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Adult goats infect kids
by a variety of routes.
It is believed
that the vast majority of M. paratuberculosis infections
in goats occur while they are kids. As goats mature, their resistance
to becoming infected increases although complete resistance is unlikely
and adult goats, 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 kids as they shed the
organism in manure, milk and colostrum.
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This contagious mycobacterial
disease spreads from infected adults to the most susceptible age group (kids)
by a number of routes. Although less susceptible than young animals, adult goats
can also acquire the infection. The organism can be shed by an infected animal
into milk and manure; the manure then can contaminate water and solid feed. It
is likely goats can acquire the infection from and transmit it to other species,
such as cattle and sheep.
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 Most
kids 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 goats,
as has been shown to be the case in cattle, kids can become infected
by nursing from or being bottle-fed milk from an infected doe. The
risk that the organism is being shed into the milk is believed to
be higher in goats that have moved into later stages of the infection.
Thus clinically affected goats (as shown by weight loss) are more
likely to infect their offspring than goats 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). At that time, if the animal is
pregnant, the fetus can also become infected. This infected and infectious kid
appears completely healthy at birth, although spontaneous abortion of fetuses
with this infection has been reported in cattle.
These young animals, although infected, are likely to remain healthy for months to years. During this extended period they are clinically normal and the infection is invisible, yet they are still capable of transmitting the infection to their young herd mates. This is why many producers are taken by surprise when a case is diagnosed: a doeling may have been purchased at auction, may have produced some kids herself and then been sold. The owner never saw any signs of illness in the doe, yet the kids she produced on the premises may years later move into clinical Johne’s disease. Without excellent herd records, it can sometimes be difficult to figure out where the infection came from (and sometimes people spend too much time trying to trace back the source when their energies are better focused on preventing new cases from occurring).
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