By: Phillip Sponenberg, DVM, PHD 2009
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!
