– Dr. Michelle Arnold, DVM, MPH UK Ruminant Extension Veterinarian
Figure 1: Recently calved cow with classic 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.
Johne’s (pronounced Yo-knees) Disease is a slow, debilitating disease of adult cattle characterized by profuse, watery diarrhea and progressive weight loss or “wasting” (Figure 1), despite remaining bright and alert with a good appetite. It is caused by the bacterium Mycobacterium avium subsp. paratuberculosis, commonly referred to as the “MAP” bacteria. This disease begins when calves (not adult cattle) are infected by eating or drinking manure containing the MAP bacteria. This most often happens around the time of birth, although infection can occur up to 6 months of age but very rarely after. Once MAP enters a calf, the organism takes up residence within the cells lining the ileum (the last portion of the small intestine) where it multiplies, causing the intestinal lining to slowly thicken. Over time, the thickened intestine loses the ability to absorb nutrients, resulting in watery diarrhea. There is no blood or mucus in the feces and no straining to defecate. Affected cattle typically begin to display clinical signs of diarrhea and progressive weight loss at 2-5 years of age or older. There is no treatment available for this condition and the animals that develop chronic diarrhea eventually die due to starvation and dehydration. The MAP organism is “shed” in the feces into the environment before the diarrhea starts and continues until the animal’s death. Map bacteria are very hardy due to a protective cell wall that allows survival for long periods, sometimes years, in the environment where it can infect young calves.
How do calves get infected with MAP bacteria?
Johne’s disease begins with MAP infection, most often when calves ingest MAP-contaminated feces from nursing dirty teats. In beef cattle, dirty teats are common when mud and manure are splashed on the udder in high traffic areas (for example, around hay rings and feeding areas), when calving cows in dirty sheds or barns, or when cattle are overcrowded or kept indoors in close confinement. MAP may also be transferred to calves in colostrum and milk from infected cattle, and from drinking manure-contaminated water, especially in shallow ponds. There can be some spread from an infected cow to her fetus during pregnancy, but this is not common. Transmission by bulls from semen has not been proven but infected bulls still contaminate the environment with their MAP-infected feces.
How did Johne’s Disease get on my farm?
In almost all cases, the MAP bacteria arrived when an infected animal that looked perfectly normal was 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 grafted on a cow. Since symptoms typically develop in mature cattle, it is easy to buy and sell infected, young, breeding age animals with no obvious symptoms yet they are already incubating the disease. Unfortunately, testing cattle for Johne’s prior to purchase is not the easy answer. Because the disease progresses so slowly, the tests will generally not detect the disease in animals under 2 years of age and may not show a positive result until the animal is much older. In other words, a negative test result does not mean the animal is not infected; it may mean the disease is not yet detectable in the infected animal. As the infected animal matures, this “subclinical” animal (subclinical=before diarrhea and weight loss develop) can and often does shed millions of microscopic MAP bacteria in feces, contaminating the farm long before there is evidence of a problem. Colostrum sourced from other herds, especially from dairies, is another potential avenue for MAP entry to a farm.
Diagnosing a clinical case: Does this animal exhibiting weight loss and diarrhea have Johne’s disease?
Options for testing individual sick cattle:
- Best test: If Johne’s disease has never been diagnosed on the farm previously, it is best to begin with submission of a suspected case to a veterinary diagnostic laboratory. The affected animal should be humanely euthanized by a veterinarian then promptly taken to the lab for a necropsy (like a human autopsy). Examination of the tissues at the cellular level (histopathology) using special stains can confirm the presence of the MAP organism.
- Best test in a live animal: PCR on a manure (fecal) sample can be used as a primary diagnostic test to confirm the clinical signs of diarrhea and wasting suggestive of Johne’s disease. PCR is an “organism detection test” meaning it detects the genetic material of the MAP bacteria present in the feces. The PCR result is also a good indicator of the amount of MAP being shed in the feces (see Figure 2). Before PCR testing was developed, a fecal culture in which MAP bacteria was grown in the lab was required for diagnosis but it was quite slow. Johne’s liquid culture had to be incubated 42 days while solid media culture was incubated 13 weeks before results were known.
Figure 2: Sample result from a Mycobacterium paratuberculosis real time PCR test for detection of the MAP organism (UKVDL)
- 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 and is much better utilized as a herd screening test. The test is not perfect; healthy, uninfected animals can test positive (a “false positive”) so cattle with positive blood test results should be confirmed truly positive with a fecal PCR.
Why should I care if I have Johne’s Disease in my 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. This is why Johne’s is often called an “iceberg disease”; since it takes years to develop the symptoms of diarrhea and weight loss, just the tip of the problem is visible, but most of the problem is invisible because infected cattle look healthy but are spreading the MAP bacteria all over the farm. 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.
Seed stock operators (including farms that sell any breeding stock, registered or commercial) should enter a rigorous testing program to eradicate this disease once identified. Many 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 an incurable disease into a buyer’s herd. Not only a tarnished reputation but litigation could potentially result from transactions when the source herd is known to be MAP-infected.
Once a diagnosis of Johne’s Disease is made, what are the next steps?
Once a diagnosis is made, the first step is to determine the goals for the operation. If selling seed stock, the goal should be to classify the herd as test-negative or work towards it as quickly as possible. Commercial operations may opt to reduce the disease prevalence gradually through testing and management changes. After the goal is established, decisions on which animals to test and what test to use often come down to how much money the herd owners are willing to spend on testing and how fast they want to see progress. A recent study of test strategies for control of Johne’s in cow/calf herds found the best method for reducing prevalence but with the lowest total testing costs was collecting individual fecal samples from all cattle over 2 years of age in the herd and submitting them to a veterinary diagnostic lab for either 1) individual fecal PCR testing every other year or 2) pooled PCR testing every year. Remember that herd testing is done on healthy animals so decisions should be made in advance on how positive results will be handled. If no changes will be instituted, then testing is a waste of time and money.
Where can I learn more about Johne’s Disease?
The Johne’s Information Center at the University of Wisconsin maintains an excellent website with good producer-level information at https://johnes.org. An easy-to-understand video about Johne’s can be found at https://www.youtube.com/watch?v=u0Y0ew5yMo8 although it is dairy-oriented.