Prevention is the most cost-effective way to manage Johne's disease. It is far less expensive to block introducing Johne's disease into a herd/flock than it is to control or eradicate the infection once it creeps in and invisibly starts to spread.
Risk management is the foundation of any good animal care program; the risk of becoming infected by bringing in infected animals is manageable. Smart buyers of animals will apply the concepts of risk management and require diagnostic test results for the source herds or flocks to limit their risk as much as possible. If sellers refuse to allow their herds or flocks to be tested before you purchase their animals find another source of animals 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 cannot provide any information about their Johne's disease history.
For dairy enterprises (in the US, the domestic agriculture industry is believed to have the greatest prevalence of the infection – 68% of herds), owners must assume that they are buying M. paratuberculosis-infected animals on a regular basis if they do not purchase from test-negative herds. The buyers must then be sure to have animal husbandry systems in place to try to control the further transmission of the infection.
Other routes by which Johne's disease may be introduced to herds/flocks 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.