Pasteurella in cattle refers to a group of bacteria that live harmlessly in the nasal passages and upper airways of healthy animals but can invade the lungs and cause severe, sometimes fatal pneumonia when cattle are stressed or weakened by viral infections. The two species that matter most are Mannheimia haemolytica (formerly classified as Pasteurella haemolytica) and Pasteurella multocida, both major contributors to bovine respiratory disease, or BRD. This disease complex costs the U.S. beef industry alone over $1 billion annually in treatment costs, death losses, and reduced performance.
How Pasteurella Turns From Harmless to Deadly
Under normal conditions, these bacteria are part of the natural microbial community in a cow’s upper respiratory tract. They sit in the tonsils, nasal passages, and throat without causing problems. The shift from commensal organism to aggressive pathogen typically requires a trigger, and that trigger is almost always stress, a viral infection, or both acting together.
Transport and commingling are the classic stressors. When calves are hauled to auction, mixed with unfamiliar animals, and shipped to a new facility, BRD cases typically peak in the first weeks after arrival. This pattern is so well recognized that pasteurellosis in cattle has long been called “shipping fever.” Research from Oklahoma State University found that stress hormones (glucocorticoids) released during transport alter the immune response in the airways. Interestingly, recent findings show that transport stress doesn’t simply suppress immunity. Instead, it triggers an exaggerated inflammatory response in the lungs, with increased neutrophil recruitment even before any pathogen has colonized the lower airway. That overreaction can damage lung tissue and create conditions for bacteria to establish a deep infection.
The Role of Viral Infections
Pasteurella rarely acts alone. The most common scenario is a primary viral infection followed by bacterial superinfection. The viruses most frequently involved are infectious bovine rhinotracheitis (IBR, caused by bovine herpesvirus-1), bovine viral diarrhea virus (BVDV), bovine parainfluenza virus type 3, and bovine respiratory syncytial virus.
These viruses damage the lining of the airways, strip away protective mucus, and impair the immune cells that would normally keep Pasteurella in check. The result can be dramatic. In one experimental study, calves infected with BVDV or Mannheimia haemolytica alone developed pneumonic lesions covering 2 to 15% of total lung volume. Calves infected with BVDV first and then exposed to the bacteria five days later developed severe fibrinopurulent bronchopneumonia affecting 40 to 75% of total lung volume. Coinfected animals showed the most severe clinical signs, and some were unable to fully recover.
What Pasteurella Does to the Lungs
Mannheimia haemolytica produces a toxin called leukotoxin, which is considered the primary factor driving lung injury in pneumonic pasteurellosis. This toxin is unusually specific: it targets bovine white blood cells while largely sparing cells of other species. When leukotoxin destroys these immune cells in the lungs, the dying cells release their own destructive enzymes and inflammatory chemicals, creating a cascade of tissue damage. The lungs fill with fluid, fibrin, and dead cells, making it progressively harder for the animal to breathe.
Pasteurella multocida causes a different pattern. In its most severe form, hemorrhagic septicemia, the bacteria enter the bloodstream and cause overwhelming infection. Animals develop a high fever and can die rapidly, with a near-100% fatality rate if untreated. This form is less common in North America but remains a serious concern in tropical regions.
Signs of Infection
The early signs are easy to miss if you’re not watching closely. Affected cattle typically show decreased feed intake, lethargy, and a droopy appearance before more obvious respiratory signs develop. As the infection progresses, you’ll see nasal discharge (starting clear and becoming thick or yellow), watery or crusty eyes, coughing, and labored breathing. Fever is a hallmark, and rectal temperatures above 104°F are common. Some animals, particularly with hemorrhagic septicemia or peracute Mannheimia infections, can be found dead without any prior signs.
The speed of onset matters. Cattle that looked fine at morning feeding and are severely depressed by afternoon likely have an aggressive bacterial pneumonia rather than a mild viral infection. Catching these animals early, within the first 24 hours of visible illness, significantly improves treatment outcomes.
Treatment With Antibiotics
Pasteurella infections in cattle are treated with antibiotics, and early intervention is critical. Several drug classes are effective, including macrolides, tetracyclines, and fluoroquinolones. In 2024, the FDA approved a new fluoroquinolone product (pradofloxacin) specifically for BRD associated with Mannheimia haemolytica, Pasteurella multocida, and other respiratory pathogens. It’s given as a single injection, which reduces handling stress on sick animals.
All antibiotics for treating BRD in cattle now require a veterinary prescription. This shift reflects growing concern about antimicrobial resistance. Both Mannheimia haemolytica and Pasteurella multocida have shown increasing resistance to commonly used antibiotics over time, partly driven by widespread antimicrobial use in the cattle industry. Working with a veterinarian to select the right drug based on local resistance patterns gives you the best chance of a successful outcome.
Even with appropriate antibiotic therapy, cattle that develop severe lung damage may never fully recover. Chronic cases often become “poor doers” with permanently reduced lung capacity, slower weight gain, and lower feed efficiency.
Prevention Through Preconditioning
The most effective strategy for preventing pasteurellosis is reducing the stress and immune compromise that allow the bacteria to invade in the first place. Preconditioning programs address the major risk factors before calves ever leave the ranch. A well-designed program includes vaccination, castration, dehorning, and a minimum 45-day weaning period before sale. Deworming and bunk training (teaching calves to eat from a feed trough and drink from a water source) are also commonly recommended so that calves arriving at a new facility don’t face the additional stress of unfamiliar feed and water systems on top of transport and commingling.
Vaccination programs for BRD typically include both viral and bacterial components. Vaccines targeting Mannheimia haemolytica are standard, while Pasteurella multocida vaccines may be added based on regional risk. In one Australian feedlot study, cattle vaccinated against Pasteurella multocida at arrival showed a 27% lower likelihood of developing subclinical BRD compared to unvaccinated controls. Vaccinated cattle also had 27% lower odds of lung consolidation and 35.5% lower odds of pleurisy (scarring of the lung lining) at slaughter. However, vaccination did not significantly reduce mortality in that study, reinforcing that vaccines are one layer of protection, not a complete solution.
Why Timing and Management Matter Most
Because Pasteurella bacteria already live in virtually every herd, you can’t eliminate them. The goal is managing the conditions that allow them to cause disease. That means minimizing transport distances and duration when possible, avoiding commingling cattle from multiple sources, ensuring adequate nutrition before and after stressful events, and providing clean, well-ventilated housing. Overcrowded, poorly ventilated barns with high humidity create ideal conditions for respiratory pathogens to spread and for stressed immune systems to fail.
Newly arrived cattle should be monitored closely for the first three to four weeks, the window when BRD risk is highest. Checking cattle twice daily and pulling any animal with early signs of illness gives you the best chance of treating successfully before permanent lung damage occurs. In a modeling study from France, eliminating BRD in beef calves would boost national beef-sector productivity by roughly 5.1%, equivalent to €95.5 million in annual revenue, a figure that underscores how much this single disease complex drags on the industry even in well-managed systems.