Understanding Non-enteric Gram-Negative Bacteria in Veterinary Medicine

Atrophic Rhinitis (Swine)

In swine, P. multocida is a key player in Atrophic Rhinitis, often working in tandem with Bordetella bronchiseptica. The pathogenesis involves the B. bronchiseptica toxin damaging the epithelium, allowing P. multocida to adhere and secrete the Pmt toxin (a Rho-activating toxin). This results in the progressive destruction of the nasal turbinates.

Differential Diagnosis Note: It is critical to distinguish Atrophic Rhinitis from Necrotic Rhinitis. While Atrophic Rhinitis involves P. multocida, Necrotic Rhinitis follows mucosal injury and is typically associated with Fusobacterium necrophorum.

Hemorrhagic Septicemia (Cattle)

In cattle, P. multocida is a component of the Bovine Respiratory Disease (BRD) complex. It can cause hemorrhagic septicemia, often triggered by stressors such as transportation or weaning.

Avibacterium paragallinarum and Infectious Coryza

Avibacterium paragallinarum is the causative agent of Infectious Coryza in chickens. This is primarily an upper respiratory disease characterized by sneezing, nasal discharge, and significant facial swelling, particularly in the infraorbital sinuses. In laying hens, a significant drop in egg production is a common clinical sign.

A defining microbiological feature of A. paragallinarum is its requirement for V factor (nicotinamide adenine dinucleotide, NAD) for growth. In the laboratory, this is identified through a phenomenon known as satellitism. When cultured on blood agar alongside a "nurse colony" (such as Staphylococcus aureus) that excretes V factor, A. paragallinarum will only grow in small, "dewdrop-shaped" colonies immediately adjacent to the nurse colony.

Mannheimia haemolytica and the BRD Complex

In cattle, Mannheimia haemolytica is a major contributor to the Bovine Respiratory Disease (BRD) complex, commonly referred to as "shipping fever". This complex is often initiated by viral infections (such as BHV-1 or BRSV) and exacerbated by the stress of transport, weaning, or crowding.

Clinical signs include lethargy, fever (> 104°F), and respiratory distress such as coughing and dyspnea. Post-mortem lesions typically show cranioventral lung consolidation (bronchopneumonia) and significant fibrin deposition in the pleural sac.

Key Laboratory Distinction: To differentiate Mannheimia haemolytica from Pasteurella multocida on a culture plate, look for hemolysis. M. haemolytica produces a distinct zone of hemolysis on bovine blood agar, whereas P. multocida does not.

Diagnosis, Treatment, and Prevention

Laboratory Diagnosis

Definitive diagnosis can be achieved through:

• Culture: Isolation of the organism from deep nasopharyngeal swabs.
• Molecular Methods: PCR is highly effective, especially if V-factor independent strains are suspected.
• Genotyping: 16s rRNA gene sequencing and capsular/somatic typing.

Treatment and Management

While these organisms are susceptible to various Gram-negative antimicrobials, management must be strategic:

• Antibiotics: Tetracyclines, macrolides, and fluoroquinolones are used in cattle. In poultry, sulfonamides (like sulfamethazine) are effective, but must be used with extreme caution in egg-laying hens due to potential residues and toxicity in breeders.
• Metaphylaxis: In cattle, mass treatment of a population experiencing early signs of disease can reduce overall morbidity.

Prevention

Prevention relies heavily on biosecurity and immunization:

1. Biosecurity: Excluding wild birds and rodents is vital for poultry to prevent the introduction of P. multocida.
2. Vaccination: Bacterins and toxoids are available for most of these diseases. However, vaccines must be specific to the serotypes prevalent in the local area to be effective.
3. Stress Reduction: Minimizing stressors like overcrowding, poor ventilation, and abrupt changes in diet is essential for preventing the onset of BRD in cattle.