Donald Coover, DVM
Johne’s disease is highly contagious and spreads quickly. Up to 500 billion organisms can be shed through fecal movement by a single infected animal in one day. Johne’s disease is a contagious, chronic, and usually fatal infection that affects the small intestine of ruminants. It is a very common problem, and is found worldwide. Good herd management and a regular testing program can control Johne’s disease. Many producers are recognizing that their herds have a Johne’s problem. The typical telltale signs that a herd is infected are animals (usually over 3 years of age) that suddenly lose weight, develop a persistent diarrhea and waste away to virtually nothing but skin and bone. The animals will eventually die after the dramatic weight loss. Appetite and attitude are initially pretty normal. As the extreme weight loss continues the animals will become slow and depressed. There is no vaccine or treatment available so the disease must be managed to avoid loss.
Johne’s is caused by Mycobacterium avium paratuberculosis, is highly contagious and can spread quickly. Animals infected will shed billions of the disease causing bacteria daily in the later stages of the disease. This contaminates pastures and lots and spreads the disease to other animals in the herd. Young animals are especially susceptible to infection. Unfortunately, the infective organism can persist for years in the environment making the disease a real challenge to control.
In order to control Johne’s disease, knowledge, management, and a good testing strategy is necessary. One of the primary goals for every producer who realizes he or she has Johne’s problem is to stop the shedding of the organism so their lots and pastures can, over time, clean up. Testing is the key. There are two tests used to detect Johne’s infected animals, and both are useful and both have serious limitations as both will often provide false negative diagnosis. To explain, you test an infected animal, and the result may say the animal is not infected. With the blood test, you test for the animal’s reaction to the infection. If the animal is in the very early stages of the disease, the animal’s body will not have mounted a defense so the test will read negative. Some animals will never mount much of a defense, (even though the infection is killing them) and the blood test will continue to show a negative diagnosis, or only a suspected positive. The blood test results will either read negative, suspect, or positive. At some point in time, regardless of the blood test results, the affected animal will start to shed the Mycobacterium, further contaminating lots and pastures. These animals will usually be showing clinical signs by then, like dramatic weight loss and persistent diarrhea.
The other test is a fecal PCR test. A feces sample is submitted to the lab and a test is run to detect the Mycobacterium organism in the feces. If it is detected, for certain the animal is Johne’s positive. If not detected, the test will come back negative, but since the animal will shed intermittently, false negatives are a problem with this test too. The other shortcoming with this test is that it does not forecast the very thing you are trying to avoid which is fecal shedding of massive amounts of the disease causing organism.
My recommendation is you use both tests to help you get a handle on the Johne’s problem in your herd. Set your program up like this – test all animals in the herd that are 3 years of age or older with the blood test and do it annually. After you get your initial results from the first year’s blood test, check to see if you have any suspect animals and any positive animals. Take a fecal sample from the suspect animals and positive animals and submit them for the fecal test. Keep the suspects in one testing group and do not pool more than 5 samples. Keep the positives in another testing group and again do not pool more than 5 samples. Should any of the stool testing result in a positive result at least one of those animals in the pool is already shedding and contaminating your lots and pastures.
My advice is to look at the results of your testing and consult with your veterinarian and/or lab to come up with a culling plan. My recommendation would be to cull any animal that is suspect or positive on the blood test. I use the fecal PCR test only to confirm that a producer has a real and immediate problem. After you get your results be aware that any offspring, especially recent offspring of positive females, are at much greater risk of being infected with Johne’s disease. Marketing them as feeders and not keeping them for replacements should be seriously considered. If you are keeping offspring out of positive cows, I would test those animals as early as 2 years of age and at least annually. If I had suspect animals in my herd and they were valuable and not shedding , I might try to keep them around for another calf or for another flush. This can be risky and if I were to keep a suspect animal around, I would test them more frequently. Testing suspects every 6 months would be best to guard against having positive animals that are a lot more likely to be shedding.
The last point I would make is that I would want to have 3 or 4 years of blood testing without any suspects or positives before I tried to convince myself I was Johne’s free. Of course, any animal showing clinical signs I would test at least with the blood test and probably with the fecal PCR test too. When selling suspect or positive animals, do everyone a favor and sell them for slaughter only. Selling an animal you know or suspect has a Johne’s problem into another herd is bad for your reputation, bad for the industry, and certainly bad for whoever buys it. If you have questions talk to your vet or call Dr. Don Coover, DVM at (620) 763-2211.