Actinobacillus pleuropneumoniae

• Contagious porcine pleuropneumonia especially of pigs under 6 months
• Endemic in UK
• 12 serotypes causing the same disease
• Different serotypes in different regions, with serotypes 3,6 and 8 the most common in the UK
• Pathogenesis and pathogenicity:
  • Virulent strains possess capsules which are antiphagocytic and immunogenic
  • Fimbriae allow the bacteria to attach to cells of the respiratory tract
  • Damaged neutrophils in the lungs produce lytic enzymes
  • The sustained inflammatory response causes tissue necrosis
  • Lungs consolidated and necrotic with fibrinous pleuisy at post mortem
  • Produce three cytotoxins which belong to the repeats-in-structural-toxin (RTX) cytolysin family
  • RTX toxins:
    • Several peptide repeats within the molecules
    • Produced by various Gram-negative bacteria
    • Possess four contiguous genes, A, B, C and D
    • A is the structural gene; B and D are required for secretion; C allows post-translational activation of the gene product of A into a functional product
    • ApxI is a strong haemolysin with cytolytic activity
    • ApxII is a weak haemolysin
    • ApxIII is a cytotoxin
    • Different Actinobacillus pleuropneumonia serotypes secrete a particular combination of toxins; American serotypes secrete ApxI and II; European serotypes secrete ApxII and III
    • Toxins introduce pores into cell membranes
• Clinical signs and epidemiology:
  • Acute disease in susceptible herds with high morbidity and mortality (up to 50%)
  • In acute outbreaks, pigs may be dyspnoeic, pyrexic or anorexic
  • Blood-stained froth surrounding nose and mouth
  • Cyanosis
  • Pregnant sows abort
  • Causes pneumonia in pigs
  • Carrier herds have some immunity, protecting from acute disease, where lesions are often subclinical, and deaths sporadic
  • Lung scarring and pleural adhesions in many recovered animals
  • Solid immunity develops in recovered animals to all serotypes
  • The disease is spread between herds by carrier pigs
• Diagnosis:
  • Haemorrhagic consolidation close to the main bronchi and fibrinous pleuritis may be suggestive
  • Specimens are cultured on chocolate agar and blood agar in 5-10% carbon dioxide for 2-3 days
  • Small colonies surrounded by clear haemolysis
  • No growth on MacConkey agar
  • Positive CAMP reaction with Staphylococcus aureus
  • Most strains are NAD-dependent (grow on Heated Blood agar)
  • Immunofluorescent- or PCR-based techniques
  • The bacteria on the palatine tonsil may remain undetected by serological tests and swabbing, and can therefore cause an outbreak in naive pigs
• Treatment:
  • Antibiotics depending on the strain of bacteria
  • Prophylactic antibiotics for in-contact pigs
• Control:
  • Killed and polyvalent bacterin vaccines as well as a subunit vaccine are available
  • Improve ventilation, avoid chilling and overcrowding