Experimental infection of pregnant cattle with bluetongue virus serotype 11 between postbreeding days 21 and 48

Department of Pathology, School of Veterinary Medicine, University of California, Davis 95616.

Four bluetongue virus (BTV)-seronegative heifers and 2 BTV-seropositive heifers were inoculated with the virulent strain UC-8 of BTV-11 between postbreeding days (PBD) 21 and 30. The heifers were observed for 10-18 days after inoculation for clinical signs, and pregnancy was monitored by ultrasound examination of the uterus and by plasma progesterone levels. Blood samples were collected daily after inoculation and processed for virus isolation and titration. Heifers were euthanized between PBD 31 and PBD 48, and tissues were collected for virologic and pathologic examination. All but 1 heifer inoculated on PBD 21 remained pregnant after BTV inoculation. A cystic corpus luteum was found in the ovary of the nonpregnant heifer, but BTV was not isolated from the reproductive tract of this heifer. Three of the inoculated heifers that remained pregnant showed mild multifocal areas of perivascular lymphocytic infiltration in the ovary. BTV was reisolated from spleen and prescapular and peribronchial lymph nodes 10 days after inoculation from 3 of the 4 BTV-seronegative heifers. BTV was also reisolated from the uterus of 1 of the heifers that remained pregnant, but microscopic lesions were not found in this organ.