Pathogenesis ofActinobacillus pleuropneumoniae infections
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Actinobacillus pleuropneumoniae causes an acute lobar pneumonia in pigs. This infection is reported to be endemic in most pig-rearing countries. Pigs are infected aerogenously, and often only a subclinical infection develops. When clinical signs occur, pneumonia is often peracute and fatal. Pigs with severe pleuropneumonia develop cardiac failure that is characterised by decrease of right ventricular ejection fraction and consecutive decrease of cardiac output. Observed cyanosis of extremities is mainly due to poor peripheral circulation, and not to insufficient blood gas exchange in the lungs. Pigs that survive pleuropneumonia develop a protective immunity. This immunity proved to be mainly humoral, as it can be passively translated by serum or colostrum. Pigs, experimentally infected by endobronchially instilled bacterial suspensions, show serious signs of disease within 9–12 h, even when only 50 bacteria were used. A unimodal relationship was demonstrated between infection dose and size of the lobar lung lesion. This suggests an optimal infection dose, below which functional clearance of these bacteria from lung still has effect, and above which rapid necrosis of lung tissue demarcates the infection site. By nose-only exposure ofA. pleuropneumoniae aerosols, higher numbers of bacteria are needed to induce clinical pleuropneumonia than by endobronchial instillation, and it takes 24 h before clinical signs appear. It seems thatA. pleuropneumoniae when instilled in suspension can easily reach the host's target cells, and may circumvent the host's defence mechanisms.
Early pathological changes of infected lungs are oedema of alveolar septa, platelet activation, and thrombosis of capillaries. Shortly thereafter bacteria can be found at the pleural site. AlthoughA. pleuropneumoniae is non-motile, it translocates effectively across lung epithelium. The next pathological feature of acute infection is influx of macrophages. Infected lungs show local accumulation of macrophages with phagocytisedA. pleuropneumoniae surrounded by haemorrhage. After several hours a dense zone with polymorphonuclear neutrophils demarcates this haemorrhagic necrotic lesion. Organisation of the necrotic areas in the lungs and of the fibrinous pleuritis by young granulation tissue starts from 4–5 days after infection and may result in encapsulated necropurulent lesions, abscesses and fibrosis. The bacterium can be isolated from the necrotic areas of the lungs, from the bronchial lymph nodes, from the pleura, but usually not from other organs.
Differences in virulence occur betweenA. pleuropneumoniae serotypes (Kamp and van Leengoed (1989) but also between strains belonging to one serotype. During log phaseA. pleuropneumoniae secretes cytolysins that belong to the RTX family. Until now three distinct cytolysins have been described: ApxI, ApxII, and ApxIII. ApxI lyses erythrocytes, but ApxII and ApxIII have little or no haemolytic activity. All three toxins are lyric for alveolar macrophages and polymorphonuclear neutrophils. Their toxicity for cells is calcium dependent as is their binding to target cells. As ApxI toxin in vitro is also toxic for bovine, sheep, and horse red blood cells, the species specifity ofA. pleuropneumoniae is probably independent of these RTX toxins. EachA. pleuropneumoniae strain produces one or more Apx proteins, and is related to the virulence of the strain. Endobronchial instillation of sterile Apx-containing culture supernatant fluid produces similar lung lesions as viableA. pleuropneumoniae. Vaccination of pigs with this culture supernatant produces an homologous protection against challenge with viableA. pleuropneumoniae. There are indications that some other antigens than Apx contribute to the protective immunity as endotoxin or iron-repressible proteins.
Lung macrophages may be involved in translocating the bacterium across lung epithelium as they are able to phagocytoseA. pleuropneumoniae, but are unable to kill this bacterium. Moreover, alveolar macrophages are killed by cytolysins that are produced byA. pleuropneumoniae, and viable bacteria are released. Polymorphonuclear neutrophils, however, can effectively killA. pleuropneumoniae. Surprisingly, there is no influx of polymorphonuclear neutrophils in peracute pleuropneumonia. Peripheral white blood cell counts of pigs with acute pleuropneumonia do not show a leucopenia, but from 6 h on a steady increase of the neutrophil count with only a slight left shift. In vitro, alveolar macrophages with phagocytosedA. pleuropneumoniae produce a polymorphonuclear cell recruiting factor only when protected by convalescent pig serum, which protects these macrophages against cytolysins ofA. pleuropneumoniae. This in vitro experiment mimics the convalescent stage with protective immunity. To protect pigs against pleuropneumonia a rapid clearance ofA. pleuropneumoniae by polymorphonuclear neutrophils seems essential.
Without convalescent pig serum that neutralises cytolysin activity, PAMs show within 15 min irreversible degeneration caused byA. pleuropneumoniae secreted cytolysins. Without specific opsonins phagocytic efficacy of PAMs forA. pleuropneumoniae is low. PAMs incubated with App in the presence of 6% convalescent serum cannot killA. pleuropneumoniae, but stay viable for several hours. Within 90 min PAMs produce mediators that strongly attract PMNs in an under-agarose chemotaxis assay. Studies in vivo revealed that the early pneumonic lesion (6–12 h p.i.) is characterised by a starting ischaemic or hemorrhagic necrosis surrounded by a dense demarcation zone of primarily macrophages and no PMNs. Differential cell counting in peripheral blood does not show leucopenia, but a steadily increasing neutrophil count from 9 h p.i. onwards. It is concluded that the absence of a rapid influx of PMNs into the infected area impairs the clearance ofA. pleuropneumoniae from the lungs resulting in pleuropneumonia.