This is because there is an age dependent increase in resistance as animals grow older. Experimental evidence has shown, however, that the resistance is never complete. Even adult animals, if given a sufficiently large dose of M. paratuberculosis bacteria can become infected. But under normal animal husbandry conditions, exposure to such large doses probably rarely occurs. This means, for example, that an infected adult dairy cow in a stanchion or tie stall barn is not going to infect the cow standing next to her.
Infected cows are primarily just a source of infection for calves.
The vast majority of infections in young animals are acquired by ingestion of M. paratuberculosis . This happens accidentally when they consume manure containing the bacterium. Sucking on manure contaminated teats on the udder of their mother, licking contaminated bars in the stall where they are born, or being housed in a location where they have access to manure from the adult herd are all ways young animals have opportunities to ingest this bacterium. A second method of exposure is by drinking infected milk. M. paratuberculosis is excreted in the milk of infected lactating animals, particularly when the infection is in the more advance stages.