Conceptual model of pestivirus spread. Squares: health states, solid arrows: transitions between health states, dashed arrows: reproduction (production of newborns), S
: protected by maternal immunity, S: susceptible, T: transiently infected, Rg: resistant with a possibility of pregnancy with a risk of vertical transmission, R: resistant without a possibility of pregnancy with a risk of vertical transmission, P: persistently infected. This scheme is broken down for each age class and sex with some variations: compartment S
is present only for newborns and young animals; compartment Rg is present only for sub-adult and adult females.