Mannheimia haemolytica in Cattle: Pathogenesis and Control of Shipping Fever

Introduction

Mannheimia haemolytica is a Gram-negative coccobacillus belonging to the family Pasteurellaceae and is the primary bacterial agent associated with the bovine respiratory disease complex (BRDC), commonly known as shipping fever. This condition represents a multifactorial syndrome in which viral and bacterial pathogens, environmental stressors, and host immune status converge to produce severe fibrinous pleuropneumonia in cattle. The term "shipping fever" reflects the strong epidemiological link between transport stress and the onset of clinical disease. Understanding the pathogenesis of M. haemolytica and implementing integrated control measures are essential for reducing morbidity, mortality, and economic losses in feedlot and dairy operations.

Etiology

Mannheimia haemolytica is classified into multiple serotypes based on capsular polysaccharide antigens. Serotype A1 is the most frequently isolated from clinical cases of bovine respiratory disease, followed by serotype A2 and, less commonly, serotype A6. The bacterium is a normal commensal of the upper respiratory tract in cattle, residing primarily in the nasopharynx and tonsillar crypts. Under conditions of stress or viral co-infection, M. haemolytica undergoes rapid proliferation and colonizes the lower respiratory tract, where it triggers an intense inflammatory response.

Key virulence factors include:

• Leukotoxin (LktA): A pore-forming cytotoxin belonging to the RTX (repeats in toxin) family. Leukotoxin specifically targets ruminant leukocytes, including neutrophils, macrophages, and lymphocytes, by binding to the CD18 subunit of beta-2 integrins. At sublytic concentrations, leukotoxin induces degranulation and release of pro-inflammatory mediators; at lytic concentrations, it causes cell death and release of cytotoxic contents.
• Lipopolysaccharide (LPS): Endotoxin that activates Toll-like receptor 4 (TLR4) on alveolar macrophages and epithelial cells, triggering a cascade of cytokine release including tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1beta), and interleukin-8 (IL-8).
• Capsular polysaccharide: Antiphagocytic properties that impede opsonization and complement-mediated killing.
• Outer membrane proteins (OMPs): Involved in adhesion to respiratory epithelium and iron acquisition.
• Neuraminidase: Cleaves sialic acid residues from host glycoproteins, potentially enhancing bacterial adherence and biofilm formation.

Epidemiology

Mannheimia haemolytica bovine respiratory disease shipping fever occurs worldwide and is most prevalent in intensively managed beef cattle during the first weeks after arrival at feedlots. The incidence peaks in autumn and winter, correlating with weaning, transport, commingling, and exposure to inclement weather. Morbidity rates in high-risk feedlot cattle can reach 30-50%, with case fatality rates of 5-10% in untreated animals. Dairy calves are also susceptible, particularly during the neonatal period and after transport to rearing facilities.

Risk factors include:

• Viral co-infections with bovine respiratory syncytial virus (BRSV), bovine parainfluenza virus 3 (BPIV-3), bovine herpesvirus 1 (BoHV-1, infectious bovine rhinotracheitis), and Bovine Coronavirus. These viruses impair mucociliary clearance, damage epithelial barriers, and suppress local immune responses, facilitating M. haemolytica invasion.
• Stressors such as weaning, castration, dehorning, overcrowding, poor ventilation, and nutritional deficiencies.
• Commingling of animals from multiple sources, which increases pathogen diversity and transmission.
• Genetic susceptibility: Certain cattle breeds and individual variations in leukotoxin receptor expression may influence disease severity.

Pathogenesis

The pathogenesis of shipping fever begins with stress-induced immunosuppression and viral damage to the respiratory epithelium. In healthy cattle, M. haemolytica is cleared by mucociliary escalator activity and alveolar macrophages. However, when viral infection compromises epithelial integrity and ciliary function, the bacterium adheres to exposed basement membrane and extracellular matrix components.

Once established in the lower airways, M. haemolytica proliferates and releases leukotoxin and LPS. Leukotoxin binds to CD18 on neutrophils and macrophages, causing cell lysis and release of proteolytic enzymes, reactive oxygen species, and additional pro-inflammatory cytokines. This creates a vicious cycle of inflammation, tissue necrosis, and fibrin deposition. The resulting lesion is a fibrinous pleuropneumonia characterized by consolidation, fibrin clots in the pleural space, and necrotic foci.

The host inflammatory response, rather than direct bacterial invasion, is responsible for most of the pulmonary damage. Neutrophil degranulation releases matrix metalloproteinases and elastase, which degrade lung parenchyma. Thrombosis of pulmonary capillaries and lymphatics contributes to ischemic necrosis and edema.

Clinical Signs

Clinical signs of shipping fever typically appear within 7-14 days after transport or stress exposure. Affected cattle present with:

• Depression, anorexia, and reduced rumen fill.
• Fever (40-42 degrees Celsius).
• Tachypnea, dyspnea, and open-mouth breathing.
• Serous to mucopurulent nasal discharge.
• Coughing, often dry and non-productive initially, becoming moist as pneumonia progresses.
• Drooling and reluctance to move.
• In severe cases, extended head and neck, abducted elbows, and audible expiratory grunt.

Subacute or chronic cases may exhibit poor growth, intermittent cough, and exercise intolerance. Peracute death can occur within 12-24 hours of onset, particularly in animals with overwhelming leukotoxin-mediated toxemia.

Pathology

Gross postmortem findings are characteristic of fibrinous pleuropneumonia. Lesions are typically bilateral and cranioventral, affecting the apical and cardiac lung lobes. Affected lung tissue is firm, dark red to gray, and consolidated. Fibrin strands cover the pleural surface, and the pleural cavity may contain serosanguinous fluid with fibrin clots. Cut surfaces reveal a marbled pattern of red (congestion), gray (hepatization), and yellow (necrosis) areas.

Microscopically, the hallmark is a fibrinosuppurative pneumonia with extensive neutrophil infiltration, fibrin exudation, and necrosis of alveolar septa. Thrombosis of pulmonary vessels is common. Gram-negative coccobacilli can be observed within neutrophils and macrophages. Chronic lesions may show organization of fibrin into fibrous adhesions between visceral and parietal pleura.

Diagnostics

A definitive diagnosis of Mannheimia haemolytica bovine respiratory disease shipping fever requires integration of clinical, pathological, and laboratory findings.

Clinical diagnosis is based on the characteristic history of stress, fever, and respiratory signs. Thoracic auscultation reveals crackles, wheezes, and areas of dullness. Ultrasonography can identify consolidated lung fields and pleural effusion.

Necropsy and histopathology provide strong supportive evidence, but bacterial culture or molecular detection is needed for confirmation.

Bacteriological culture from nasopharyngeal swabs, transtracheal washes, bronchoalveolar lavage fluid, or lung tissue at necropsy yields M. haemolytica on blood agar or chocolate agar after 24-48 hours of aerobic incubation. Colonies are grayish, smooth, and produce a characteristic sweet odor. Biochemical identification can be performed using commercial systems or matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).

Molecular diagnostics using polymerase chain reaction (PCR) targeting the leukotoxin gene (lktA) or 16S rRNA gene provide rapid and specific detection. Real-time PCR assays can quantify bacterial load and differentiate serotypes. Multiplex PCR panels that include other BRDC pathogens (e.g., Mycoplasma bovis, Histophilus somni, Pasteurella multocida) are increasingly used in diagnostic laboratories.

Serology is of limited value for acute diagnosis due to the time required for seroconversion. Paired serology may be used for retrospective confirmation or herd-level surveillance.

Antimicrobial susceptibility testing is recommended given the emergence of resistance to commonly used drugs. Disk diffusion or broth microdilution methods should follow Clinical and Laboratory Standards Institute (CLSI) guidelines for veterinary pathogens.

The following Mermaid diagram outlines a diagnostic workflow for suspected shipping fever cases.

flowchart TD
    A[Clinical suspicion: fever, dyspnea, stress history], > B{Thoracic ultrasound/auscultation}
    B, > C[Consolidation/pleural effusion present]
    C, > D[Sample collection: transtracheal wash or BAL]
    D, > E[Gram stain and cytology]
    E, > F[Gram-negative coccobacilli, neutrophils]
    F, > G[Culture on blood agar]
    G, > H[Isolate identification: MALDI-TOF or biochemical]
    H, > I[PCR for lktA gene]
    I, > J[Antimicrobial susceptibility testing]
    J, > K[Confirm M. haemolytica as primary agent]
    B, > L[No consolidation]
    L, > M[Consider other BRDC pathogens or non-respiratory causes]

Treatment

Antimicrobial therapy is the cornerstone of treatment for shipping fever. Selection should be based on susceptibility patterns, pharmacokinetics, and withdrawal times. Commonly used antimicrobial classes include:

• Tetracyclines: Oxytetracycline and chlortetracycline are widely used but resistance is increasing.
• Fluoroquinolones: Enrofloxacin and danofloxacin have excellent lung penetration and activity against M. haemolytica.
• Macrolides: Tulathromycin, gamithromycin, and tilmicosin provide prolonged activity and are effective in single-dose regimens.
• Cephalosporins: Ceftiofur (third-generation) is commonly used in feedlot settings.
• Phenicols: Florfenicol is effective and often used in combination with other agents.

Supportive care includes non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine to reduce fever and inflammation, and fluid therapy for dehydrated animals. Severely affected cattle may benefit from oxygen supplementation and isolation in well-ventilated pens.

Treatment failure may result from delayed initiation, inadequate dosage, antimicrobial resistance, or concurrent infections such as Mycoplasma bovis in Feedlot Cattle. In chronic cases, pulmonary abscesses or pleural adhesions may preclude full recovery.

Control

Control of shipping fever relies on reducing stress, enhancing immunity, and minimizing pathogen exposure. A comprehensive program includes:

Management practices:

• Minimize transport duration and provide rest stops with access to water and feed.
• Avoid commingling cattle from different sources; maintain stable groups.
• Provide adequate pen space, bedding, and windbreaks.
• Implement a gradual weaning protocol (e.g., fence-line weaning) to reduce stress.
• Ensure proper ventilation in barns and trailers.

Vaccination:

• Modified-live or killed vaccines containing M. haemolytica leukotoxin toxoid and capsular antigens are available. Vaccination should be administered prior to high-risk periods, ideally 2-3 weeks before weaning or transport.
• Intranasal vaccines may provide more rapid mucosal immunity.
• Autogenous vaccines can be used when field strains are not covered by commercial products.
• Vaccination against viral co-pathogens (BRSV, BPIV-3, BoHV-1, BVDV) is essential to reduce predisposing infections.

Metaphylaxis:

• Mass antimicrobial administration (e.g., tulathromycin or ceftiofur) to high-risk cattle upon arrival at feedlots can reduce morbidity. However, this practice must be balanced against concerns for antimicrobial resistance and is best guided by risk assessment.

Biosecurity:

• Quarantine new arrivals for at least 14 days.
• Isolate sick animals promptly.
• Clean and disinfect transport vehicles and facilities between groups.

Nutritional support:

• Ensure adequate energy, protein, and trace minerals (zinc, copper, selenium) to support immune function.
• Provide high-quality feed and clean water to encourage early feed intake after arrival.

Conclusion

Mannheimia haemolytica remains the most significant bacterial pathogen in the bovine respiratory disease complex. Its pathogenesis is driven by the synergistic effects of stress, viral infection, and bacterial virulence factors, particularly leukotoxin. Effective control requires a holistic approach combining stress reduction, vaccination, strategic antimicrobial use, and biosecurity. Advances in molecular diagnostics enable rapid identification and susceptibility profiling, supporting targeted therapy and antimicrobial stewardship. Continued research into leukotoxin neutralization, host genetics, and vaccine development will further improve outcomes for cattle at risk of shipping fever.

References

1. Griffin D. Economic impact associated with respiratory disease in beef cattle. Veterinary Clinics of North America: Food Animal Practice. 1997;13(3):367-377.
2. Rice JA, Carrasco-Medina L, Hodgins DC, Shewen PE. Mannheimia haemolytica and bovine respiratory disease. Animal Health Research Reviews. 2007;8(2):117-128.
3. Confer AW, Ayalew S. Mannheimia haemolytica in bovine respiratory disease: immunogens, potential immunogens, and vaccines. Animal Health Research Reviews. 2018;19(1):79-99.
4. Highlander SK. Molecular genetic analysis of virulence in Mannheimia (Pasteurella) haemolytica. Frontiers in Bioscience. 2001;6:D1128-D1150.
5. Welsh RD, Dye LB, Payton ME, Confer AW. Isolation and antimicrobial susceptibilities of bacterial pathogens from bovine pneumonia: 1994-2002. Journal of Veterinary Diagnostic Investigation. 2004;16(5):426-431.