At the initiation of immuno-competence, the established virus appear be treated as 'self and all clones of immune cells, reactive to BVDV, are depleted. This depletion tolerance would account for the failure of the animal to make BVDV antibody to clear the virus. They are persistently infected for life.
The second event is the superinfection of PI animals with the second biotype, BVDVc. This usually occurs between 6-18 months old but can occur within weeks of birth and as late as 7-8 years old. Following superinfection, the animal develops fatal mucosal disease within 3 weeks. It is now evident that antigenic 'homology' between the two biotypes is important for the rapid onset of mucosal disease. Once BVDVc has arisen, it will spread quickly amongst PI animals by direct contact.