Johnes Disease is a chronic wasting disease caused by the bacterium Mycobacterium avium paratuberculosis . It can affect any ruminant. Young animals are more susceptible than adults to contracting the disease. It is primarily spread by the fecal-oral route but may also be transmitted across the placenta and through milk and colostrum of infected does. Transmission through milk and across the placenta are more common in animals showing clinical signs of disease. After an animal is exposed it will either clear the organism or develop a chronic persistent infection, depending on its immune status.
The most consistent clinical sign in sheep and goats is chronic weight loss despite a good appetite. Although profuse diarrhea is common in cattle with Johne’s Disease, this sign is not common with goats or sheep. It is important to differentiate this disease from internal parasites vecause these condctions may look similar. This wasting condition eventually results in death. Many infected animals may take months to years to show clinical signs of illness. Meanwhile, an infected animal can be shedding the organism in its feces, contaminating the environment and other animals in the herd.
Diagnosis: Culture of the organism from feces is the official test; however, this process may take eight weeks or longer. Newer diagnostic methods available are blood testing by the enzyme-linked immunosorbent assay (ELISA) and agar-gel immunodiffusion (AGID). Both these tests are rapid and results are available within 48 hours. These tests can be useful as screening tools for whole-herd management, ELISA as a whole-herd test and AGID as a test for individuals. The results from the AGID are more reliable than the ELISA in sheep and goats.
Treatment: There is no effective treatment, so prevention and control are very important. Preventing the introduction of the Johne’s organism into a herd can be difficult. The best prevention is by maintaining a “closed herd” but this may not be practical. Blood testing of animals is helpful when non-infected animals are purchased, but it is not 100-percent reliable. The herd history should be obtained before purchase, with special attention paid to the age and body condition of the herd’s oldest animals. Newly-purchased animals should be cultured every six months due to the subclinical nature of the disease. Infected animals should be culled from the herd.
Control: Control is a combination of management, education and periodic screening. Control of the fecal-oral route of infection is a vital part of any herd management plan. Consult a veterinarian or livestock extension agent to help develop effective management plans and confirm and cull any suspect animals. A clean environment is important, especially the kidding barn and pastures. Using above-ground feeders and waterers and cleaning does’ udders before nursing young are two methods that will help control the fecal-oral route of infection.
For More Information: The Johne’s Disease Q & A for Goat Owners booklet produced by the National Johne’s Education Initiative is available at http://www.johnes.org/handouts/files/JohnesGoatBooklet.pdf. Detailed information about Johne’s Disease is available at the University of Wisconsin College of Veterinary Medicine’s Johne’s Information Center at http://johnes.org/.