Johne's disease is caused by Mycobacterium paratuberculosis, a slow-growing bacteria that invades the animal's small intestine causing a thickening of the intestinal wall. Johne's impairs the function of the digestive tract reducing nutrient utilization necessary for body weight gain, milk production, reproduction and eventually life itself.

Johne's disease has been found on 20-25 percent of U.S. dairy operations, affecting 5-10 percent of all U.S. dairy cows. Prevalence is assumed to be lower in the beef industry, however the beef industry has not been testing to the same extent as the dairy industry.

Johne's disease impacts the overall health and productivity of the animal. For dairy cattle, the loss of milk production is the most direct loss of income. However other economic factors such as premature culling, reduced slaughter value, treatment and cost of replacement animals also affect the economic viability of the dairy farm. In addition, researchers have shown a relationship between Johne's disease and reproductive performance, which leads to further economic losses.

Calves are the most susceptible to infection, especially during the first few weeks of life. The majority of Johne's infections in young animals are acquired by ingestion of M. paratuberculosis. This happens when calves consume infected manure or infected milk/colostrum. A third, but less common route of infection is by in-utero transmission.

The National Johne's Working Group (NJWG) has identified specific management areas that should be addressed to reduce or prevent infections in a herd. Management practices to control Johne's disease are not complicated and are generally simple and inexpensive to implement. The NJWG recommendations include: manure management, colostrum and milk management, and identification and removal of infected animals and their offspring.

There are nearly a dozen tests that can be performed to diagnose Johne's disease. Tests that detect antibodies to M. paratuberculosis, such as the blood or milk ELISA are most often recommended as screening tests to determine the herd-level prevalence of Johne's and identify high-risk animals. Tests that detect the M. paratuberculosis organism, like traditional fecal culture or the Rapid Fecal Test, are useful to confirm Johne's in high-risk animals and to identify the most problematic animals for transmission.

Frequency depends on how aggressively producers want to reduce or eradicate Johne's disease. The Johne's disease picture in a herd will not change significantly from month to month and thus testing intervals should be closer to a year. Testing may even be discontinued in certain groups of animals that have been designated Johne's free with a high degree of confidence by testing negative over repeated testing intervals.

Johne’s disease is a state mandated reportable disease, however it is not actionable, and the state regulatory agency is not out to remove the test-positive animals from the farm. Reporting assists the state in managing Johne’s by determining statewide disease incidence and location. Furthermore, through the Freedom of Information Act (FOIA) reportable disease information can be requested, however information is released on a countywide basis only, and information pertaining to a specific operation is kept strictly confidential. This protects producers who take the initiative and manage for Johne’s and who are actively using smart management practices to protect their herd and the industry.

Sensitivity refers to a tests ability to detect infected (positive) animals on a percentage basis. Sincemany diseased animals are in the early stages of Johne's, current tests are unable to detect these animals and thus, test sensitivity is approximately 50 percent.

Specificity refers to the tests ability to detect non-infected (negative) animals on a percentage basis. Specificity is high for Johne's disease tests (> 95 percent) and a single positive result in herds confirmed with Johne's disease is likely correct.

More evidence is accumulating that Mycobacterium paratuberculosis is associated with Crohn's disease. However, a cause and effect has not been established. It is uncertain if Mycobacterium paratuberculosis in Crohn's patients caused the disease or if Mycobacterium paratuberculosis is just able to survive in Crohn's patients because of compromised immune systems. Mycobacterium paratuberculosis is found throughout the environment, not just in milk and meat from infected animals.

Licensed tests are monitored for their consistency in manufacturing and their performance against specified claims. On the other hand, certification or accreditation refers to the laboratory conducting tests and it acknowledges the lab's ability to correctly conduct specific tests. USDA accreditation/certification is only available for serum and fecal Johne's tests. The USDA has not yet devised a certification/accreditation program for the milk ELISA.