Over the past several months, I have examined several beef cows that presented with chronic weight loss and diarrhea. Based on clinical findings, I decided to test several for Johne’s disease, and found cows from four separate farms that tested positive for the disease.
Johne’s (pronounced Yo-nees) disease is a form of inflammatory bowel disease caused by the bacterium Mycobacterium avium ss. Paratuberculosis (MAP). First described by a German veterinarian in 1895, this disease is generally contracted by very young animals, but no clinical symptoms are seen for months or even years. The bacteria are spread by fecal-oral transmission; most animals are infected soon after birth after contact with their infected dam or another infected cow in the herd that is in close proximity. Cattle are the primary species infected, but any ruminant species can be infected.
The MAP bacteria is very slow growing in the intestinal tract, thus taking, in many cases, years to develop enough to cause clinical disease. The foremost clinical symptoms associated with the disease are profound diarrhea and weight loss. Diarrhea will be very watery, but normal manure can be seen in some animals with advanced disease. Weight loss occurs as a result of chronic damage to the small and large intestine. This damage inhibits the intestinal ability to absorb nutrients; thus the animal eventually dies of starvation. Most infected animals will have normal appetites even with severe clinical symptoms. Research has shown that non-clinical cows are not as productive, producing less milk and lower reproductive efficiency compared to healthy cows.
Spread of Johne’s occurs mainly by ingestion of fecal material from an infected animal. Animals can be infective for months to years before they exhibit signs of disease, and can spread bacteria into every farm environment that they contact. Many calves become infected in calving areas, whether in calving barns seen on dairy farms, or in pasture situations where bedding is put out in winter and cows congregate in small areas to calve. Johne’s disease can also be transmitted through colostrum obtained from infected cows. Beef farms that obtain colostrum from the local dairy can transmit bacteria and infect calves in this way.
Testing for Johne’s disease can be difficult to interpret. Simple blood serologic tests are inexpensive and fast, but the accuracy when used as a screening method in cattle not showing disease symptoms is approximately 70 percent for detecting an infected animal. Fecal culture is the gold standard test method, but cultures can take weeks to get adequate growth for identification; newer methods of culture have shown promise for quicker results. PCR testing of blood or manure is highly accurate, but can be expensive if several animals are tested. If you decide to test, your veterinarian can outline a testing program that is best for your herd.
There is currently no effective treatment for Johne’s disease, so prevention is the best method to limit the disease in your herd. Maintain clean calving areas and keep bedding as clean as possible to limit exposure. Try to limit feeding colostrum obtained from another herd unless you know the status of the donor cow. Test any animal that is losing weight, has diarrhea, and continues to have a normal appetite. It is a good idea to check for internal parasites at the same time to rule out parasitism as a cause of weight loss and diarrhea.