Early Events Following Oral Administration of Newcastle Disease Virus Strain V4

Abstract

Heat-resistant strains of Newcastle disease virus, such as strain V4, are being used as vaccines to protect flocks of rural chickens in developing countries. Sometimes these vaccines are administered on food, a procedure that has yet to be optimized. Experiments were undertaken to determine the anatomical sites of initial infection with orally administered V4 virus, and the sites of initial viral replication. During the first 24 hours after introduction of V4 into the mouth of 3-weeks-old chickens, virus was isolated by egg inoculation from oesophagus, crop and trachea and less frequently from proventriculus. V4 virus was not isolated from other parts of the digestive tract, nor from blood. Evidence for viral replication was sought by egg inoculation and by immunohistochemistry 4 to 10 days after V4 strain was placed in the crop. Virus was detected most frequently and at highest titre from jejunum, ileum and caecum 6 days after infection. There were also isolations from blood, 4 days after infection. Viral antigens were detected in epithelial cells in most parts of the digestive tract and in some cells, possibly lymphocytes or macrophages, in lamina propria. It is suggested that V4 vaccine virus, given orally, attaches to cells in the upper digestive tract. Viral replication occurs mainly in cells of the lower digestive tract, probably reaching these sites as the result of a viraemia.