Transplacental Transmission of Bluetongue Virus Serotype 1 and Serotype 8 in Sheep: Virological and Pathological Findings

Abstract

The Bluetongue virus serotype 8 (BTV-8) strain, which emerged in Europe in 2006, had an unusually high ability to cause foetal infection in pregnant ruminants. Other serotypes of BTV had already been present in Europe for more than a decade, but transplacental transmission of these strains had never been demonstrated. To determine whether transplacental transmission is a unique feature of BTV-8 we compared the incidence and pathological consequences of transplacental transmission of BTV-8 to that of BTV-1. Nine pregnant ewes were infected with either BTV-8 or BTV-1. The BTV strains used for the infection were field strains isolated on embryonated chicken eggs and passaged twice on mammalian cells. Blood samples were taken to monitor the viraemia in the ewes. Four weeks after the infection, the foetuses were examined for pathological changes and for the presence of BTV. BTV-8 could be demonstrated in 12 foetuses (43%) from 5 ewes (56%). %). BTV-1 was detected in 14 foetuses (82%) from 6 ewes (67%). Pathological changes were mainly found in the central nervous system. In the BTV-8 group, lympho-histiocytic infiltrates, gliosis and slight vacuolation of the neuropil were found. BTV-1infection induced a severe necrotizing encephalopathy and severe meningitis, with macroscopic hydranencephaly or porencephaly in 8 foetuses. In our experimental setting, using low passaged virus strains, BTV-1 was able to induce transplacental transmission to a higher incidence compared to BTV-8, causing more severe pathology.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): bluetongue

“…Oral BTV infection of both ruminant livestock and wild or zoo carnivores has been described, including infection of calves via the feeding of infective colostrum (Alexander et al., ; Mayo et al., ) and experimentally via cultured viruses or urine from an experimentally infected sheep. At least some BT strains (including members of serotypes 1, 2 and 8) can be transmitted directly between ruminants in close contact (van der Sluijs et al., ; Rasmussen et al., ; van der Sluijs et al., ). The mechanisms of contact transmission are still uncertain.…”

Bluetongue Virus in wild ruminants in Europe: Concerns and facts, with a brief reference to bluetongue in cervids in Greece during the 2014 outbreak

“…Additionally, during the parturition period, a direct contamination or digestion of contaminated placental or foetal tissue may lead to infection and disease of individual animals (De Clercq et al, 2008;Menzies et al, 2008). Finally, transmission of the virus to pregnant females may lead to transplacental infections, resulting in foetal deaths, stillbirths or births of neonates that would die in their early life (Osburn, 1994;Mayo et al, 2010;Rasmussen et al, 2013;van der Sluijs et al, 2013).…”

Virological, immunological and pathological findings of transplacentally transmitted bluetongue virus serotype 1 in IFNAR1-blocked mice during early and mid gestation

“…The infection thereafter spread to most parts of the northern Europe and caused disease in sheep and cattle 10,11 . TPT rate varies according to the species, around 10% 10 to 41.7% 12,13 in cows, 56 to 69% in ewes 14,15 and 33% in goats 16 , and stage of gestation 7 . Most important consequences of TPT are birth of BTV positive foetuses and viral persistence in heifers resulted in introduction of BTV in disease free regions 11 .…”