Recently in the headlines, Johne’s Disease— also known as Tuberculosis—is a bacterial disease caused by Mycobacterium avium subspecies paratuberculosis that is both contagious and chronic in nature. Affected animals often do not demonstrate any clinical signs until the condition has progressed as they have aged. Typical signs may include: emaciation, weakness, anorexia, decreased production and weight gain, and a moist cough that does not resolve with routine antibiotic therapy.
The disease can be easily transferred into a herd via new animals, especially when multiple sources of new herd mates are involved. The transfer of bacteria results from breathing in aerosols, contaminated food and water, and from drinking raw milk—colostrum for neonates in particular. Infected deer are also a common source of contamination if they are residing with, or in the same region, as your herd. Although risk of disease transmission is more related to close confinement and contact between animals, any saliva or discharge that contaminates group water and food areas are potential instigators.
Suspect animals should be housed in an isolation stall until further diagnosis can be conducted by your veterinarian. A commonly utilized test for infection is called the tuberculin test. The test subject is injected with tuberculin and analyzed again 72 hours later for a reaction spot under the skin.
A recent study was presented in the Journal of the American Veterinary Medical Association (JA- VMA) regarding the economic differences in calf weaning weights between beef cows that were infected with M. paratuberculosis and those that were not. The analysis included nearly 5,000 animals enrolled in the USDA National Johne’s Disease Demonstration Herd Project. Calves born from M. paratuberculosis-positive cows were found to have significantly lower adjusted weaning weights than those calves from negative test cows. Previous studies have also noted that infected cows took longer to conceive and had lower birth weights.
Overall, these studies report data that many producers can tell us from experience—the economic factor is paramount, as accumulated deaths and sales of underweight cows infected with M. paratuberculosis can cause a severe loss of income for herd managers.
APHIS documented over 1 million animals being tested annually in the United States. The national surveillance program targets testing on adult animals (over 18 months of age) that are suspected to have disease, are changing ownership, are registered for exhibition, those that may be transported interstate, and as a part of routine slaughterhouse monitoring. The prevalence of disease in beef herds varies depending on the method used to identify positively infected cattle. One survey noted in JAVMA stated that approximately eight percent of herds contained cattle that had tested positive for Johne’s Disease.
Testing can be conducted by your veterinarian in several ways. A less utilized option is the bacterial culture of manure samples due to the long growth period of the bacterium. Direct PCR to detect M. paratuberculosis genetic material can also be run on manure samples; this test will often reveal results in less than 10 days from sample submission. Antibodies may be detected in blood or milk from contaminated cows, as well.
Farm biosecurity is paramount to control disease in your herd and to minimize economic losses.
Wildlife should be prevented from utilizing herds’ water sources as much as possible; also, strict storage of feed sources is helpful.
More information on testing, treatment and your regional disease risk can be found from your local veterinarian, the Department of Natural Resources, and at the Johne’s Information Center website, www.johnes.org. — Dr. Genevieve Grammer (Dr. Genevieve Grammer is a mixed-species veterinarian practicing in eastern Colorado. Please direct correspondence to email@example.com)