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It is believed that the vast majority of M. paratuberculosis infections
in sheep occur while they are lambs. As sheep mature, their resistance to becoming
infected increases although complete resistance is unlikely and adult sheep, if
given a sufficiently large dose of M. paratuberculosis at a period of immune
insufficiency, can become infected. Infected adults serve as the source of infection
for lambs as they shed the organism in manure, milk and colostrum.
Most lambs acquire the organism by sucking on manure-contaminated teats, by licking
contaminated flooring/fencing/feed bunks or by eating off of ground contaminated
by infected manure. Since M. paratuberculosis is thought to be excreted
in the milk of infected lactating sheep, as has been shown to be the case in cattle,
lambs can become infected by nursing from or being bottle-fed milk from an infected
ewe. The risk that the organism is being shed into the milk is believed to be
higher in sheep that have moved into later stages of the infection. Thus clinically
affected sheep (as shown by weight loss) are more likely to infect their offspring
than sheep still in good condition. They also can consume the organism by drinking
water contaminated by infected manure.
Another, but less common, route of exposure occurs in utero. Again during the
later stages of infection, the organism can disseminate beyond the initial location
of the infection (i.e. the ileum, a portion of gastrointestinal tract ileum).
At that time, if the animal is pregnant, the fetus can also become infected. This
infected and infectious lamb appears completely healthy at birth, although spontaneous
abortion of fetuses with this infection has been reported in cattle.
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