Ontogeny of host responses in ovine fetuses infected with bluetongue virus.

Ovine fetuses were inoculated (IM) with bluetongue virus (BTV) between the 50th and the 130th days of a 150-day gestation. At 10 to 15 days after virus was inoculated, the interstitial tissue of the fetal lung was diffusely infiltrated with large mononuclear leukocytes. By 20 days, unitized nodules of macrophages were seen in the interstitial tissue of the lung. The only gross alterations observed in infected fetuses were large lymph nodes and prominent lymphoid nodules within the spleen. Fetuses inoculated on the 80th day developed increased serum concentrations of immunoglobulin G (IgG) within 20 days after inoculation, whereas fetuses inoculated on the 100th gestational day and later developed increased concentrations of IgG by 10 days. Fetuses inoculated on the 80th gestational day also developed serum neutralizing antibodies by 20 days after infection. The BTV was recovered from fetuses which had serum neutalizing antibody. The sequential development of responses to BTV infection during ontogeny in ovine fetuses is discussed.