BVD virus biotypes
Volker Moennig
BVD type 1 and type 2 are different genotypes which means that they differ with regard to their genomes. However, BVD virus can also be divided into two biotypes. The biotypes are identified with regard to their phenotypic characteristics.
While one biotype is cytopathic, meaning it damages cells in a cell culture, the other one does not and is therefore a non-cytopathic biotype. Culture cells which are infected with a non-cytopathic variant of BVD virus are not destroyed by the virus. Both genotypes, BVD type 1 and BVD type 2, can be cytopathic and non-cytopathic. The non-cytopathic biotype is circulating in nature and bovine populations. Since it is able to manipulate the bovine immune system, it is very well attenuated to cattle. Both biotypes can cross the placenta of pregnant cattle, infect the foetuses and cause the considerable damage which bovine viral diarrhoea is notoriously known for.
Cytopathic biotypes on the other hand, arise by mutation from non-cytopathic virus types. These events are very rare but have severe consequences when they occur as they lead to mucosal disease, which is fatal. The molecular mechanisms by which the cytopathic variants arise are quite well known. They typically emerge from persistently infected animals because they shed huge amounts of virus at any given time. Hence, in the course of weeks, months or even years it may well happen that a cytopathic biotype arises from such a situation.
