Some new aspects of the pathology, pathogenesis, and aetiology of disseminated lung lesions in slaughter pigs

From 40 pigs rejected for human consumption at slaughter due to an apparent presence of pyemic lung lesions (defined as disseminated processes containing pus and/or necrotic material), the lungs, spleen, liver, and kidneys were subjected to an extended macroscopic examination. Several lung lesions were sampled from each animal for histological and bacteriological examination. Samples from the kidneys and spleens were also subjected to bacteriological examination. At gross level, four groups of lung lesions were identified: 1) disseminated foci with contents of pus and/or necrotic material (n=26); 2) disseminated or multifocally located ecchymoses with a central area of fibroplasia (n=9); 3) non-pneumonic lesions, i.e., disseminated areas of atelectasis (n=1) or haemorrhagic areas developing due to the process of slaughter (n=1); and 4) suppurative lesions without a disseminated distribution pattern (n=3). Histologically, the disseminated suppurative/necrotic foci were identified as: A) abscesses (n=10); B) necrotic lesions (n=6); and Q ectatic or ectatic-like bronchioles with contents of pus and necrotic material (n=10). The macroscopic observation of disseminated centres of fibroplasia with peripheral ecchymoses (n=9) was confirmed histopathologically. The livers of five pigs contained multiple areas of chronic interstitial fibrosis related to migration of Ascaris suum larvae ("milk spotted liver"). Such hepatic lesions were significantly (p<0.01) related to the simultaneous occurrence of disseminated pulmonary ecchymoses with a central area of fibroplasia. Generally, all lung lesions of each individual animal contained identical monocultures of bacteria following this pattern: Staphylococcus aureus (abscesses); Actinomyces hyovaginalis (necroses); S. aureus, A. hyovaginalis, and Arcanobacterium pyogenes (ectatic and ectatic-like bronchioles). Areas with fibrosis were sterile or contained bacteria considered to be a result of contamination. Apart from one kidney, from which S. aureus was cultured, all other organs were sterile. It is concluded that difficulties exist in differentiating pulmonary pyemic lesions from non-pyemic lesions at the gross level. Thus, it was not possible to distinguish between abscesses/necroses and ectatic bronchioles, the pathogenesis of the latter being uncertain. However, the chronic non-pyemic lesions related to the migration of A. suum larvae should be identified by the absence of pus/necrosis. S. aureus was predominantly isolated from abscesses, whereas, and most surprisingly, A. hyovaginalis was the dominant bacterium isolated from the pulmonary necroses.