Mannheimia haemolytica is a primary bacterial pathogen in cattle, widely recognized for its role in severe pneumonia. This condition is a significant component of the Bovine Respiratory Disease (BRD) complex, often called “shipping fever.” The disease imposes substantial financial burdens on the beef and dairy industries, including veterinary treatments, production losses, and animal mortality. Annual losses in North American feedlots alone can reach an estimated $1 billion.
The Role of Stress and Co-Infection
Mannheimia haemolytica commonly exists in the upper respiratory tract of healthy cattle. It becomes disease-causing and invades the lungs when specific stressors are present. These stressors compromise the animal’s natural immune defenses, allowing the bacterium to proliferate rapidly and colonize the lower respiratory tract.
Stressors include long-distance transportation, weaning, and mixing with new animals. Sudden weather changes, like extreme cold or rapid temperature fluctuations, also suppress immunity. Inadequate nutrition can deplete energy reserves, making animals more vulnerable to disease.
Co-infection with various viruses often precedes M. haemolytica pneumonia, exacerbating its severity. Viruses like Infectious Bovine Rhinotracheitis (IBR), Bovine Viral Diarrhea Virus (BVDV), Parainfluenza-3 virus (PI3), and Bovine Respiratory Syncytial Virus (BRSV) can damage the respiratory tract lining. This damage impairs the lung’s defense mechanisms, providing an entry point and a favorable environment for M. haemolytica to establish infection.
Clinical Signs and Diagnosis
Cattle affected by Mannheimia haemolytica pneumonia often show rapid onset of clinical signs, usually 1 to 2 weeks after a stressful event like transport. Early signs can be subtle, including slight depression and lack of interest in feed. As the disease progresses, animals may become more lethargic, exhibiting a lowered or drooped head and ears.
A sudden increase in body temperature, often reaching 104.0°F to 107.0°F (40.0°C to 41.67°C), is a common and early sign. Breathing becomes rapid and shallow, and a soft, moist cough may be observed. Nasal discharge often begins as clear and watery, but quickly transitions to a thick, cloudy, or even purulent consistency as the infection advances.
In severe cases, affected animals may stand with their elbows positioned away from their chest walls, indicating pain and difficulty breathing. They might also be reluctant to move or stand with their head and neck extended. A veterinarian diagnoses M. haemolytica pneumonia by observing these clinical signs and listening for abnormal lung sounds, such as crackles or wheezes, using a stethoscope. To confirm the pathogen’s presence, deep nasopharyngeal swabs, transtracheal washes, or bronchoalveolar lavage samples may be collected for bacterial culture or PCR testing.
Treatment Approaches
Once Mannheimia haemolytica infection is confirmed or strongly suspected, prompt treatment is important. Managing affected cattle involves antimicrobial therapy and supportive care. A veterinarian’s guidance is necessary to select an effective antibiotic, as antibiotic resistance in M. haemolytica strains has been observed. Administering an appropriate antimicrobial early in the disease course is associated with a more favorable outcome.
Commonly used broad-spectrum antimicrobials include macrolides and phenicols, often employed as first-line treatments. The specific choice of antibiotic should be based on the pathogen’s susceptibility patterns to improve treatment efficacy. Supportive care measures are also important to aid recovery and alleviate discomfort. Non-steroidal anti-inflammatory drugs (NSAIDs) are often given to reduce fever, mitigate pain, and lessen lung inflammation.
Providing sick animals with easy access to fresh, clean water and palatable feed helps maintain their hydration and nutritional status, supporting their immune response. Isolating affected cattle from the rest of the herd is another management practice that reduces stress on the sick animal and helps prevent further spread of the infection to healthy herd mates.
Prevention and Management Strategies
Preventing Mannheimia haemolytica pneumonia relies on strategic vaccination and management practices to minimize stress. Vaccines help cattle develop an immune response to M. haemolytica, preparing them to fight off the bacterium if exposed. For optimal protection, these vaccines should be administered with proper timing, such as several weeks before calves are subjected to known stressors like weaning or transportation to a feedlot.
Many preconditioning programs recommend a two-dose vaccination series, with the first dose given at branding or 2-4 weeks prior to weaning, followed by a booster at weaning or according to label instructions. These programs often include vaccines against common viral pathogens that predispose cattle to M. haemolytica infection, such as IBR, BVDV, PI3, and BRSV. This proactive approach aims to build strong immunity before calves encounter high-risk environments.
Effective management practices further reduce the likelihood of disease outbreaks. “Preconditioning” calves involves weaning them, administering necessary vaccinations, and acclimating them to feed bunks and water troughs before they are transported to new environments. Maintaining adequate ventilation in cattle housing helps remove harmful gases like ammonia and carbon dioxide, reduces humidity, and disperses airborne pathogens, thereby improving air quality and supporting respiratory health. Avoiding overcrowding in pens and ensuring a high-quality nutrition program with sufficient energy and minerals helps maintain a strong immune system, allowing cattle to better resist infections.