Summary
Bluetongue (BT) viruses (BTV) have been propagated in laboratory culture systems for more than 50 years, especially for the production of vaccines. An important outcome of the adaptation of these viruses to laboratory culture is attenuation of their virulence. As a consequence of this modification, it has been possible to produce vaccines that have reduced the clinical impact of BT in a number of countries. Unfortunately, the adaptation of these viruses has also introduced undesirable properties. In particular, modified live BT vaccine viruses have a high capacity to cross the ovine placenta and cause congenital abnormalities in the foetus. Modified strains of BTV have also been found in the semen of bulls and rams. It is possible that there are also other undesirable properties, including the potential to infect non-ruminant hosts. Because these characteristics are not properties of naturally occurring BTVs, the use of laboratory-adapted strains is not recommended when the biological properties of BTV are being studied.

Keywords
Attenuation, Bluetongue, Cell culture, Laboratory modified, Foetal defect, Semen contamination, Virus.