Fetal infection with Schmallenberg virus — An experimental pathogenesis study in pregnant cows

Since its first appearance in 2011, Schmallenberg virus (SBV) has been repeatedly detected in aborted ruminant fetuses or severely malformed newborns whose mothers were naturally infected during pregnancy. However, especially the knowledge about dynamics of fetal infection in cattle is still scarce. Therefore, a total of 36 pregnant heifers were experimentally infected during two animal trials with SBV between days 60 and 150 of gestation. The fetuses were collected between 10 and 35 days after infection and virologically and pathologically investigated. Overall, 33 heifers yielded normally developed, macroscopically inconspicuous fetuses, but abundant virus replication was evident at the maternal/fetal interface and viral genome was detectable in at least one organ system of 18 out of 35 fetuses. One heifer was found to be not pregnant at autopsy. One of the animals aborted at day 4 after infection, viral RNA was detectable in the lymphatic tissue of the dam, in the maternal and fetal placenta, and in organs and lymphatic tissue of the fetus. In another fetus, SBV typical malformations like torticollis and arthrogryposis were observed. The corresponding dam was infected at day 90 of pregnancy and viral genome was detectable in the cerebellum of the unborn. Interestingly, no common patterns of infected fetal organs or maternal/fetal placentas could be identified, and both sites of virus replication and genome loads varied to a high degree in the individual fetuses. It is therefore concluded, that SBV infects in many cases also the bovine fetus of naive pregnant cattle, however, the experimentally observed low abortion/malformation rate is in concordance to the reported low rates in the field during the first outbreak wave following the introduction of SBV. This observation speaks for a natural resistance of most bovine fetuses even during the vulnerable phase of early pregnancy, which has to be further studied in the future.