BRD in Cattle: Symptoms, Prevention, and Treatment

Bovine respiratory disease is the single most common and costly illness in the beef and dairy cattle industries, responsible for more treatment costs, more deaths, and more lost productivity than any other health problem feedlot and cow-calf operators face. BRD is not one infection but a syndrome driven by the collision of viral and bacterial pathogens, environmental stress, and compromised immune defenses. Understanding how it develops, what it looks like, and how to fight it has occupied veterinary researchers for decades, and the picture that has emerged is more complicated than “cattle catch pneumonia.”

What Actually Causes BRD

BRD is a multifactorial disease, meaning no single germ is the villain. Typically, one or more viruses weaken the respiratory tract first, and then opportunistic bacteria move in and cause the severe pneumonia that does the real damage. The key bacterial players are Mannheimia haemolytica (the most common and most damaging), Pasteurella multocida, Histophilus somni, and Mycoplasma bovis. Mixed infections involving more than one of these bacteria are common, and while each species has its own set of virulence strategies, the resulting lung damage often looks similar on necropsy.¹

On the viral side, the usual suspects include bovine herpesvirus 1 (IBR), bovine respiratory syncytial virus (BRSV), and parainfluenza virus type 3 (PI3). But a metagenomics study found that bovine adenovirus 3, bovine rhinitis A virus, and bovine influenza D virus were also significantly associated with BRD, appearing alone or together in about 62% of affected animals.² The viral cast is broader than many producers realize, which partly explains why no single vaccine program eliminates the problem.

Most of these bacteria live harmlessly in the nasal passages of healthy cattle. They become dangerous only when the animal’s respiratory defenses break down due to stress, viral infection, sudden exposure to cold air, poor ventilation, or elevated cortisol from handling and transport.³ That is why BRD is sometimes called “shipping fever”: it tends to erupt after calves endure the stresses of weaning, trucking, and commingling with unfamiliar animals.

Why Transport and Stress Are Central

The link between transportation and BRD has been recognized for a long time. During shipping, calves face a pile-up of stressors at once: food and water deprivation, novel social groups, temperature swings, dust, fatigue, and the physical jostling of the truck itself. Researchers broadly agree that these combined stressors cause immunosuppression, giving opportunistic pathogens an opening to invade the respiratory tract.⁴ The result is that BRD cases spike in the first few weeks after cattle arrive at a feedlot or stocker operation.

Commingling is especially risky because calves from different sources carry different pathogen populations. When they are thrown together, each animal is exposed to microbes its immune system has never encountered. Add the cortisol surge from handling and the disrupted eating patterns that come with a new environment, and you have the perfect storm for respiratory disease.

Recognizing the Signs

The classic symptoms of BRD include nasal discharge, coughing, labored breathing, drooping ears, depression, and reduced feed intake. In feedlot settings, pen riders typically watch for animals that look dull, lag behind the group, or breathe with visible effort. A study monitoring feedlot steers found that animals exhibiting visual signs like difficult breathing and coughing were pulled for examination, and those that ultimately died from BRD had lower weight gains, higher visual BRD scores, and required more treatment rounds than animals that recovered.⁵

The trouble is that cattle are prey animals. They are wired to hide illness until it becomes severe. By the time a steer is obviously sick enough for a pen rider to spot, the lung damage may already be extensive. Clinical respiratory scoring systems, which assign points based on temperature, nasal discharge, eye discharge, ear position, and cough, are the standard tool for on-farm diagnosis. But their sensitivity is limited. Estimates of how well these scoring systems catch true BRD cases range from about 30% to 72%, while specificity runs from roughly 86% to 94%.⁶ In practical terms, in a group where one in five calves has BRD, a scoring system with 30% sensitivity would miss seven out of ten sick animals.

The Subclinical Problem

Some cattle develop lung damage from BRD without ever showing obvious clinical signs. These subclinical cases are invisible to pen riders but still hurt the animal’s performance. A study of over 1,100 cattle at slaughter found that about 18% had lung lesions consistent with pneumonia. Animals with those lesions had significantly lower carcass weights, with the effect being most pronounced in male animals and in veal calves.⁷ This means a substantial portion of cattle carry silent BRD damage that erodes profitability without anyone knowing until the carcass hits the rail.

Subclinical BRD is one reason the disease’s true economic toll is so hard to pin down. The visible costs, treatment drugs and labor, are real enough. But the invisible costs from reduced feed efficiency, lighter carcasses, and lower quality grades in animals that were never diagnosed may be even larger.

Better Diagnostic Tools

Given the limitations of visual scoring, researchers have been looking for more objective ways to catch BRD earlier. Thoracic ultrasound has emerged as a promising option, especially for young calves. A study of pre-weaned calves found that ultrasound was a more reliable classifier of BRD than clinical respiratory scoring, and was particularly useful for picking up subclinical cases in newborns.⁸ Using a threshold of lung consolidation at least 3 centimeters deep behind the heart, ultrasound achieved estimated sensitivity around 89% and specificity around 95% for active pneumonia in calves.⁹

Blood biomarkers can add another layer. A comparison of ultrasound, clinical scoring, and blood analysis in transported calves found that calves with more severe ultrasound scores had lower lymphocyte counts, higher serum amyloid-A (an acute-phase protein that rises during inflammation), and lower total immunoglobulin levels.¹⁰ These patterns suggest that combining ultrasound with targeted blood work could give producers and veterinarians a much more accurate picture of which animals are in trouble.

On the technology frontier, sensor-based systems that track feeding behavior, movement, and social interactions are being tested for early BRD detection. One study using machine learning on behavioral data from pre-weaned calves achieved moderate accuracy, correctly flagging about 63% of BRD cases while maintaining high specificity.¹¹ The technology is not yet reliable enough to replace a trained eye, but it points toward a future where automated monitoring catches illness before it becomes clinically obvious.¹²

How BRD Is Treated

Antibiotics are the backbone of BRD treatment once an animal is diagnosed. The most commonly used classes include macrolides (like tulathromycin and tilmicosin), fluoroquinolones (like enrofloxacin), phenicols (like florfenicol), and tetracyclines (like oxytetracycline). A meta-analysis pooling 93 trials across 12 antibiotics confirmed that there are meaningful differences between drugs in how well they resolve BRD, including comparisons that had never been tested head-to-head in a single trial.¹³

What may matter as much as the first drug choice is the sequence of treatments when an animal does not respond. A feedlot study found that calves receiving bactericidal antibiotics (drugs that kill bacteria directly) for both their first and second treatments had roughly a 50% chance of needing four or more total treatments, compared to over 70% for calves whose first drug was bacteriostatic (drugs that slow bacterial growth without killing outright). The bactericidal-first calves also gained weight faster and produced higher-quality carcasses.¹⁴ The implication is that the pharmacological properties of the drug, not just its spectrum of activity, should factor into treatment decisions.

The Role of Anti-Inflammatory Drugs

Because BRD involves serious lung inflammation, it seems logical that adding a non-steroidal anti-inflammatory drug (NSAID) to antibiotics would help. The answer is complicated. A single-dose trial of carprofen alongside tilmicosin found that the combination lowered fever and respiratory rates faster within the first six hours and improved animal comfort over the observation period, even though the final clinical resolution was similar to the antibiotic alone.¹⁵ From a welfare standpoint, the animal felt better sooner.

But a systematic review and meta-analysis of 17 randomized trials covering nearly 5,000 animals found that adding an NSAID to antibiotic treatment did not reduce the need for retreatment. The combined risk ratio was 0.94, meaning essentially no difference.¹⁶ And in an experimental BRSV infection study, early NSAID treatment had no significant effect on virus replication, clinical signs, or lung lesion extent, leading researchers to conclude NSAIDs should not be relied on to prevent the clinical expression of viral respiratory disease.¹⁷ The upshot: NSAIDs may help an animal feel more comfortable during acute illness, but they do not meaningfully change clinical outcomes or reduce the need for additional antibiotic treatment.

Prevention Strategies

Because BRD is so difficult and expensive to treat once it develops, prevention has always been the preferred approach. The main pillars are preconditioning, vaccination, nutrition, and management practices that reduce stress.

Preconditioning

Preconditioning programs prepare calves for the stresses of weaning and shipping before they leave the ranch. A typical program includes weaning calves at least 45 days before sale, training them to eat from a bunk and drink from a trough, administering respiratory vaccines, deworming, and castrating. The goal is to build the calf’s immunity and reduce the shock of transitioning to a feedlot environment.¹⁸ Preconditioned calves consistently get sick less often and perform better on feed, though the cost of preconditioning falls on the cow-calf producer while many of the benefits accrue to the feedlot.

Vaccination

Respiratory vaccines target the major viral culprits (IBR, BRSV, PI3, and sometimes bovine viral diarrhea virus) and sometimes bacterial pathogens like Mannheimia haemolytica. Both injectable (parenteral) and intranasal routes are used, and research suggests they stimulate the immune system in different ways. Intranasal vaccines boost local airway defenses more directly, increasing neutrophil activity and immunoglobulin A (IgA) levels in the fluid lining the lungs within days of administration, whereas injectable vaccines did not produce the same local airway changes in one heifer study.¹⁹

A field trial in pre-weaned dairy calves showed that intranasal vaccination at three weeks of age primed the immune system so that a follow-up injectable vaccine at 15 weeks triggered a stronger recall response to BRSV and PI3 antigens than in calves that only received the delayed injectable vaccine.²⁰ This “prime-boost” approach, intranasal first then injectable later, is gaining traction in practice. However, comparing vaccine programs head-to-head in feedlot heifers, one trial found that morbidity and BRD mortality rates were similar across intranasal, parenteral, and combination protocols, though total death loss tended to be lower in heifers that received the combination.²¹ Vaccines reduce risk but do not eliminate it.

Nutrition and Mineral Status

Micronutrient deficiencies can quietly undermine a calf’s ability to fight off respiratory infection. Dietary mineral levels that seem adequate for growth may be insufficient during times of physiological stress like weaning and transport, especially when feed intake drops. Copper and selenium deficiencies, for instance, impair the ability of white blood cells to kill ingested bacteria. Supplemental vitamin E, an intracellular antioxidant, has been shown to reduce illness rates in stressed calves.²² This is an area where the payoff from a good mineral program may not be obvious until calves face a health challenge.

Metaphylaxis

Metaphylaxis, the practice of giving antibiotics to an entire group of at-risk cattle upon arrival at a feedlot, is one of the most debated tools in BRD prevention. In a randomized trial of medium-risk feedlot calves, metaphylaxis-treated cattle had roughly half the BRD morbidity of calves that only received treatment when they showed signs of illness (about 7% versus 17%), along with lower total mortality.²³ However, metaphylaxis-treated calves used about five times as many antibiotic doses per animal, and the net economic returns per head were not significantly different between groups. That last detail is worth noting: the health advantage did not consistently translate into a profit advantage.

Observational feedlot data have complicated the picture further. A study using propensity score matching on real-world feedlot records could not identify a clear positive effect of metaphylaxis, with point estimates actually going in the wrong direction for daily gain, death loss, and feed efficiency.²⁴ The authors attributed this partly to the difficulty of controlling for all the factors that lead feedlot managers to choose metaphylaxis in the first place (sicker-looking cattle are more likely to get it). The randomized evidence supports a health benefit in at-risk groups, but the economic case is not always clear-cut, and the practice contributes to antibiotic use volumes that fuel resistance concerns.

Antimicrobial Resistance

The heavy use of antibiotics for BRD prevention and treatment has accelerated resistance among the very bacteria that cause the disease. A systematic review found clear trends: resistance among Mannheimia haemolytica isolates was highest to tulathromycin (around 24%) compared to florfenicol and tildipirosin (each around 6-7%). For Pasteurella multocida, tildipirosin resistance ran highest at about 22%. Critically, resistance was far higher in samples collected after cattle arrived at a feedlot than in samples taken on arrival, suggesting that antimicrobial exposure during the feeding period selects for resistant populations.²⁵

One lingering question is whether bacteria in healthy cattle carry silent resistance genes that only become expressed once antibiotics are administered. Diagnostic lab data show a clear decline in susceptibility over time for the three major BRD pathogens, but studies testing healthy cattle report much lower resistance levels. Whether that reflects a true absence of resistance genes or dormant genes that switch on under antibiotic pressure remains unclear.²⁶ Either way, the practical consequence is that drugs that worked reliably a decade ago may not work as well today, and veterinarians increasingly need culture and sensitivity data to guide treatment choices.

Breeding for BRD Resistance

Because management alone has not solved the BRD problem, there is growing interest in whether genetics can help. Studies have estimated that BRD resistance is heritable, though at a low level, with heritability estimates ranging from about 0.04 to 0.2. That means genetics plays a modest but real role in whether an animal gets sick. Using genomic selection, researchers working with over 1,200 Angus feedlot steers achieved moderately accurate predictions of which animals would need BRD treatment.²⁷ It is not a silver bullet, but over many breeding cycles, selecting for BRD resistance alongside other traits could meaningfully reduce disease incidence. A genomic analysis of dairy calves identified significant genetic markers on multiple chromosomes associated with BRD resistance, suggesting there are many small-effect genes at play rather than a single “resistance gene.”²⁸

The Nasal Microbiome Connection

Researchers are also looking beyond individual pathogens to the entire community of microbes living in the bovine respiratory tract. The emerging picture suggests that BRD is not simply about “bad” bacteria invading, but about a disruption of the normal microbial balance. Calves with respiratory disease tend to have lower diversity in their nasal microbiome, and overgrowth of Mycoplasma species has been linked to illness.²⁹

Interestingly, the known BRD-causing bacteria can be found in the noses of both sick and healthy calves at similar levels. One study tracking calves from diagnosis through recovery found no difference in the relative abundance of BRD pathobionts between affected and apparently healthy animals, and the pathogen profile was unique to each individual calf rather than following a predictable pattern.³⁰ Geography, the specific niche sampled (nose versus lung), and transportation history all shape the respiratory microbiome in ways that likely affect disease susceptibility.³¹ This line of research is still young, but it reinforces the idea that BRD is fundamentally about the collapse of a complex ecosystem rather than the presence of any single villain. Future interventions, perhaps probiotics or targeted microbiome management, could eventually complement vaccines and antibiotics rather than replace them.