Q & A:  Johne's Disease (Paratuberculosis)
Dr. Susan Stehman

Extension Veterinarian, Vet Diagnostic Lab, Cornell University

 

What is Johne's Disease? 

It is a chronic disease of primarily the digestive tract in ruminants, including goats, caused by the bacterium Mycobacterium paratuberculosis (M.para. TB). Clinical weight loss in adult goats is the only symptom. Diarrhea, a predominant sign in cattle, is seen in fewer than 20% of affected goats.

How is it transmitted?

It can be transmitted several ways. The primary means of transmission in through ingestion of feces, by  fecal-oral transmission. Adult animals shed the organism in their feces and the susceptible young ingest it. Infected goats can carry the infection in a dormant state into adulthoold and at some point begin to shed the organism in the feces.

The disease is not isolated in the digestive tract and in the early subclinical and late subclinical stages the disease can be disseminated through the body, including the uterus. We see around a 25% incidence of in utero infection.

Another 20% can be infected via the milk or colostrum of an infected animal. Heat treating colostrum does not kill this organism. Evidence suggests that it is quite heat resistant. The organism can live up to one year, maybe longer, in the soil and environment. [Editor's note: Pasteurization may not kill the bacteria in milk.]

A brief explanation of the timeline involved in the disease may be helpful. A kid may be exposed to the disease by any of the above means, but the dose, age of the kid, and its genetic make-up are what determines the course of the disease in later years. For example, if the kid receives a high dose at an early age, it would be more likely to start shedding the disease and showing clinical signs at an earlier age. Conversely, a kid that received low doses may take longer to become subclinical or clinical and may never reach the final stages. Younger kids are more susceptible. There does seem to be an age-related resistance to this disease but older animals can become infected, especially in overcrowded and unsanitary conditions.

In the first year goats can be considered simply infected. They do not start shedding until one year of age and can continue to shed without showing outward symptoms for up to 7 or 8 years.

The next stage is when an animal reaches the subclinical stage where it is infected and shedding the organism but there are no clinical signs. These are the real Trojan Horses of the disease.

Clinical animals are those that begin showing signs of weight loss with no decrease in appetite until the disease becomes all-consuming. These animals are shedding large amounts of organisms and are typically 2 to 4 years old.

Once acquired, what are the symptoms?

There are no symptoms until the goat has reached the clinical stage where the owner may notice weight loss. The animal can shed for years at low numbers and if the animal is managed well, with good nutrition, no overcrowding and good manure management, it can control the symptoms. Stress,  such as poor nutrition or concurrent disease, lead to accelerated symptoms. Overcrowding, kidding, and moving are other contributing stressful factors. Crowding contributes in a number of ways; not only is overcrowding stressful, but by the sheer numbers you will see a rapid buildup of the organisms, thus exposing more animals at a higher rate.

Once the animal begins to show signs, it will live a year or less and ultimately dies from the inability to absorb any nutrients.

Is there an accurate test for the disease when it is not showing signs and not shedding the bacteria in its feces?

No. There is no test for an infected animal that has not turned subclinical.

Is there an accurate test for the disease when it is not showing signs but shedding the bacteria in its feces?

This is called the subclinical stage and we can refer to the light shedders and the heavy shedders. There is an antibody test (blood or serum test) that is fairly accurate. The Agar-Gel Immune Diffusion Test (AGID) serum test is a good test to use on an individual basis and there are no false positives. The Enzyme-linked ImmunoSorbent Assay test (ELISA) is fairly accurate but can cross react with Corynebacterium pseudotuberculosis, the organism found in caseous lymphadenitis, to give a false positive. None of these tests are 100% accurate. The ELISA test is best used as a herd screening test and not on a individual basis. It is not as specific in goats as it is in cattle. The sensitivity of a test reflects the ability to detect a truly infected animal as positive. Antibody tests in general have poor sensitivity because antibodies show up relatively late in the disease. The ELISA test is more sensitive but the AGID test has more specificity and ability to detect an animal that is truly negative. The fecal test discussed below can also be used.

Is there an accurate test for the disease when an animal is clinical?

When an animal is clinical (showing signs of the disease), 85% will be antibody positive by the AGID test and greater than 95% will be shedding the organism in the feces that can be cultured. If the animal dies you can see the organism in histopathogy.

Light shedders will be harder to detect, with about a 20-40% accuracy in both the AGID and ELISA tests. Moderate shedders will have more antibodies and a 40-50% test accuracy can be achieved. For heavy shedders there is a 60-80% accuracy. This is based on my best scientific guess.

For a fecal you need at least 100 organisms per gram of feces to detect the disease. If it is lower than that, the test will be negative. This organism cannot be cultured in the feces of sheep. The goat strain of paratuberculosis can be cultured (grown), but the disadvantage of that test is that it takes from 6-12 weeks to grow,  in contrast to 24-48 hours for a blood test. And, since the fecal test takes from 6-12 weeks to culture, labs usually charge a bit more (around $10) for the test. The advantages of using the fecal tests are that no other organism looks like Mycobacterium paratuberculosis so you don't get any false positives. It also gives you some important information. It can tell you how much of the organism is being shed. The fecal test is 40-45% accurate in light shedders and in the clinical stages and heavy shedders it is 95-98% accurate.

The disease can be seen as a pyramid. You can assume that for every clinical animal that is tested positive, there are probably 10 subclinical animals that are actively shedding but not showing symptoms. The exception is when you bring an infected animal into a clean herd. It takes longer to build up that type of pyramid.

One quick way to monitor your herd is bleed everyone and use the ELISA test and continue to test routinely. Testing in this way, however, should be considered a snapshot in time. The greater certainty comes with multiple snapshots over time.

The history of your herd and where you purchased your animals is important. If herds you have purchased animals from have had Johne's in their herds, your herd is more suspect. If an animal dies for an unknown reason, I encourage you to do a post-mortem.

There are other tests on the market but their accuracy is limited. The rectal scraping is about 30% accurate even in a heavily infected animal. The DNA probe is, unfortunately, less sensitive than the fecal culture because there are inhibitors in feces that mask the detection.

Using body condition scoring may help to detect possible weight loss problems early.

Is it possible to get a false positive?

Yes, with the ELISA test because of the cross activity with C. pseudotuberculosis, as discussed earlier. AGID rarely gives a false positive. A false negative? Yes, in all cases.

I should also mention the laboratory quality issue here, too. Typically, university and state labs have check tests done that monitor their accuracy and validate known positives against known negatives. You also need to ask the lab if it provides interpretation of the results.

Where did the disease originate?

It was first diagnosed in cattle in 1895 and in goats in the early 1900s. It has worldwide distribution and no one knows the origins.

What makes a goat finally break with the disease?

Stress, nutrition, concurrent disease, and so on. Of course, all of this is dependent on the dose and age the kid was exposed to the disease, as well as the characteristics of the individual kid. Overcrowding and poor management intensify the disease.

Are there any vaccinations?

There is a vaccine used in Norway and Iceland. It is not available in this country because it cross-reacts with Tuberculosis testing.

If you have not introduced any new animals into your herd in past two years, is your herd safe?

No, due to the lengthy dormant stage of the disease.

Can an infected animal that comes over for breeding infect your herd?

It would be difficult unless her feces has contact with the younger kids. As discussed earlier, it takes more of a dose over a longer period of time to infect adults. However, if you track feces on your boots from one barn to another, it is possible. Managing the fecal-oral transmission is really the key. Raise those feeders. Make a foot bath from one barn to another. Manage manure so that it has no contact with the kids. Wash feed pans and utensils from one barn to another. A great quote from a local dairy farmer that we like to use is, "All manure is suspect." The same holds true for taking your doe for outside breeding.

How can breeders protect their herds themselves from this disease?

Ask questions about purchased animals. Ask the seller if they have lost any animals that were undiagnosed but that showed signs of weight loss. You need a good history.

When you bring animals in, segregate them for at least 2-3 weeks and until they can be tested. If they are shedding during that time (not an infected but a passing through type shedding) as a result a heavy contamination at the other farm, you are alleviating the possibility of infection. Then, when those animals are one year of age, use the ELISA test to monitor your herd on a yearly basis.

What do you suggest when a person finds out their herd is infected with the disease?

Define their goals. What is important to them? Breeding and selling? Then they need to continue to eliminate the disease in their herd before selling any animals. Think of the heart break you could cause. There are some real ethical issues here. Let the buyer beware, but sellers must be as honest as they can be.

Would you suggest that everyone test their herd?

 It's a good insurance policy. If your herd is closed you can feel more comfortable after several years of testing.

How long can the bacteria stay in the soil and the surroundings?

Up to and possibly over a year.

Is there anything you can put on the soil to kill the bacteria?

No.

Can other animals infect goats?

Grazing with other infected animals, such as cattle, sheep or deer, even farm gear, can transmit the disease. Colostrum from dairy cattle that are infected is suspect, and spreading manure on your property from an infected animal can create problems. Places where there are mixed species are at risk for the disease being transmitted from one species to another.

Is this a potential zoonotic disease (one that can be transmitted from goats to people)?

 No definitive causative link has been established but it is being investigated because of the potential association with Crohn's disease.

Are there any restrictions being considered for the disease?

No.

Why haven't restrictions been important for Johne's Disease?

Animal diseases are not usually restricted unless there is a high mortality rate such as seen in Hoof and Mouth Disease. Johne's Disease is not a high mortality rate disease because of the prolonged nature of the disease.

Do you test many Pygmy goats with Johne's?

Using a diagnostic lab as a basis for this statement is not very accurate because there are biases built in. No one has done a prevalence study, so there is no way to tell whether or not Pygmy goats have a higher incidence. Is it because the Pygmy goat breeders are more aware and do more testing? There are increasing numbers of Johne's Disease, but there is no way to tell why.

What would you consider epidemic proportions?

I would consider epidemic proportions to be significant levels of infection across many infected herds.

Is there a cure for the disease?

No.

What is the typical age when an animal starts showing clinical signs?

Minimum of one year, with 2-4 years of age being the norm and 7 to 8 years old the upper limit.

Is there any way of knowing the animal has the disease BEFORE it starts showing signs or is shedding the organism is its feces?

No.

But the animal can still be infected?

Yes.

If two herds have no contact with each other and one was diagnosed with Johne's, could the disease "cross the line to a clean herd?"

It all depends on the management. If you go into one pen and step in manure and carry it to the other barns, yes, the disease may infect the other herd. If you clean one barn and head to the other without first washing your shovel or pitchfork, yes. You can manage the disease, but not cure it. You can break the cycle by testing, culling and manure management.

Should anyone consider purchasing an animal from a herd that has been diagnosed with Johne's?

 Not unless they are willing to segregate that animal with stringent management and test the animal for years. All goats in a known-infected herd must be considered at risk of infection.

 

Related Reading

Johne's Disease Facts
Johne's Disease Facts 2
How Do I Keep Johne's Disease Out of My Herd?
Living With Johne's Disease


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