Histopathological findings
Microscopic lesions of cases 1-8 were tabulated in Table 2. All eight cases showed haemorrhage with variable severity in the splenic tissue (Fig 3a-c). In 3/8 pigs (cases 20, 58 and 59), severe haemorrhage replaced the red pulp parenchyma and many tissues surrounding the lymphoid follicles, rendering the evaluation of necrotizing tissue difficult. In the liver tissue, the hemorrhagic lesion was seen in different levels of severity in 6/8 cases. The mild cases showed focal haemorrhage in zone 2, and severe cases showed multifocal to more extensive haemorrhages (Fig 3d-f). In the tonsillar tissue, all cases did not show any obvious significant changes in the tonsillar morphology, and the lymphoid follicles were easily identified in low magnification (x40, light microscope, field number 18) (Fig 4a). In higher magnification, apoptotic tissues, characterized by numerous nuclear debris and karyorrhexis, were more intense within the diffuse lymphoid tissue (T-cells area) compared to the lymphoid follicles (Fig 4b-c). Extensive necrosis or haemorrhages were not seen in all cases. In the lymph node tissue, sinus haemorrhages, apoptotic bodies and necrosis were remarkable in all cases (Fig 5a). Like the tonsil, lymphoid follicles were easily identifiable in the lymph node tissue.
In the lung tissue, interstitial pneumonia was seen in all 8 cases. Mononuclear inflammatory cell infiltration increased the thickness of the interstitium (Fig 5b). In many cases, pulmonary intravascular macrophages were easily detectable in the small blood vessels and capillaries. No prominent alveolar macrophages were seen in all cases. Pulmonary oedema was seen in 5/8 cases, while pulmonary congestion (3/8) and haemorrhage (2/8) were uncommon. In the brain tissue, meningitis (8/8) and perivascular cuffing (5/8) were characterized with 2–4 layers of mononuclear cells with frequent apoptotic bodies or nuclear karyorrhexis (Fig 5c). Rarely, neutrophils were found in the meninges among the mononuclear cell infiltration. Severe congestion of the meningeal capillaries was common, but haemorrhages were subtle and not present in most of the cases. Perivascular cuffing was commonly accompanied with swollen endothelial cells. Glial nodule was absent in all cases.
In the cardiac tissue, epicardial and myocardial haemorrhage was seen in 3/8 cases (Fig 5d). The haemorrhagic areas were sometimes infiltrated with aggregates of mononuclear inflammatory cells. Microscopically, haemorrhagic lesions were seen in various locations of the gastrointestinal tract tissue. Case 58 showed haemorrhage in the lamina propria of gastric tissue with oedema in the submucosal layer (Fig 5e). Case 18 showed multifocal haemorrhages in the outer muscularis layer of the small intestinal tissue (Fig 5f). Case 19 showed haemorrhages in the deep lamina propria, dilated capillaries with congestion, and oedema in the submucosa layer of large intestinal tissue (Fig 5g). In the renal tissue, all cases showed haemorrhage in the renal pelvis (Fig 5h), but only cases 57 and 59 showed oedemas in the area. Petechial haemorrhages of the renal cortex were seen in 5/8 cases, with the lesion occasionally extended into the capsular surface (Fig 5i). The haemorrhagic lesion was absent in the renal medulla. Multifocal interstitial nephritis was seen in 5/8 cases.