A study of the pathogenesis of egg drop syndrome in laying hens.

Conventionally reared and SPF laying hens were infected with egg drop syndrome (EDS) virus and killed at intervals during the first 13 days post inoculation (pi). Tissues were collected and studied using histo-pathological and immunoperoxidase techniques. EDS viral antigen and intranuclear inclusion bodies were detected in the surface epithelium of the nasal cavity of conventional hens 2 to 6 days pi. Low levels of viral antigen were detected in lymphoid tissue throughout the body 2 to 5 days pi and inflammatory lesions and viral antigen were observed in the infundibulum 3 to 5 days pi. Viral replication was first detected at a low level in the pouch shell gland (PSG) 8 days pi with extensive replication occurring in the PSG of hens killed subsequently. There was a good correlation between the presence of viral antigen and the presence of lesions. Viral antigen was never detected in the surface epithelium of the alimentary tract. These findings suggest that following EDS virus infection, viral replication occurs in the nasal epithelium, viraemia occurs and that the pouch shell gland is the main site of EDS virus replication in the laying hen. It is concluded that the reproductive tract is the most important source of virus for lateral spread in the laying hen.