Reference Library

JOHNE'S DISEASE
The Vet's Bag, September, 1997
by Stuart J. Burns, D.V.M

I had a nice letter from a Kentucky breeder a few weeks ago inquiring about Johne's disease (pronounced as yo-nees). This is a topic worth review, but not a favorite of veterinarians because it presents many difficulties in diagnosis, treatment, and recovery is not forthcoming.

Johne's disease is widespread in cattle in Europe, and purebred seedstock from these herds was its probable source of introduction to American herds years ago. Its incidence is highest in cattle kept under close confinement, but the frequency of infection is difficult to measure due to the uncertainty of diagnosis. Death loss in infected herds may approach 10%. These losses, accompanied by long periods of poor health and reduced performance, may evoke serious economic consequences in affected herds.

Most common in cattle, it can also affect sheep, goats, water buffalo, deer, antelope, camels and yaks! The disease is caused by Mycobacterium paratuberculosis, which may persist in infected pastures for up to a year. h is relatively susceptible to sunlight and drying, and soils high in calcium content with a high pH. It is interesting to note that herds on alkaline soils, particularly in limestone areas, may have a high incidence of infection, but little clinical disease. Adults from these herds often develop severe Johne's disease when moved to areas with acid soils. Young animals are most susceptible to infection, but clinical signs of disease may not appear for two years. Natural infection is the result of exposure to and ingestion of contaminated pastures, water or feed. (Of interest, there is some cross reaction in tests when herds are checked for TB, as some Johne's carriers will produce a weak positive for TB even though they are not infected with TB. Once clinical signs are observed, most of these cows will not react as a false positive for TB.) There is a long incubation period and the infected cow may spread the infection in her manure for 15 to 18 months before clinical signs of disease are observed. Some animals may become chronic carriers of the organism and never develop clinical signs of infection. These animals may remain undetected in the herd for years while they continue to contaminate the pastures.

After ingestion, the bacteria invades the tonsils, the intestinal lining and associated lymph nodes. The final result is chronic inflammation of the intestinal lining which causes hypermotility and a reduced ability to absorb fluids. This is the cause of the profuse diarrhea observed in clinical cases of Johne's disease. This diarrhea is usually not evident until the cow is two to six years old and leads to progressive weight loss and emaciation. These cows do not have a fever and appear otherwise healthy. The diarrhea is thin and watery with no offensive odor. A temporary improvement can be gained by removing the affected cow from pasture and putting it on dry feed. Some of these cows will appear to be improved in late pregnancy only to renew clinical diarrhea after calving.

Diagnosis is usually made on the basis of clinical signs. Pre-clinical animals can be difficult to identify, but an intradermal (in the skin) injection of yonin or avian TB in the neck region might identify an early case by causing a local swelling at the injection site. Cultures of the diarrhea are usually not rewarding because the bacteria is a slow and difficult organism to grow. Multiple efforts may he necessary to identify the bacteria on culture.

M. paratuberculosis is markedly resistant to antibiotics, and the response to treatment has been uniformly unrewarding. The lack of accurate tests and the long incubation make this a difficult disease to control. On a herd basis, control is attempted by removal of all infected animals that can be identified, fencing off water supplies, so that the cattle must drink from clean sources, not contaminated ponds or streams. These efforts may reduce the frequency of cases, but will usually not succeed in reaching a clean herd. The disease can be removed by destocking the ranch for one to three years, but this is sometimes not practical. Intensive grazing management should be avoided as this tends to increase bacterial contamination of the pastures by concentrating the manure in the grazed strips.

Efforts have been made to attempt vaccination with moderate results. Vaccination is not widely used in the US because it is not very effective and vaccinated animals become reactive to TB testing even though they do not have TB. The vaccine is not useful in an animal already infected with the disease. Johne's disease appears to be a little more common than it once was, but it is not yet widespread enough to attract regulatory efforts. It is unfortunate, but true, that if you have the misfortune of having a cow with Johne's disease, your best course is probably to cut your losses and send the cow to slaughter. This is not a solution that is pleasing to the owner, the veterinarian or the cow. In short, Johne's disease is a bad deal!

If you have questions you would like to have Dr: Burns answer please let us know at the Trails office or write him at P0. Box 4J, Pans, KY 40362.

Reprinted with permission of Texas Longhorn Trails Magazine   and/or TLBAA (Texas Longhorn Breeders Association of America) Lucky S&L Ranch P.O. Box 18757 Corpus Christi, TX 78480-8757 Phone: (361) 949-7197(H) or (361) 949-6919(O) Fax: (361) 949-7405