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A definite position of transmissible gastroenteritis (TGE) virus of pigs in the viral classification has not yet been established. The present author studied, with WITT~ and EASTERDAY (15), the morphology and development of TGE virus by electron-microscopic techniques of negative staining and thin sectioning. They suggested that TGE virus was similar, in size, shape, and possible mode of replication to the myxoviruses and some oncogenic viruses. However, neither surface projections nor internal helical structure characteristic of the myxovirus could be recognized in their preparations. Recently, OI~A~IWA et al. (11) demonstrated the presence of surface projections on TGE virus, and mentioned that most particles in samples partially purified by the sucrose density gradient centrifugation method have lost their projections. They pointed out the similarity between TGE virus and the myxovirus because of the same size range of both viruses, and the presence of surface projections and of an envelope. The results described here suggest, together with other properties previously reported, that TGE virus would be placed in the group of coronaviruses. In the present study, considering the fragility of surface projections of TGE virus in the purification procedure, the negative staining method was applied directly to infected culture fluid which contained about 106.5 TCIDs0 of virus per ml. Primary cultures of pig kidney cells were infected with TGE virus, TO strain, at a multiplicity of about 10 infective units per cell. After 2 days, the culture fluid was collected and clarified by low speed centrifugation. A small amount of the supernatant was placed on a carbon-Formvar coated grid, and excess fluid was removed with filter paper. The grid was washed with 1 ~o ammonium acetate to remove residual salts. One drop of 2~o phosphotungstic acid adjusted to pH 6.2 with normal potassium hydroxide was then applied for negative staining. Most of the samples were examined immediately after harvesting the culture fluid, but some of them, frozen and stored at-20~ for about one month, were subsequently thawed and processed as described above. A JEM-6S electron microscope was used.

ABSTRACT

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Highly virulent infectious bursal disease virus (IBDV) was isolated from field cases, and the pathogenicity of the isolates was examined in specific-pathogen-free chickens. Chickens inoculated with the isolates developed severe clinical disease with a high mortality rate. Histopathologically, infectious bursal disease was characterized by bursal and thymic necrosis, aplastic anemia, acute hepatitis with fatty change, and systemic inflammatory response. In addition to functional abnormalities in the liver, a hypoxic state was induced by aplastic anemia and severe inflammation in the pulmonary air capillary walls. These pathological changes appeared to be closely related to the cause of death.