Experimental Infection of Pregnant Cattle with Bluetongue Virus Serotype 11 between Postbreeding Days 21 and 48

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 PBD31 and PBD48, and tissues were collected for virologic and pathologic examination. All but 1 heifer inoculated on PBD21 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. One major cause of infertility in animals is early embryo loss. 16 As a consequence of embryonic death, the length of calving intervals is increased. 36 Data of the Dairy Herd Improvement Association indicate that the economic losses due to an increase in calving in- tervals in the USA was more than $500 million per year 29 In spite of this detrimental effect, little is known about the causes of embryo mortality. In cattle, tricho- moniasis, 3 vibriosis, 3 bovine viral diarrhea, 2,30 and in- fectious bovine rhinotracheitis 4,22,23,31 have been con- sidered causes of embryo mortality. Epidemiologic and in vitro studies suggest that bluetongue virus (BTV) could also be associated with embryo death in cat- tle. 5,6,14 pathogenesis of viral infections of the embryo at this stage of gestation can be carried out with greater as- surance of the events occurring in the uterus.