmes, which primarily target well-established viral and bacterial agents. The role of emerging pathogens in BRD pathogenesis remains incompletely understood, and their inclusion in vaccination protocols has not yet been realized. This represents a critical research gap that warrants further investigation to elucidate their epidemiology, pathogenicity, and potential impact on vaccine efficacy. Developing vaccines targeting these novel agents could enhance disease control strategies and reduce the overall burden of BRD in cattle populations.

Effective prevention of BRD requires a multifaceted approach centred on reducing stress [166], implementing timely vaccination protocols, and managing concurrent health burdens such as parasitism [167,168]. In the past, prophylactic antimicrobial treatments were commonly used in high-risk weanlings to mitigate the effects of pneumonia, often compensating for suboptimal management. However, with the implementation of EU Regulation 2019/6 [169], the routine preventive use of antimicrobials in healthy animals has been banned. As a result, meticulous planning and management around the weaning period have become essential to BRD control. While vaccination remains a critical component in enhancing immunity against key BRD pathogens, it is not a substitute for sub-optimal husbandry management practises. Optimal nutrition, appropriate housing and ventilation, stress reduction, good Stockpersonship, and consideration of environmental factors such as weather and seasonal changes are all essential elements that influence BRD risk and animal resilience. Good husbandry practises including, ensuring adequate colostrum intake [170], minimizing transport stress [171,172], and maintaining clean, well-ventilated housing environments [173], help reduce the incidence and severity of BRD.

In the context of BRD prevention, the role of stress as a predisposing factor cannot be overstated. Stress disrupts normal immune function, rendering calves more vulnerable to respiratory pathogens [174]. While the physiological and psychological stresses of weaning are well-documented [175,176,177,178], it is equally important to consider the compounding impact of husbandry procedures such as dehorning [179] and castration [180] when these are carried out in close temporal proximity to weaning [2]. Both dehorning and castration are known to induce acute stress responses in calves, including elevated cortisol levels, behavioural distress, and inflammation [179,180]. These responses may compromise immune function, creating an immunosuppressive window during which the risk of BRD increases significantly. When these procedures coincide with other stressors, such as weaning, housing, or transport, the cumulative burden can overwhelm the calf’s ability to mount an effective defence against respiratory pathogens [6,7,28,34,142]. To mitigate this risk, best practise guidelines recommend performing castration and dehorning at least four weeks prior to weaning, preferably during early life stages when the stress response and risk of complications are reduced [179,180]. Integrating the timing of these interventions into a comprehensive weaning management plan is therefore essential. This not only improves calf welfare but also supports the effectiveness of other BRD prevention strategies, such as vaccination and nutritional support. Additionally, monitoring animal behaviour and early clinical signs supports timely intervention, which is key to effective disease control [181].

Although this review focuses primarily on clinical diagnostic methods and vaccination strategies, we emphasize that these should be implemented within an integrated management framework. Addressing underlying management and environmental factors is fundamental to successful BRD prevention and to reducing reliance on antimicrobial treatments. Future reviews could explore in greater depth the roles of nutrition, housing design and air microbiome, Stockperson training, environmental management in BRD control, as these factors are critical in supporting overall herd health and welfare and reducing AMR development.

BRD remains a critical health challenge in cattle worldwide, significantly impacting animal welfare and industry profitability. Key viruses such as BoHV-1, BRSV, BPIV-3, BVDV, and BCoV are highly prevalent and play significant roles in the onset and progression of BRD. Emerging viruses like Influenza D and C are increasingly recognized as important contributors. The endemic nature of these viruses, especially BoHV-1 and BVDV, alongside their capacity for latency and immunosuppression, complicates disease control.

BRD is also associated with a diverse range of bacterial pathogens, including Pasteurella multocida, Mannheimia haemolytica, Histophilus somni, M. bovis, and Mycoplasma bovirhinis, each exhibiting varying roles in disease progression and prevalence across regions. Emerging evidence highlights the complexity of the bovine respiratory microbiome, where opportunistic pathogens such as Moraxella spp. and M. bovirhinis may also contribute to respiratory health or disease, depending on host and environmental factors. The complexity of the multifactorial causes of BRD and the frequent presence of subclinical infections complicate timely and accurate diagnosis. While traditional clinical scoring methods provide a useful framework, their limitations in sensitivity and subjectivity highlight the need for more objective diagnostic tools. Advances such as TUS and behavioural monitoring show promise in improving early detection and disease management. Addressing BRD effectively requires integrated approaches combining improved diagnostics, targeted treatments, vaccination and preventive strategies to reduce antimicrobial use and mitigate economic losses. Continued research and innovation in diagnostics and management practises are essential to control BRD and support sustainable cattle production.

The challenges posed by BRD have significant implications for animal health, farm economics, and public health. Inaccurate or delayed diagnosis can lead to untreated or improperly managed infections, increasing mortality rates and reducing growth performance in affected cattle. This results in direct financial losses for farmers due to decreased productivity and increased treatment costs. Moreover, reliance on antimicrobials for BRD treatment raises concerns about AMR, which threatens both animal and human health globally. In summary, the implications of BRD extend beyond immediate animal health issues, influencing economic sustainability, public health, and animal welfare, underscoring the urgent need for improved diagnostic and preventive strategies.

Effective control of BRD requires an integrated, multifactorial approach. While current strategies have improved early detection and mitigation, several areas warrant further investigation to optimize long-term outcomes.

Diagnostic tools including TUS, CALA, and CRS require validation across diverse production systems. Research should focus on tailoring these tools to different animal populations (dairy and beef), housing types, animal ages, and management conditions to ensure diagnostic accuracy and consistency. For example, TUS protocols may vary depending on whether calves are restrained or not, and CRS tools validated in dairy systems often lack sensitivity and specificity when applied to feedlot beef calves. Future work should focus on tailoring and validating these diagnostic protocols to the unique handling and environmental conditions found in beef feedlots, dairy farms, and veal production systems.

Automated behavioural monitoring, remote temperature sensing, and AI-driven health alert systems offer promise for real-time BRD detection. These technologies require validation across commercial settings and integration with decision-support frameworks to enable early, targeted interventions.

Environmental conditions—including ventilation, dust levels, humidity, ammonia, and other noxious gasses—substantially impact respiratory health. In addition to these abiotic factors, the air microbiome is increasingly recognized as a potential contributor to BRD risk, influencing pathogen exposure and immune responses. Future studies should characterize airborne microbial communities in cattle housing environments and assess their association with BRD incidence.

Emerging pathogens such as IDV are increasingly implicated in BRD but remain unaddressed by current vaccines. Future research should prioritize defining the epidemiology, virulence, and interactions of pathogens with co-infecting agents to inform the development of multivalent or platform-based vaccines. Treatment approaches, especially those integrated with advances in molecular and laboratory diagnostics of BRD pathogens, are beyond the scope of this review. These interventions, along with improved molecular diagnostics, hold significant potential to reduce antimicrobial use—a critical step in combating AMR.

The substantial economic burden of BRD highlights the urgent need for improved prevention and control strategies, particularly in the context of vaccination. Variability in vaccine efficacy, age-dependent responses contribute not only to ongoing treatment costs but also to long-term productivity losses. Integrating these challenges into economic modelling frameworks will provide a more accurate assessment of cost-effectiveness, thereby informing optimized vaccination protocols and resource allocation strategies tailored to specific production systems.

The following abbreviations are used in this manuscript:

Conceptualization, S.O., B.E., S.M.W., and D.W.M.; methodology, S.O. and B.E.; writing—original draft preparation, S.O., B.E., S.M.W., and D.W.M.; writing—review and editing, S.O., B.E., S.M.W., and D.W.M.; visualization, S.O., B.E., S.M.W., and D.W.M.; supervision, S.M.W., B.E., and D.W.M.; funding acquisition, S.M.W., and B.E. All authors have read and agreed to the published version of the manuscript.

Ethical approval was not required.

Data are contained within the review paper.

The authors declare no conflicts of interest.

The authors gratefully acknowledge funding from; Department of Agriculture, Food and the Marine (DAFM) US-Ireland R&D partnership call (Project 16/RD/US-ROI/11), the US-Ireland Tri Partite Grant (2018US-IRL200), and the European Union Horizons 2020, HoloRuminant project (Grant agreement No. 101000213).