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AdvertisementHerd expansion leads to more Johne'sIn just over 10 years, the contagious bacterial disease of the intestinal tract known as Johne's disease has become prevalent in as many as 70 percent of U.S. dairy herds. National Animal Disease Center (NADC) microbiologist Judy Stable said this increase is due to the fast expansion of large dairy herds throughout the country. "Studies showed that in 1996, Johne's disease was in about 22 percent of the U.S. herds, but because of rapid expansion of herds across the country, producers unknowingly purchased young heifers and older cows which were infected with the disease and thus has raised the prevalence of Johne's to be present in nearly 70 percent of the herds," she said. Stable along with other scientists at NADC and the Agriculture Research Service (ARS) are performing research on the main organism causing the disease--Mycobacterium paratuberculosis. The organism is also referred to as Mycobacterium avium subspecies paratuberulosis (MAP). MAP is a relative of the bacterium that causes tuberculosis in humans and animals. MAP only grows in animals. It cannot grow and multiply in nature; however, if soil or water is contaminated with this bacterium, it can survive over a year because of its resistance to heat, cold and drying. Stable said her initial research, when she joined ARS in 1992, was to learn more about the basic immunology and pathogen interaction of MAP. But now, that research has changed to move more toward making an early-detection diagnostic test for producers to use and eventually a vaccine that will work against Johne's. "Back in 1992, we were more anti-vaccine, but now we realize we need to use the tools we can, to fight against disease in the least expensive approach for producers. Ultimately, that is a vaccine," she said. "A vaccine will be much more cost-effective than testing a whole herd, culling affected animals and replacing them with disease-free animals." A killed vaccine is currently available, but not all states allow its use. Stable said that along with using the vaccine, a herd must also be on a very stringent management program.
Johne's testing Currently, several different tests are available to determine if an animal is infected with Johne's. There are two types of tests for Johne's disease in routine use today--tests that measure antibodies in blood serum, and tests that find the MAP organism in manure by fecal culture. Stable said the trouble with these tests is the waiting time. While some of the testing time has been decreased to eight weeks, some can take as many as 16 weeks to get a result. "Producers need to be able to make management decisions before that amount of time," she said. The difficulty in detecting the MAP organism is that different animals shed the organism at different amounts and at different times in their life cycle. That is why she, along with several other scientists, is trying to develop a test that can be used at as early as six months of age. This is before the animals are showing the organism in their feces and before they are showing clinical signs of the disease. She hopes that this type of test will be available soon to producers, but has no timeline on when that might be. With current testing procedures costing $5 to $20 per animal, she also hopes an early-detection test will be economical for producers.
Diagnostics Johne's disease has always been difficult to diagnose because each animal has different host immune responses and many factors come into play when the activators are broken down and stressed, allowing the disease to take over. "We don't know what exactly causes the depression from sub-clinical to the clinical disease," she said. "Many stressors play a large role in the takeover." She said studies have shown that parturition can be a major activating stressor, especially in older cows--when a cow gives birth, many of her other disease-fighting responses are weakened, allowing the MAP organism to take over. "At some point, the immune cells can't handle the load of bacteria," Stable added. Each animal is different and, while some animals may be in the sub-clinical state their entire lives, others may become clinically infected at the very young age of 2 to 4 years old. Stable said the dam of the calf plays a large role in how strong the animal's immune responses will be. How much shedding of organisms a cow does to her young can determine the amount of immune response the calf will have to a disease. Besides shedding of the disease from dam to calf, Johne's can be spread through improper management practices, also.
Other research Another NADC microbiologist, John Bannantine, is studying information from the MAP genome to assemble an array of 96 proteins that might be useful in confirming a Johne's disease diagnosis. Stable said Bannantine's research is identifying gene sequences that might be useful in confirming a Johne's disease diagnosis, or might serve as targets for disease intervention and treatment. His findings will also be useful in the eventual development of a diagnostic tool for Johne's disease. Meanwhile, Stable's research is working with as many as 500 proteins to find the protective abilities and which proteins are important in disease diagnostics. This work is being done with mice first; and then, she will eventually use a goat and sheep model in hopes of getting the early-detection test. She said the smaller ruminants tend to reach clinical disease state slightly quicker than cattle. Therefore, they will know the efficacy of the test sooner, before moving on to a calf model. Together, their research should eventually mean a test can be used to determine if an animal has been infected with MAP organisms at a very young age and definitely prior to clinical signs of the disease. While they are testing for this in a lab environment, they will ultimately have to see if their research will work on infected cattle in real-world dairy herds.
Management is key Stable said that while they are working as quickly as they can to develop a test to determine Johne's at a young age, it is up to the producer to use proper management techniques to prevent the spread of the disease. "A vaccine to prevent Johne's would be great for producers, but if they have a problem now, they should find contact points that might be causing the spread of the disease," she said. Simple, good management practices, such as using different equipment to scrape manure from holding areas and to mix feed, can help with prevention since manure is the main cause for the spread of disease. "Proper manure management practices can prevent not only Johne's disease, but also other bacterial diseases," she said. Another contact point is from dam to fetus through milk. Calves should only be fed colostrum from non-infected cows or a powder milk replacer should be used. Growing new heifers in the proper environment and away from older cows which might be infected can help prevent spread to younger animals. "Separation of younger animals from older animals can sometimes be a challenge for producers. However, it may be the most cost-effective way to prevent further spread of diseases like Johne's," Stable said. She said producers need to know what they will do with animals that are infected. Getting rid of the clinically infected animals is very important. This is usually a low percentage, but the most important group to depopulate. The sub-clinical animals are an important group to keep an eye on, as they can become clinical at any time. With the high prevalence of Johne's in U.S. dairy herds today, she said a complete cull and replace method may not always work; but if they are purchasing new animals for their herd, they should know the Johne's status of the animals. "It is hard to find the status level four herds these days, but if at all possible, producers should think about spending a little more money to prevent a disease like Johne's in their herd," Stable said. "Once Johne's is present in a herd, it may be hard to completely get rid of it, but you could markedly reduce its presence." Stable said the MAP organism is a very difficult one to work with since the biology of it is so complex. "We continue to work toward getting a usable test for producers as quickly as we can." Jennifer Bremer can be reached by phone at 515-833-2120, or by e-mail at jbremer@hpj.com. Advertisement
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