Risk management is the foundation of any good animal care program; the risk of becoming infected by bringing in infected cows is manageable. Smart buyers will apply the concepts of risk management and require diagnostic test results for the source herds to limit their risk as much as possible. The dairy industry has the greatest prevalence of the infection in the United States (up to 68% of herds are believed infected). Owners must assume that they are buying MAP-infected animals on a regular basis if they do not purchase from test-negative herds. If sellers refuse to allow their entire herds to be tested before you purchase their animals, find another source from whom to buy if possible or at least test the individual animals you are contemplating for purchase (and their dams!). You are better off buying a test-negative animal from a dairy herd with a known low test prevalence than buying an animal from a herd that has no clue about its Johne's disease history.
In addition to buying it in, other routes of introducing Johne's disease may exist, but they are of much lower risk (although data is limited quantifying these risks). These other routes include spreading manure from potentially infected farms on land the herd owner uses for grazing or forage production, use of colostrum or milk from dairy herds of unknown status for hand-rearing orphaned neonates, or animal access to run-off water from adjacent farms. These routes are theoretically important, but the risk of acquiring the infection from them is likely much lower than through the purchase of animals.
For animal industries, self-regulation to encourage open discussion of Johne’s disease and marketing of animals from test-negative herds is the best way to manage it.