M. haemolytica, formerly Pasteurella haemolytica, is a Gram-negative bacterium frequently found in the upper respiratory tract of healthy cattle. Although it normally exists without causing harm, this opportunistic pathogen is the primary bacterial agent responsible for severe, acute pneumonia in cattle. Its presence is a major contributing factor to the economically devastating condition known as Bovine Respiratory Disease (BRD).
The Role of M. haemolytica in Bovine Respiratory Disease
Mannheimia haemolytica is the principal bacterial species associated with Bovine Respiratory Disease (BRD), often called “shipping fever.” The bacterium is the most frequent cause of the severe, acute phase of pneumonia. The organism typically resides innocuously in the nasopharynx and tonsils of cattle as a commensal.
When the animal’s immune system is compromised, the bacteria proliferate rapidly and are inhaled into the lungs. Once there, M. haemolytica produces a virulence factor called leukotoxin (Lkt). This leukotoxin specifically targets and destroys white blood cells, such as neutrophils and macrophages, impairing the host’s defense mechanisms.
The destruction of these immune cells and the subsequent inflammatory response lead to characteristic severe lung damage, specifically a fibrinous, hemorrhagic, and necrotizing bronchopneumonia. This rapid tissue destruction results in irreversible lesions and consolidation in the cranioventral portions of the lungs. BRD is responsible for significant economic losses, estimated to cost the livestock industries billions of dollars annually due to mortality, treatment expenses, and reduced performance.
Recognizing Symptoms and Diagnosis
The clinical signs of a M. haemolytica infection, which often follow a preceding viral infection, escalate quickly in severity. Early indicators include listlessness, depression, and a fever that can range from 104 to 106°F (40 to 41°C). As the disease progresses, cattle may exhibit a serous to mucopurulent nasal discharge, a moist cough, and a rapid, shallow respiratory rate.
In severe cases of pneumonia, animals show signs of extreme difficulty breathing, including open-mouth breathing and an extended head and neck position. The acute nature of the disease means that an animal can progress from appearing mildly ill to deceased within a single day. Veterinarians diagnose the condition through a physical examination, which may reveal abnormal lung sounds upon auscultation, particularly in the cranioventral lung field.
Definitive diagnosis requires laboratory confirmation, often using samples collected from deep nasopharyngeal swabs or lung tissue. Bacterial culture can isolate the organism, while techniques like Polymerase Chain Reaction (PCR) are used to quickly identify the specific pathogen. Post-mortem examination of affected animals typically reveals a characteristic fibrinous pleuritis and extensive reddish-black consolidation of the lung tissue.
How Cattle Contract the Infection
M. haemolytica transitions from a harmless resident to a pathogen when the host’s defenses are compromised by various stressors. The most significant factor is the stress associated with management practices, which is why the resulting disease is commonly called “shipping fever.”
Long-distance transport, commingling, and the stress of weaning all suppress the immune system. These events create an opportunity for the bacterium to invade the lower respiratory tract. Environmental stressors, such as poor ventilation, overcrowding, or sudden changes in weather and temperature, further weaken the animal’s resistance.
Co-infection with common respiratory viruses is a frequent trigger for the disease. Viruses like Bovine Viral Diarrhea Virus (BVDV), Infectious Bovine Rhinotracheitis (IBRV), and Bovine Respiratory Syncytial Virus (BRSV) damage the lining of the respiratory tract. This viral damage clears the way for M. haemolytica to take hold, acting as a secondary opportunistic invader that causes the acute, destructive pneumonia.
Strategies for Treatment and Control
The cornerstone of treating active M. haemolytica pneumonia is the prompt administration of broad-spectrum antibiotics. Fast-acting antibiotics that are rapidly distributed to the lung tissue, such as gamithromycin, are often selected for their effectiveness against the bacterial pathogen. Due to the potential for antimicrobial resistance, veterinary consultation is necessary to select an appropriate treatment, and sensitivity testing may be performed if the animal does not respond initially.
Supportive care, including the use of non-steroidal anti-inflammatory drugs (NSAIDs) to reduce fever and inflammation, is also an important part of the treatment protocol. For control and prevention, a multi-faceted approach focusing on minimizing stress and maximizing immunity is employed. Management practices like preconditioning, which involves vaccinating and adjusting calves to feed before transport, help to mitigate the stress of arrival at the feedlot.
Vaccination is a fundamental preventative measure, targeting both the bacterial pathogen and the common viral triggers. Modern vaccines against M. haemolytica often contain leukotoxoid components to stimulate antibodies that neutralize the destructive leukotoxin. In high-risk groups of cattle, metaphylaxis—the mass treatment of healthy animals with antibiotics—is a common industry practice to prevent disease outbreaks before clinical signs appear.