– Dr. Michelle Arnold, Ruminant Extension Veterinarian, University of Kentucky
Figure 1: Cow with signs of Johne’s disease; dull hair coat, profuse watery diarrhea and weight loss. Photo used with permission from Dr. Amy Jennings, The University of Edinburgh, Scotland.
What is Johne’s Disease? Johne’s (pronounced Yo-knees) Disease is a slow, progressive disease of adult cattle characterized by profuse, watery diarrhea and weight loss or “wasting” (Figure 1). It is caused by the bacterium Mycobacterium avium subsp. paratuberculosis, also known as “MAP”. This disease begins when calves (not adult cattle) are infected with MAP-contaminated colostrum, milk, feed or water, most often around the time of birth. Once MAP enters a calf, the organism lives permanently within the cells of the large intestine where it multiplies and causes the intestinal lining to slowly thicken. Over years of time, the thickened intestine loses the ability to absorb nutrients, resulting in watery diarrhea and weight loss despite continuing to eat well. These symptoms do not show up in adult cattle until 2-5 years of age or even older. There is no treatment available, and the animal eventually dies due to starvation and dehydration.
Why should a commercial beef producer care if they have Johne’s Disease in the herd? Economically, Johne’s disease can be costly in a beef operation. For every clinical (sick) cow with Johne’s in a herd, there are potentially 10-20 more who are infected but not yet showing signs.
Obviously, death loss and premature culling will mean higher replacement costs to keep herd numbers stable. Perhaps less obvious is that MAP-infected cows showing no evidence of disease are less fertile and produce less milk, resulting in lighter calves at weaning and more open cows at pregnancy check.
Why worry about buying Johne’s Disease? No beef producer in his or her right mind would buy a cow or bull with diarrhea and weight loss to add to their own herd! Very true but the infection is present long before diarrhea and weight loss begins. In almost all cases, the MAP bacteria arrive on the farm when an infected but healthy-looking animal is purchased and added to the herd. These bacteria can be hiding in replacement heifers, cows, breeding bulls, recipients used for embryo transfer, or even in an infected calf purchased to graft on a cow. Although these cattle show no obvious symptoms, they may already be spreading the disease. In a typical case, an infected cow starts shedding the MAP bacteria in her manure after delivering her first calf but she doesn’t develop diarrhea until after her third calf, resulting in 2 years of “silent” disease spread. Infected cattle shed the MAP organism in increasing numbers as the disease progresses, contaminating the farm environment and the risk of spread within the herd.
How do calves get infected with MAP bacteria? Johne’s infection occurs mainly by calves ingesting MAP-contaminated manure when nursing dirty teats. Most infections (75+%) occur around the time of birth up to 6 months of age and the remainder up to 1 year of age. In beef cattle operations, mud and manure are frequently splashed on the udder when calving cows in dirty sheds or barns, in high traffic areas (around hay rings, feeding areas) or when cattle are held in close confinement or overcrowded conditions. MAP is also passed in colostrum and milk of infected cattle. There is great opportunity for disease transmission in nursing beef calves since they remain with dams 6-7 months or more. Colostrum obtained from other herds, especially from dairies, is another potential source of MAP. There can be MAP transmission from an infected cow to her fetus during pregnancy but this rarely happens unless the dam has an advanced case and has already developed diarrhea. To date, transmission by bulls through infected semen has not been proven but infected bulls can contaminate the environment with their MAP-infected feces .
How do I find out if a cow or bull with diarrhea and weight loss has Johne’s disease?
- Best test if animal is dead or near death: Submission of a dead animal to a veterinary diagnostic laboratory. The affected animal should be humanely euthanized then promptly taken to a veterinary diagnostic lab for a necropsy for definitive confirmation of Johne’s.
- Best test in a live animal: If this is the first suspected case of Johne’s disease in the herd, PCR on a fecal sample is the best test to confirm the disease. PCR is an “organism detection test” meaning it detects the DNA of the MAP bacteria in the feces. The PCR result is very accurate and a good indicator of the amount of MAP being shed in the feces (see Figure 2). If the PCR is positive, the first question to consider is “was this infected animal born and raised in this herd?” If the answer is yes, there are likely to be other infected animals in the herd. If no, spread within the herd may be limited.
- The blood test (known as a “Serum ELISA”) is an “antibody detection test”. It is considered a good herd screening test for MAP antibodies and positive results can be confirmed with PCR. The fecal PCR will usually detect the disease earlier than the blood test.
Figure 2: Sample result from a Mycobacterium paratuberculosis real time PCR test for detection of the MAP organism (UKVDL)
Once a diagnosis of Johne’s Disease is made, what are the next steps? Once a diagnosis is made, the next steps depend on the type of beef operation involved. For registered herds selling seedstock, the goal should be to classify the herd as test-negative using fecal PCR to eradicate this disease once identified. Seedstock producers are often reluctant to test for Johne’s Disease for fear that a positive diagnosis will ruin their reputation. However, a seedstock herd’s reputation may be damaged much more severely by selling a MAP-infected animal to a customer when that animal is eventually diagnosed with Johne’s disease. Commercial operations, on the other hand, may opt to reduce the disease prevalence gradually through blood testing and herd management changes to clean up the environment, especially during calving.
After the goal is established, decisions on which animals to test and what test to use will depend on the answers to the following questions. What management changes will be made based on test results? Will positive animals be culled, or will test-positive and test-negative herds be established based on results? How much money will be allocated for testing? How quickly is progress needed towards goals? Remember that herd testing is performed on healthy animals over multiple years so decisions should be made in advance on how positive results will be handled. It takes at least 5 years of consistently following a written plan of action to control this disease. If no management changes will be instituted, then testing is a waste of time and money.
How does a producer avoid buying MAP-infected cattle? It is not always easy because infected animals will often test negative when young and may not show a positive test result until they reach 3-5 years old.
The safest option is to have a “closed” herd with no purchased animals. If this is not possible, the next best option is to buy from low-risk herds with Johne’s test results on the adult herd available within the last year. Since low-risk beef herds are uncommon, a good rule of thumb is to purchase animals from as few different herds as possible and test all new animals prior to mixing with the home herd. Purchased animals should be tested at least 3 times (begin testing no younger than 2 years of age) before calling them “negative” or “low risk”. The number of cattle purchased is not as important as the number of herds these animals came from. Buying randomly sourced cattle from many different herds is a guarantee your herd will become MAP-infected at some point.
Where can someone learn more about Johne’s Disease? First and foremost, talk with your veterinarian about detection and prevention strategies tailored for your specific situation. The Johne’s Information Center at the University of Wisconsin maintains an excellent website full of information at https://johnes.org.