Effects of Border disease virus (genotype 3) naturally transmitted by persistently infected sheep to pregnant heifers and their progeny

Eight heifers pregnant between days 47 and 73 were kept together with nine healthy persistently Border disease virus (BDV)-infected sheep allowing natural contact comparable to field conditions. All heifers seroconverted between days 23 and 38 after exposure. Besides a mild increase in body temperature in four heifers, no clinical signs of infection were observed, but 5 animals aborted between days 54 and day 202 after exposure. BDV was detected in the aborted fetuses of four heifers and in the placenta of the fifth (the only material available). Foetal mummification was seen in three foetuses, aborted between days 113 and 116 of gestation, with a crown rump length (CRL) of between 11 and 12 cm. The associated placentas showed dystrophic calcification. The foetus aborted on day 267 of gestation had a CRL of 70 cm and a body mass of 16 kg. The brain in this case was normal in terms of gross morphology, but histologically slight lymphocytic meningeal and perivascular infiltration, slight demyelination in the cerebellar white matter and slight focal acute liquefactive necrosis in the thalamus were seen. Three heifers delivered clinically healthy calves. Two healthy calves were pestivirus negative, of these one was serologically positive and one negative (precolostral). The third calf was pestivirus positive and antibody negative at birth, suggesting immunotolerance and persistent infection as has been described for BVDV. But surprisingly when retested at an age of seven months the calf had seroconverted and was pestivirus negative. Post-mortem examination of the heifers and the calves born alive revealed no abnormalities, pestivirus specific RNA was not detected in any of the examined organ samples of the eight heifers and the three calves. In this study, BDV-type 3 infection had a disastrous impact on fertility of pestivirus naïve heifers. The observed abortion rate exceeded 50% and dramatically underlines the need for a pestivirus control program which includes BDV and small ruminants.