Schematic of model for Mycobacterium avium subsp. paratuberculosis in cattle herds. Animals exist in one of 11 states: susceptible calves (S1), transiently shedding calves (T1), susceptible heifers (S2), transiently shedding heifers (T2), latent heifers (E2), susceptible adults (S3), latent progressing adults (EH), low-shedding progressing adults (LH), high-shedding adults (H), latent non-progressing adults (EL), and low-shedding non-progressing adults (LL). All parameters are defined in Table 1.