Johne's
Disease In Goats
By: Phillip Sponenberg
Johne's (pronounced YO nees) disease is a serious
disease... and one that will only become more and more important to
goat breeders in the coming decade. My suspicion is that this is much
more widespread in the Tennessee Myotonic goat breed than most breeders
are currently aware. We all need to be doing what we can to identify
this disease, and then to try to eliminate it whenever it is found.
It is a tough disease in many regards, and one we can all live without!
Johne's disease is an insidious bacterial disease that causes wasting.
This usually occurs in middle-aged animals, although the course of
the disease varies considerably and it should be considered whenever
a goat has chronic wasting. The best strategy for any goat producer
is to try to get a diagnosis on any chronically wasting goat. Some
of the old ones will be due to bad teeth, but any wasting goat under
10 years old should ideally be necropsied to find out what is causing
the wasting. The usual culprits are teeth, Johne's, caseous lymphadenitis
that has gone internally, or tumors. Any of these is worth knowing
about!
Most of the information on Johne's disease comes from cattle, and
that needs to be considered as we discuss the goat disease. Johne's
is generally spread from a dam to her kids. The disease can spread
to older animals but with much more difficulty than to youngsters
a few days old. The most susceptible time period is the short timespan
right after birth, and the environment needs to be heavily contaminated
so that oral exposure occurs. Infected does generally have the organism
on their udders, which is how the youngsters get an early and infectious
exposure. The organism can last in soil up to a year and maybe two,
which represents a certain but low risk in extensive pasture-based
systems. One of the confounding issues with goats is that the organism
is found in parasite larvae, and this is one potential mechanism for
spread from adult goat to pasture and back to adult goat. This is
in contrast to more closely housed dairy situations (especially dairy
cattle) where significant environmental contamination is assured throughout
most facilities, and most youngsters are closely housed in confined
groups.
However it is spread, it is essential to try to reduce shedding of the
organism and then contamination of the farm. This is best done by
quickly diagnosing chronic wasting. Most goats do not shed many
organisms until they start to waste away, and some of these also have
diarrhea. At this stage of the disease most animals are shedding many
organisms, and the resulting contamination can be severe.
The tests for Johne's disease all have drawbacks, but are useful if
used appropriately. The AGID test is very specific, meaning that animals
that are positive by this are nearly all infected. This test does
miss some infected goats, though, and so is not that good at eliminating
all risk. The ELISA test, in contrast, picks up more of the infected
animals - not all of them, but most. This test also tends to pick
up as positive some that are not infected, so that some uninfected
goats could end up being culled on the basis of this test. Fecal culture
can also be done, but takes months and the availability and cost of
this vary considerably state to state. Fecal culture is the "gold
standard" test, although even this can be misleading as some
goats that shed the organisms do so irregularly, and a single culture
might miss them.
The ELISA test, though useful, has a few details that can cause it to
be misleading. Two ELISA tests are available, one from IDEXX, and one
from Biocor. These tests are designed for use in cattle, and the Biocor
test turns out to be much better for goats than is the IDEXX test. So,
it is important when using ELISA to figure out which specific ELISA the
lab is using. There is also some indication that ELISA results can be
false positive much more frequently when goats also have caseous
lymphadenitis. So, in herds with CL, or in herds vaccinated with CL,
the results must be interpreted with great caution.
Interpretation of the test results is important to consider when evaluating
the status of a herd - a negative test coming from a herd with a number
of positives is much less reassuring than a negative test coming from
a herd that is all negative. Animals are also only likely to be positive
by either test if actually shedding the organisms. So, an animal with
an early, noncontagious case might well be negative and then become
positive only later as it develops the clinical disease and becomes
infectious. It is therefore important to evaluate test results on
the basis of the entire herd and not only the individuals in the herd.
It makes the most sense, when testing, to test everything one year and
older. Goats younger than this are very unlikely to show positive, even
if they are in fact infected. The good news is that they are unlikely
to be shedding, and tests in future years can hopefully identify them
before they become a threat to the herd and the farm.
Test results can guide breeder decisions. A few outcomes are likely. A
large herd that has all negative tests is very likely to not have the
organism at all. In this situation it is probably most sensible to
retest in a year, and then if they are all negative to then decide how
often to retest. At least for cattle, the recommendation is to ELISA
test the first year, then ELISA test the second year, then culture the
third year, then ELISA test the fourth year. If these are all negative,
then the cattle herd is considered negative. This is pretty arduous for
a large goat herd, especially the culture step, and it may be
impractical in some situations. At any rate, herds that are negative on
test can continue to test annually or every other year to stay vigilant
as well as to document their negative status to buyers.
A large herd with a few positive goats does have the organism, but
minimally. This situation needs to be monitored, and one strategy
is to test annually, and remove all positive goats quickly. This strategy
helps to avoid goats that shed because the annual test should identify
nearly all of them before they shed many organisms. The result is
a slow and sure reduction (and finally - elimination) of contamination
of the farm. Annual retesting is necessary, though, because
of the long incubation period of the disease, as well as the relative
inaccuracy of the test. After three or four completely negative annual
tests the herd is very low risk, and likely negative.
A large herd with anything over 5 or 10% positives on the ELISA test
likely has a pretty extensive problem with Johne's disease. After
the first test it makes sense to eliminate the positive goats, and
probably most of these should have necropsies in order to determine
how many are truly positive. An alternative strategy would be to do
fecal cultures on all of the positive goats. Testing at six-month
intervals is also wise until the rate of infection goes down, for
this helps to progressively decrease the level of contamination of
the farm as the positive (and shedding) goats are removed before they
can contribute much to contamination.
Johne's is a serious disease, and a serious threat to goats, cattle,
and sheep. Breeders should tackle it head on, and in my opinion it
is one of the diseases that everyone should strive to eliminate. This
is especially true of breeders that are selling breeding stock. Buyers
should insist on being informed of test results, and sellers should
be eager to provide these. At our farm we test annually, and I try
to make sure that every buyer reads our "goat health status"
sheet that provides information on our Johne's status, as well as
our approach and vigilance towards caseous lymphadenitis, footrot,
caprine arthritis encephalomyelitis, and other diseases. The goal
of all breeders should be that buyers buy goats and not diseases!
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