– Dr. Michelle Arnold, Ruminant Extension Veterinarian, University of Kentucky
Figure 1: Recently calved cow with signs of Johne’s disease; dull hair coat, profuse watery diarrhea, and weight loss. Photo from “Management and Control of Johne’s Disease in Beef Sucker Herds” by Drs.Isabelle Truyers and Amy Jennings. In Practice July/August 2016/Volume 38, page 348.
What is Johne’s Disease?
Johne’s (pronounced Yo-knees) Disease is a slow, progressive disease of profuse, watery diarrhea and weight loss or “wasting” in adult cattle (Figure 1) 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. There is no blood or mucus in the feces, no straining, and no fever. 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. It is believed that for every clinical (sick) cow with Johne’s in a herd, there may be 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 signs 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 can be spread long before diarrhea and weight loss begin. 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. 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. The 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. It is easy to buy (and sell) infected, young breeding age animals with no obvious symptoms even though they are already incubating and spreading the disease. These infected animals will shed the MAP organism in increasing numbers as the disease progresses, contaminating the farm environment and increasing the risk of infection spread within the herd. Unfortunately, infected animals will often test negative when young and may not have a positive blood test until they reach 3-5 years old.
How do calves get infected with MAP bacteria?
Johne’s infection is mainly caused 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 but very rarely after 12 months. In beef cattle, 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. MAP is also shed in colostrum and milk of infected cattle. There is great opportunity for transmission thru colostrum and milk in beef calves since they remain with dams 6-7 months or more and many calves steal extra milk from cows other than their own dams. Colostrum 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 occurs unless the dam has already developed diarrhea. Infected bulls can contaminate the environment with their MAP-infected feces. To date, transmission by bulls through infected semen has not been proven.
Diagnosing a clinical case: Does this animal exhibiting weight loss and diarrhea have Johne’s disease? Options for testing individual cattle:
• 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 the lab for a necropsy. Histopathology (with special staining) on necropsy-collected tissue is 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 primary diagnostic 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 also 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.
• The blood test (known as a “Serum ELISA”) is an “antibody detection test”. It is not the preferred test for confirmation of an individual clinical case but can be used if the herd is already known to be Johne’s-infected. The test is not perfect; cattle with Johne’s disease can test negative on serum. However, the blood test is considered a good herd screening test for MAP antibodies and positives should be confirmed with PCR.
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 seed stock, the goal should be to classify as test-negative or work towards it as quickly as possible. Farms that sell any breeding stock should enter a rigorous testing program using fecal PCR to eradicate this disease once identified. Many producers are reluctant to test for Johne’s Disease for fear that a positive diagnosis will ruin their reputation. However, a seed stock herd’s reputation may be damaged much more severely by selling a MAP-infected animal to a customer and introducing a contagious, incurable disease into a buyer’s herd. Commercial operations, on the other hand, may opt to reduce the disease prevalence gradually through blood testing and herd management changes to improve hygiene, 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 done 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 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?
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 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 them prior to mixing with the home herd. Remember that infected cattle, especially young cattle under 3 years of age, may test negative although they are actually positive. Purchased animals should test negative at least 3 years in a row (begin testing no younger than 2 years old) 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. However, buying randomly sourced cattle from random herds is not a question of if, but when, your herd will become MAP-infected.
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 operation. The Johne’s Information Center at the University of Wisconsin maintains an excellent website full of information at https://johnes.org.