Pathogenesis and Disease of Non-enteric Gram-negative Bacteria in Veterinary Medicine

Exam Tip: If a question asks about the primary mechanism of M. haemolytica pathogenesis, focus on the RTX toxin (leukotoxin) and its ability to cause hemolysis and white blood cell destruction.

Clinical Manifestations

1. Respiratory Tract Involvement

Respiratory disease is the most common clinical presentation for these pathogens.

Pneumonia and Shipping Fever

In ruminants, the Bovine Respiratory Disease (BRD) Complex, commonly known as Shipping Fever, is a major economic burden. It is caused by a combination of bacteria like Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni, often exacerbated by viruses.

• Clinical Signs: Animals exhibit lethargy, depression, reduced feed intake, high fever ($> 104^\circ\text{F}$), increased respiratory rate, dyspnea (difficulty breathing), and coughing.
• Lesions: Post-mortem examination typically reveals bronchopneumonia or pleuropneumonia, characterized by significant fibrin deposition and the exudation of proteinaceous fluid in the pleural sac.

Upper Respiratory Diseases

• Atrophic Rhinitis (Swine): This is caused by a combination of Bordetella bronchiseptica (which produces a dermonecrotic toxin that damages the epithelium) and P. multocida (which secretes a Rho-activating toxin or Pmt toxin). The result is the destruction and atrophy of the nasal turbinates.
• Infectious Coryza (Chicken): Caused by Avibacterium paragallinarum, this disease presents with facial swelling, sneezing, and nasal discharge. A unique feature is that Avibacterium requires V-factor (NAD) for growth, often appearing as "satellite colonies" near Staphylococcus on culture media.

2. Bacteremia and Septicemia

When bacteria escape the local site of infection and enter the bloodstream, they cause systemic disease.

• Hemorrhagic Septicemia: Caused by P. multocida in cattle, characterized by severe vascular disturbances and hemorrhages.
• Avian Cholera: Also caused by P. multocida in birds, this can present as a peracute form where birds die suddenly with little warning, or a chronic form involving swollen wattles and joints.
• Septicemia in Sheep: Attributed to Bibersteinia trehalosi.

3. Trauma and Wounds

P. multocida is frequently isolated from bite wounds (especially in cats) and surgical sites due to contamination from the oral flora.

Diagnosis and Management

Laboratory Diagnosis

To differentiate between these pathogens, clinicians use several methods:

• Deep nasopharyngeal swabbing: Essential for early, untreated cases.
• Culture: Growing the bacteria on specialized media (e.g., chocolate agar for V-factor dependent species).
• Molecular Methods: PCR and 16s rRNA gene sequencing provide definitive identification.

Treatment and Prevention

Treatment: Management involves the use of broad-spectrum antibiotics (such as tetracyclines, macrolides, and fluoroquinolones). Because these diseases often involve intense inflammation and high fever, NSAIDs (like flunixin meglumine) are critical for managing pain and fever.

Prevention:

• Stress Reduction: Managing environment, ventilation, and transport to prevent the "trigger" for endogenous infection.
• Vaccination: Using bacterins or toxoids to prime the immune system.
• Metaphylaxis: The mass treatment of a group of animals (via feed, water, or injection) to prevent the onset of blatant illness in high-risk populations.

Note on Metaphylaxis: When treating sick animals via metaphylaxis, if the animal is already showing signs, treatment should be parenteral (injection) rather than through feed or water to ensure efficacy.