Predicted spreadof Johne's disease in a dairy herd

The curve shown is for a herd of 100 milking dairy cows with average quality herd management (effective contact rate of 2.0), where one replacement heifer is bought per year from a source with a 10% prevalence of M. paratuberculosis infections (typical of random source dairy replacement cattle market). The model illustrates that it takes a long time for the infection to get started in a herd, but once that happens M. paratuberculosis can spread through the herd quickly if no actions are taken to control it.