HEMORRHAGIC ENTERITIS VIRUS-INFECTION IN TURKEYS - A COMPARISON OF VIRULENT AND AVIRULENT VIRUS-INFECTIONS, AND A PROPOSED PATHOGENESIS

Five-week-old specific-pathogen-free turkey poults were inoculated orally with either virulent or avirulent haemorrhagic enteritis virus (HEV) and killed sequentially from 3 h to 10 days post-inoculation. Tissues were examined by light and electron microscopy, and immunoperoxidase staining. Lesions produced were similar for both types of virus. The earliest signs of infection were seen in the spleen with proliferation of the white pulp surrounding the ellipsoids on day 3 post-inoculation. Intranuclear inclusion bodies appeared in lymphoblasts on day 3 and remained until day 5. Necrosis of the white pulp followed the proliferative phase on days 4 and 5, and plasma cells appeared in the red pulp on days 6 and 7. By day 10 the splenic archiecture had returned to normal. The liver contained focal random collections of lymphocytes with occasional hepatocullular necrosis beginning on day 3. The bursa had mild lymphoid depletion in the virulent-virus-infected birds on day 6. Bleeding in the duodenum occurred on day 5 in virulent-virus-infected birds and was associated with small numbers of necrotic cells in the villus lamina propria. All other organs remained unchanged throughout the study. A pathogenesis for HEV infection is proposed. Virus replicates in lymphoid cells within the spleen causing first proliferation and then necrosis of the cells followed by production of plasma cells which produce specific antibody. Replication does not occur in any other organ, but virus-infected lymphocytes from the spleen circulate through the other organs of the bird without causing any visible local alterations. Intestinal bleeding occurs as diapedesis without vascular damage and appears to be initiated by mediators of the inflammatory response.