after obtaining owners permission , we sampled calves born to previously infected dams during the vector - free period of december 20 , 2007 to march 15 , 2008 . farmers were also asked to report any births , abortions , or stillbirths from btv - infected dams outside the vector - free period . blood samples from live calves were taken as soon as possible after birth ( usually within 4 days ) and tested by using a real - time reverse transcription  pcr ( rrt - pcr ) ( 10 ) and the pourquier c - elisa kit ( idexx , chalfont st . when possible , information about the health of the calf was obtained , dams were sampled alongside their calves , and placenta samples were collected . calves with positive btv rna results were resampled at 23 week intervals . in total , 61 calves were tested and 21 ( including 1 set of twins ) had detectable levels of btv rna in their blood or organs ( appendix table ) . all calves except calf 21 and calf x , each of which had not consumed colostrum before sampling , had antibodies against btv . calf 21 was also negative for btv rna , but calf x showed the highest viral load in the blood ( appendix table ) . virus isolation in kc cells ( 11 ) was attempted for all calf blood samples with a cycle threshold ( ct ) < 29 , but virus was isolated from calf x only . viral rna load in all calves tested declined over time , and almost all calves were rrt - pcr negative by the end of the study ( table ) . * btv , bluetongue virus ; ct , cycle threshold ; neg , negative ; nt , not tested . estimated stage of gestation at which transplacental infection may have occurred  these calves could not be followed up for farm management reasons or because the project had ended . the rna load in the calves always exceeded that of their dams , and 7 of the 20 dams giving birth to btv - positive calves had no detectable viremia . calves y , x , and 33 were born weak and died within hours , days , and weeks after birth , respectively , and calves 13 and 29 exhibited dummy calf syndrome ( 12 ) . all calves except calf 33 were examined postmortem and had negative pcrs for bovine viral diarrhea virus ( s.w . , pers . although calf x died of colisepticemia , this illness probably resulted from the calf s weakness and inability to consume colostrum . no infectious cause for the early postnatal death of calf y , other than bluetongue , was identified ; pathologic findings for calves 13 and 29 are described elsewhere ( s.w . calf 27 , which had negative btv test results , was born with hypermobility of the fetlock joints , unilateral carpal valgus , and arthrogryposis . time windows for possible in utero infection of each calf were calculated according to the btv testing history of the dam and the birth date of the calf ( figure ) . these windows were used to investigate effect of stage of gestation on the probability of transplacental transmission . to account for uncertainty in the date of infection the probability of transplacental transmission increased with the time of gestation during which the dam became infected ( 1 0.033 ; 95% credibility interval 0.0140.063 ) . estimated gestation period at infection of the dam in relation to occurrence of transplacental transmission . bluetongue virus ( btv ) test data for the dams and birth dates of the calves were used to calculate the window of gestation when the dam could have become infected ( technical appendix , for details ) . the calculated infection windows are shown in red for btv - positive calves ( transplacental infection did occur ) and in blue for btv - negative calves ( transplacental infection did not occur ) . because calves were conceived naturally , the exact date of conception is not known , although all were considered to have been born at full term . this detailed field study , which combines data on btv infection in cows with data on transplacentally acquired infection in their offspring , demonstrates that the btv-8 strain circulating in northern europe can cross the bovine placenta in a high proportion ( 33% ) of cases and infect calves when dams are infected during pregnancy . however , because the transmission season was longer in some of these countries , many seropositive dams could have been infected before pregnancy , leading to underestimation of the probability of transplacental infection . in our study , we tested only calves from dams infected between august and december 2007 and known to be pregnant at the time of infection . furthermore , analysis of our data suggests that transplacental transmission is more likely when infection occurs later in gestation ; indeed , most of the dams in this study would have been in the second or third gestation trimester when infected ( figure ) , which may have increased our estimated rate over that found in continental europe . transplacental transmission is of particular concern for policy makers because it may result in the birth of immune - tolerant , persistent carriers , as has happened with bovine viral diarrhea virus ( 14 ) . in our study , all btv - positive calves other than x and y were tested after they had received colostrum and , hence , maternal antibodies . the presence of btv antibodies in calf y suggests that fetal antibody formed in response to in utero infection , yet calf x had no detectable antibodies against btv despite strongly positive rrt - pcr results . calf x was infected late in gestation ( figure ) , when it should have been capable of mounting its own antibody response ( 15 ) . follow - up testing is needed to assess whether such calves remain persistently infected ; however , because calf x died a few days after birth , follow - up testing was not possible . rna declined in all retested calves ( table ) ; most were pcr - negative by the end of the study , including dummy calf 13 . therefore , our results do not suggest that transplacental infection with btv-8 results in subclinical , persistent carriers . nonetheless , the finding that some calves may be born with deformaties after the virus has cleared may lead to underestimation of the economic effects of btv ; calf 27 , which was born with limb deformities to a btv positive dam , could be such a case . live virus has been successfully isolated from only 4 transpacentally infected calves ( including calf x described in this study ) , all of which received either no maternal colostrum or only pooled colostrum ( 9,13 ) . further work is needed to assess whether infectious virus can be isolated from healthy transplacentally infected calves that have colostrum - derived maternal antibodies , because infectious virus needs to be present if transplacental infection is to play a major role in overwintering . in conclusion , future emerging btv strains should be considered to have the potential for transplacental transmission until investigations show otherwise . bluetongue virus testing and results for calves and their dams , united kingdom * time of gestation at infection
to determine whether transplacental transmission could explain overwintering of bluetongue virus in the united kingdom , we studied calves born to dams naturally infected during pregnancy in 200708 . 
 approximately 33% were infected transplacentally ; some had compromised health . in all infected calves , 
 viral load decreased after birth ; no evidence of persistent infection was found .