Prevention is critical to controlling Johne's disease in a dairy herd.
Protect calves and youngstock from exposure to infected mature cows.
While many tests are available, results require careful interpretation to be used effectively.
Johne's disease is an infection caused by the bacterium Mycobacterium paratuberculosis. It has been estimated by the USDA (United States Department of Agriculture) that about 22% of U.S. herds are infected. The rate is higher for herds of 300 or more cows (~ 40%). Infection rates in other countries appear to be similar, especially ones with developed dairy industries. Johne's is a significant contributor to increased morbidity and cull rate in dairy herds. There is also increasing suspicion that Johne's is linked to Crohn's disease in humans, which has spurred greater interest in controlling and even eradicating this disease in dairy herds.
Control and Prevention
Prevention of this disease is, by far, the most effective method of controlling this disease on the farm. The most susceptible animals are the calves, as older animals develop resistance to the disease over time. Johne's is generally spread from the manure or milk of infected older cows to youngstock. The major areas of control focus on keeping older cows as isolated from youngstock as possible. This approach will also help control other infectious diseases such as: BVD, Salmonella, and S. aureus.
The best place is start is right at the beginning, in the maternity pens. Keep the pens clean and dry, as bacteria tend to flourish in wet environments. Use the "wet knee" test on maternity pen bedding. If you kneel in the bedding and your knee comes up damp, more bedding and more frequent cleaning are in order. In addition, cows that have diarrhea should not be allowed in the maternity area at all. Removing the calf immediately from the dam is important so as to avoid the ingestion of contaminated milk or manure. Colostrum and milk management is critical in controlling Johne's. Calves should receive an adequate amount of colostrum, but it should come from cows that test negative for Johnes. If there is a particularly high incidence of the disease in the herd, the use of a colostrum supplement should be considered. Either use milk replacer or pasteurize the milk that is fed to calves.
Another critical area is feed management. It is important to keep feed from becoming contaminated with manure. Never drive vehicles through feed alleys, especially if they have been in animal traffic areas. Always use separate equipment for manure handling and feed handling. Be aware of manure runoff on farms and control it from contact with feed. If runoff is a real problem, consider housing youngstock uphill from older animals. Maintain and use separate pastures and avoid using refused feed from mature cows for youngstock. There is some evidence that certain wildlife species may become infected with the Johne’s organism. However, it is currently unknown whether these species can transmit Johne’s back to cattle.
Animal Management and Testing
Culling positive cows is a major step towards eliminating Johne's in the herd. Removing them from the herd is important, as is identifying them so that their colostrum and milk are not used for feeding calves. Consider culling the calves from these cows, as they are more likely to become infected. Testing and isolation should be practiced with cows coming into the herd. If possible, buy cows only from herds that practice Johne's prevention programs.
There are two major types of tests for Johne's disease:
- fecal culture
- serum antibodies
The specificity of these tests is quite high, 100% for the fecal culture and 97% for the blood test. This means that, for the fecal test, a positive result always means that the animal does have the disease and that for the blood test, 97% of the positive tests are correct. The disadvantage to testing, however, is that test sensitivity is not very high (i.e. test results can be negative for a cow that really is infected), especially for the blood tests. Blood tests tend to be more popular since they are cheaper ($8-$12 vs $15-$25 USD) and faster (1 week vs 3-4 months). For the most part, testing should be done to determine probability of infection in the herd, instead of trying to obtain an absolute answer.
Research is also being done to develop a bulk milk test for Johne's antibodies and to develop a skin test of gamma interferon that is similar to the test used for diagnosing tuberculosis in humans.
While there are many options for testing, results require careful interpretation to be utilized effectively.
Diagnosis and Control of Johne's Disease
Board of Agriculture and Natural Resources
This e-book contains a more detailed description of the cause and impact of Johne's disease. It contains helpful appendices on interpretation of diagnostic results and descriptions of control programs in the United States.