Abstract
Transplacental transmission of bluetongue virus has been shown previously for the North European strain of serotype 8 (BTV-8) and for tissue culture or chicken egg-adapted vaccine strains but not for field strains of other serotypes. In this study, pregnant ewes (6 per group) were inoculated with either field or rescued strains of BTV-2 and BTV-8 in order to determine the ability of these viruses to cross the placental barrier. The field BTV-2 and BTV-8 strains was passaged once in Culicoides KC cells and once in mammalian cells. All virus inoculated sheep became infected and seroconverted against the different BTV strains used in this study. BTV RNA was detectable in the blood of all but two ewes for over 28 days but infectious virus could only be detected in the blood for a much shorter period. Interestingly, transplacental transmission of BTV-2 (both field and rescued strains) was demonstrated at high efficiency (6 out of 13 lambs born to BTV-2 infected ewes) while only 1 lamb of 12 born to BTV-8 infected ewes showed evidence of in utero infection. In addition, evidence for horizontal transmission of BTV-2 between ewes was observed. As expected, the parental BTV-2 and BTV-8 viruses and the viruses rescued by reverse genetics showed very similar properties to each other. This study showed, for the first time, that transplacental transmission of BTV-2, which had been minimally passaged in cell culture, can occur; hence such transmission might be more frequent than previously thought.
Highlights
Bluetongue virus (BTV) is a member of the Orbivirus genus within the family Reoviridae
Experimental infections In order to examine the characteristics of the different BTV strains in sheep, a study was performed in which two sets of field and rescued viruses (BTV-2 and BTV-8) were inoculated into pregnant ewes at about 4 months of gestation (1 month prior to expected parturition)
Most (24/28) of the lambs were born healthy with the exception of 3 which had to be euthanized for welfare reasons and one that was born dead
Summary
Bluetongue virus (BTV) is a member of the Orbivirus genus within the family Reoviridae. The virus can infect a variety of ruminant hosts including cattle, sheep and goats. Following the introduction of the pathogenic BTV-8 into Northern Europe in 2006, it was observed that transplacental transmission of this virus strain occurred in both cattle and sheep [13,21,22,23,24,25,26]. The particular features of the European BTV-8 virus which are responsible for transplacental transmission are not known This property of the virus and the subsequent birth of viremic offspring may contribute towards its ability to be maintained from one year to the (overwintering), within Northern Europe, in the absence of an active vector population during the winter months
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