In 2006 and 2007, Bluetongue virus serotype 8 (BTV-8) caused devastating outbreaks in Northern Europe; the outbreaks were controlled in 2008 and 2009 by an international vaccination policy. Remarkably, BTV-8 differs from other serotypes in that it spread transplacentally (De Clercq K et al. 2008 Transboundary and Emerging Diseases 55, 352-359). Apart from the transplacental spreading, a significant increase in the incidence of abortions was reported in Belgium (Meroc E et al. 2009 Transboundary and Emerging Diseases 56, 39-48). The aim of the present study was to investigate the susceptibility of bovine-hatched, in vitro-produced blastocysts to BTV-8. A total of 1390 immature bovine oocytes were matured and fertilized in vitro. Presumed zygotes (n = 1148) were denuded 24 h post-insemination and cultured in 50-μL droplets of modified synthetic oviduct fluid (SOF) medium with 10% fetal calf serum (tested negative for BTV antibodies) at 39.0°C in 5% CO2, 5% O2, and 90% N2. At 7 days post-insemination (dpi), blastocysts were grouped to enhance hatching. For virus incubation, BTV-8 Bel 2006/2 from Veterinary and Agrochemical Research Centre (VAR, Brussels, Belgium) was used. At 8.5 dpi, hatched embryos were placed in 800μL of minimum essential medium (MEM) containing 103.8 50% tissue culture infectious doses (TCID50) of BTV-8 and incubated for 1 h at 39°C in an atmosphere of 5% CO2 in air. At the same time, 2 groups of hatched control embryos were incubated under the same circumstances in 800 μL of SOF and 800 μL of MEM, respectively. After infection, all embryos were washed according to IETS guidelines with the exception that they were not zona pellucida intact and cultured in new SOF. At 48, 60, 72, and 96 h post-infection (hpi), one-fourth of the embryos of each group were fixed in 4% paraformaldehyde for 12 to 24 h and subsequently stained for BTV-8 with double immunofluorescent staining using a BTV-8 monoclonal antibody (8A3B.6, ID-Vet, Montpellier, France). All control embryos (CTRL and MEM) were negative for BTV-8 virus antigen at all time points. At 48 hpi, only 1 out of 7 infected embryos was positive for virus antigen (in all its cells). At 60 hpi, all remaining embryos (n = 6) were negative, whereas at 72 hpi and 96 hpi all embryos had 25% to 100% BTV-8-positive cells (n = 6 at 72 hpi and n = 7 at 96 hpi). Furthermore, 1 embryo at 72 hpi and 2 embryos at 96 hpi showed morphological signs of degeneration. This study has showed for the first time that hatched in vitro-produced blastocysts are susceptible for BTV-8 virus infection and replication in vitro. The relatively long time between virus infection and detection of viral antigen is in accordance with the slow replication cycle of the virus. Further research is ongoing to investigate the importance of BTV-8 infection in early embryonic death.
The first author is supported by Research Foundation-Flanders.
Flow cytometric analysis of in vitro bluetongue virus infection of bovine blood mononuclear cells
