Bluetongue in cattle: effects of vector-transmitted bluetongue virus on calves previously infected in utero.

Three of 7 principal calves, after a challenge of immunity exposure by bites of bluetongue (BT) virus-infected Culicoides variipennis, became latently infected with BT virus. These calves were born to heifers infected with the homologous virus by bites of C variipennis at 60 or 120 days' gestation. Latent BT virus infection was detected by isolation of BT virus from washed erythrocyte samples obtained from the calves at 57, 100 to 102, 200 to 202, 300 to 302, and 400 to 402 days after challenge of immunity and from 1 of the calves over 5 years after challenge of immunity. The 3 latently infected calves were healthy; 2 were immunologically competent and 1 was immunologically incompetent to develop detectable BT virus antibodies in their blood. Bluetongue virus infection was detected (by viral isolation) in 2 other principal calves during the challenge of immunity, but they were not considered latently infected. The latter 2 calves were immunologically incompetent to develop detectable BT virus antibodies.