This article written by Dr. Reynold Bergen, BCRC Science Director, originally appeared in the December 2022 issue ofCanadian Cattlemenmagazine and is reprinted on BeefResearch.ca with permission of the publisher.
Photo courtesy of Beef Farmers of Ontario
Johne’s disease is sometimes compared to an iceberg – the visible cases are only a tiny fraction of the problem. The biggest part is hidden beneath the surface, particularly in the early stages of disease.
In the silent stage, cattle that were infected as calves by a manure-contaminated udder, water or feed show no clinical signs. The Johne’s disease bacteria hide and multiply in the cells lining the digestive tract. In the subclinical stage the animal mounts an immune response to the bacteria. Some infected intestinal cells burst, release more bacteria into the gut, shed them in the feces, and infect other animals. At this stage, diagnostic blood and fecal tests often miss the infection because shedding is sporadic, and the immune response and numbers of bacteria shed are generally low. Infected cattle gradually shed more and more bacteria in their manure, infecting more and more herdmates. The obvious clinical stage doesn’t appear until cows are a few years old. By then their digestive tracts are too damaged to efficiently digest feed or absorb nutrients or water. They develop a “water hose” diarrhea that sheds tremendous amounts of bacteria, resulting in significant weight and body condition loss. They are generally culled at a much younger age than uninfected cows, making it a very costly disease.
In the silent and subclinical stages, producers may be completely unaware that they could be selling or buying infected herd replacements. With many medium-sized herds currently dispersing, many younger cows are going to larger, expanding herds. Some of these cows may carry undiagnosed Johne’s disease.
At the Western College of Veterinary Medicine, Cheryl Waldner led a team to determine whether Johne’s disease is becoming more common in Canada, to identify risk factors for Johne’s disease and to evaluate different testing strategies (“Estimating the sensitivity and specificity of serum ELISA and pooled and individual fecal PCR for detecting Mycobacterium avium subspecies paratuberculosis in Canadian cow-calf herds using Bayesian latent class models”; doi: 10.3389/fvets.2022.937141).
What They Did
159 herds from the Canadian Cow-Calf Surveillance Network participated in this study. Each producer provided information about their herd (commercial, purebred or both), calving season, calving management (confinement or on pasture), whether they had dairy cattle, used community pastures, purchased replacements or had experienced Johne’s disease before. Veterinarians sampled at least 20 individual cows per herd (over 3,000 blood and 3,000 fecal samples in total). Blood samples were used to test for the Johne’s disease bacteria and fecal samples were tested for bacterial DNA. Fecal samples from five cows were also combined to reduce testing costs and time. If the fecal pool tested positive, individual samples were tested.
Johne’s disease prevalence remains low but appears to be rising. Johne’s disease was detected in 6% to 8% of Eastern herds compared to 5% of Western herds. Within infected Eastern herds, 17% to 25% of cows tested positive, compared to 10% of cows in infected Western herds. The higher prevalence in Eastern Canada may be related to higher pasture stocking densities, more confined winter feeding and the larger dairy population.
Risk factors for Johne’s disease included thin cows, the presence of dairy cattle on the farm or having observed Johne’s disease symptoms within the last three years.
Testing strategies: The fecal test was more sensitive than the blood test (54% vs. 35%), meaning that if Johne’s disease was present, the fecal test was more likely to find it. But these relatively low sensitivities mean that both tests likely overlook some infected individuals. Both the blood (99%) and fecal (99.9%) tests were highly specific, so a positive result from either test is almost certainly accurate. The bottom line is that a negative test doesn’t mean an animal doesn’t have Johne’s, but a positive test almost certainly means that it does. But if you’re testing a lot of cows with the blood test, those 1% false positives can add up. In that case, the presence of multiple positives likely means the herd is infected. Another option is to retest positive cows using the more sensitive and specific individual fecal test.
So, What Does This Mean… to You?
Diseases are easier to avoid than to eradicate, so be careful how you expand your herd. Avoid anonymous bargains with no health information. Become familiar with the health management and history of the replacements you are considering. Stress can trigger the clinical signs of Johne’s, so segregating newly purchased animals for a few weeks may be a chance to detect and remove shedders before they spread disease to the main herd.
If the clinical signs of Johne’s disease sound familiar (especially the rapid loss of weight and condition and severe water-hose diarrhea), consider having your veterinarian collect fecal samples for Johne’s testing. If they test positive, sell those cows for immediate slaughter to avoid spreading the problem. There’s a good chance their daughters were infected as calves, so keep a close eye on them and strongly consider testing and/or culling them as well.
The BeefResearch.ca website hosts Dr. Waldner’s free Johne’s Testing Decision Tool (and explanatory videos) to help producers and their veterinarians understand how different animal purchase practices and Johne’s disease testing strategies (including doing nothing) impact disease transmission and herd performance.
The Beef Cattle Research Council is funded by the Canadian Beef Cattle Check-Off. The BCRC partners with Agriculture and Agri-Food Canada, provincial beef industry groups and governments to advance research and technology transfer supporting the Canadian beef industry’s vision to be recognized as a preferred supplier of healthy, high-quality beef, cattle, and genetics.
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