Mannheimia haemolytica is a bacterium found in the upper respiratory tract of healthy ruminant animals, such as cattle and sheep. It is a Gram-negative bacterium, meaning it lacks an outer membrane and stains pink when subjected to a Gram stain test. This microorganism is a facultative anaerobe, capable of growing with or without oxygen, and does not form spores. While a normal inhabitant, M. haemolytica can become a concern for livestock health under certain conditions.
Role as an Opportunistic Pathogen
Mannheimia haemolytica is an opportunistic pathogen, meaning it can live harmlessly in an animal’s upper respiratory system but cause disease when conditions allow. These bacteria commonly reside in areas like the nasal passages, nasopharynx, and tonsils of healthy cattle. The shift from a benign presence to a disease-causing agent often occurs due to stress factors that weaken the animal’s natural defenses.
Triggers include stress from transportation, which can compromise the immune system. Weaning is another stressor that can predispose calves to infection. Sudden changes in weather or co-infections with respiratory viruses can create an environment where M. haemolytica can proliferate and become pathogenic.
Bovine Respiratory Disease Complex
Mannheimia haemolytica is a primary bacterial agent in the Bovine Respiratory Disease (BRD) complex. This condition is a significant cause of economic losses in the livestock industry due to illness and death. The disease manifests with clinical signs that worsen as the infection progresses.
Affected animals exhibit a fever, with body temperatures potentially rising to 104-107°F (40-41.6°C), accompanied by depression and loss of appetite. Symptoms include a moist cough, nasal discharge evolving from thin and clear to thick yellow and viscous, and rapid, shallow breathing. In advanced stages, calves may stand with their elbows positioned away from their chest wall, indicating painful and labored breathing.
The pathogenesis begins when stress or viral infections suppress the animal’s immune defenses, allowing M. haemolytica to rapidly multiply in the upper respiratory tract. The bacteria then move into the lungs, where they produce toxins, such as leukotoxin, that damage lung tissue and impair immune cells like neutrophils and macrophages. This leads to severe fibrinous pneumonia, characterized by inflammation, tissue necrosis, and fluid accumulation in the lungs.
Diagnosis and Treatment
Diagnosing Mannheimia haemolytica infection involves a veterinarian assessing the animal’s clinical signs and history. To confirm the diagnosis, laboratory testing of samples is performed. These samples include:
- Nasal swabs
- Deep nasopharyngeal swabs
- Transtracheal washes
- Bronchoalveolar lavage fluid
Lab tests involve culturing the bacteria on specific media, such as blood agar, where M. haemolytica forms distinct hemolytic colonies. Molecular methods like polymerase chain reaction (PCR) assays can also be used for rapid and accurate identification of the bacterium. Obtaining samples from untreated animals is preferable to ensure accurate antimicrobial sensitivity testing, guiding appropriate antibiotic selection.
Treatment for M. haemolytica pneumonia relies on antibiotics. Long-acting antimicrobials, such as tulathromycin, gamithromycin, or tilmicosin, are commonly used in feedlot calves. Supportive care includes providing fluids to combat dehydration, administering anti-inflammatory drugs to reduce fever and pain, and ensuring a comfortable, low-stress environment for the animal.
Prevention and Management Strategies
Proactive measures are important for preventing Mannheimia haemolytica outbreaks in livestock herds. Vaccination is a key strategy, as vaccines help the immune system recognize and defend against the bacterium. Ideally, calves should receive vaccines, including those for M. haemolytica, several weeks before anticipated stressful events like transportation to allow for immunity to develop.
Reducing stress factors in livestock management is another key preventative approach. Preconditioning calves before shipping, which involves weaning them for at least 45 days prior to sale and training them to eat from feed bunks and drink from water troughs, reduces their susceptibility to BRD. Minimizing transportation time and avoiding the commingling of animals from different sources can also decrease stress and pathogen exposure. Maintaining good ventilation in housing facilities and implementing strong biosecurity protocols further contribute to a healthier environment, reducing the likelihood of bacterial proliferation and disease transmission.